Bioinspired Surfaces Adapted from Lotus Leaves for Superliquiphobic Properties DISSERTATION Presented in Partial Fulfillment Of

Bioinspired Surfaces Adapted from Lotus Leaves for Superliquiphobic Properties

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Samuel Graeme Martin, M.S.

Graduate Program in Mechanical Engineering

The Ohio State University

2017

Dissertation Committee:

Professor Noriko Katsube, Advisor

Professor Anthony Luscher

Professor Shaurya Prakash

Copyrighted by

Samuel Graeme Martin

2017

Abstract

Nature can be turned to for inspiration into novel engineering designs that help address scientific difficulties. Through evolution, nature has created efficient and multipurpose objects using commonly occurring materials. These objects have many applications that can aid humanity and can be of commercial interest. One technical difficulty that nature can help solve includes liquid repellency. Inspiration for extreme liquid repellency, also known as superliquiphobicity, can be found on lotus leaves

(Nelumbo nucifera) due to their extreme water repellency. The motivation for studying the surface of lotus leaves is that their unique surface features can be adapted for commercial applications to save time, money, and lives. Nature has a limited material toolbox, but by incorporating synthetic materials and better manufacturing processes, the surface properties can be enhanced. Mimicking these biological structures and using them for design inspirations is the field of biomimetics.

In this thesis, an introduction chapter on biomimetics and liquid repellency is first presented. These principles are referred to throughout the thesis for creating superliquiphobic surfaces. Next, a chapter on experimental procedure and sample characterization is presented. Afterwards, three chapters are presented containing original research on surfaces inspired by lotus leaves for liquid repellency. Lotus leaf surfaces were created with several manufacturing methods including spray coating, vapor and spin

ii coat deposition, and micropatterning. These surfaces were characterized for liquid repellency using contact angle and tilt angle with water and hexadecane and in some cases using shampoo and laundry detergent. This work provides discussion on optimal design as well as valuable insight for superliquiphobic surfaces. The objective of studying these surfaces was to understand their underlying principles for improved surface design in superliquiphobic applications. This design knowledge has applications in a wide variety of industries as surfaces with these properties continue to develop and the number of applications requiring these properties increase.

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Dedication

Dedicated to my parents

Acknowledgments

I could not have completed this dissertation without the assistance of many people in my life, and I would like to take the time to acknowledge them. I would like to thank

Prof. Katsube, who guided me in the last stages of my work. I would also like to thank my committee members, Prof. Luscher and Prof. Prakash, for their advice and comments in completing my dissertation. I also would like to thank Prof. Subramaniam and Prof.

Bons for their insight and guidance. I would like to mention my lab colleagues, Greg

Bixler, Dave Maharaj, Yongxin Wang, Philip Brown, Renee Ripley, Dev Gurera, and Joe

Cremaldi, for their advice and assistance in my research. Furthermore, I would like to thank Joe West, Kevin Wolf, Chris Adams, and Chad Bivens for their electrical and machine shop expertise. In addition, I thank the advising and administrative staff of the mechanical engineering department who have been extremely supportive during my undergraduate and graduate career.

I would like to express my gratitude to my parents, Brent and Kathy Martin, and my sister, Sara Martin, who have encouraged me throughout my academic career. I would like to thank my girlfriend, Rachel Saloman, for her unwavering support during my graduate career. Lastly, I would like to thank all my friends for their encouragement over the years.

Lastly, I thank The Ohio State University for my academic and research opportunities. I also thank the university for their support in the form of a University

Fellowship and the Department of Mechanical and Aerospace Engineering for their support in the form of a Graduate Teaching Assistantship.

Vita

2009 ...... Toledo Technology Academy, Toledo, OH

2013 ...... B.S. Mechanical Engineering, The Ohio

State University

2015 ...... M.S. Mechanical Engineering, The Ohio

State University

Publications

1. Martin, S., and Bhushan B. (2014), “Fluid flow analysis of a shark-inspired

microstructure,” J. Fluid Mech. 756, 5–29.

2. Martin, S. and Bhushan, B. (2016), “Fluid flow analysis of continuous and

segmented riblet structures,” RSC Adv. 6, 10962–10978.

3. Martin, S. and Bhushan, B. (2016), “Modeling and optimization of shark-inspired

riblet geometries for low drag applications,” J. Colloid Interface Sci. 474, 206–

215.

4. Martin, S. and Bhushan, B. (2016), “Discovery of riblets in a bird beak

(Rynchops) for low fluid drag,” Phil. Trans. R. Soc. A 374, 20160134.

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5. Martin, S. and Bhushan, B. (2017), “Transparent, wear-resistant,

superhydrophobic and superoleophobic PDMS surfaces,” J. Colloid Interface Sci.

488, 118–126.

6. Martin, S., Brown, P. S. and Bhushan, B. (2017), “Fabrication techniques for

bioinspired, mechanically-durable, superliquiphobic surfaces for water, oil, and

surfactant repellency,” Adv. Colloid Interface Sci. 241, 1–23.

Fields of Study

Major Field: Mechanical Engineering

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Table of Contents

Abstract ...... ii

Dedication ...... iv

Acknowledgments...... v

Vita ...... vii

Publications ...... vii

Table of Contents ...... ix

List of Tables ...... xiii

List of Figures ...... xiv

Chapter 1: Introduction ...... 1

1.1. Biomimetics ...... 1

1.2. Significance of the problem ...... 4

1.3. Overview of liquid repellency ...... 5

1.3.1. Wettability ...... 5

1.3.2. Fluorinated compounds ...... 10

1.3.3. Re-entrant geometry ...... 11

1.4. Objective and layout ...... 14

Chapter 2: Experimental procedure and sample characterization ...... 16

2.1. Substrate descriptions ...... 16

2.1.1. Flat substrates ...... 16

2.1.2. Micropatterned substrates ...... 17

2.2. Fabrication methods ...... 20

2.2.1. Changing coating wettability ...... 21

2.2.2. Modifying coating resin and heating ...... 26

2.2.3. Ultraviolet-ozone surface activation treatment ...... 27

2.3. Sample characterization ...... 31

2.3.1. Contact angle and tilt angle ...... 31

2.3.2. Scanning electron microscope (SEM) imaging ...... 32

2.3.3. Coating thickness ...... 32

2.3.4. Repellency of surfactant-containing liquids ...... 34

2.3.5. Wear resistance ...... 35

2.3.5.1. Microwear with AFM ...... 35

2.3.5.2. Macrowear with tribometer...... 36

2.3.5.3. Fingerprint resistance with rubber finger tip or actual thumb ...... 36

2.3.6. Transparency ...... 38

2.3.7. Self-cleaning and anti-smudge ...... 39

2.3.8. Oil–water separation ...... 41

2.3.9. High temperature durability...... 41

2.4. Summary ...... 42

Chapter 3: Lotus-leaf-inspired surfaces for superliquiphobicity using micro- and nano- scale roughness ...... 43

3.1. Introduction ...... 43

3.2. Results and discussion ...... 48

3.2.1. Micropatterned substrates ...... 48

3.2.2. Wettability of surfaces ...... 50

3.2.3. Surface morphology ...... 55

3.2.4. Transparency ...... 60

3.3. Summary ...... 62

Chapter 4: Wear-resistant, substrate-independent coatings using hydrophobic nanoparticles for superliquiphobicity ...... 65

4.1. Introduction ...... 65

4.2. Results and discussion ...... 67

4.2.1. Wettability of surfaces ...... 68

4.2.2. Surface morphology ...... 73

4.2.3. Repellency of surfactant-containing liquids ...... 75

4.2.4. Wear resistance ...... 80

4.2.5. Self-cleaning and anti-smudge ...... 88

4.2.6. Transparency ...... 93

4.2.7. High temperature durability...... 94

4.3. Summary ...... 95

Chapter 5: Superliquiphobic coating adaptations using heating, different resins, and different surface functionalities ...... 97

5.1. Introduction ...... 97

5.2. Results and discussion ...... 98

5.2.1. Coatings using oven heating ...... 99

5.2.1.1. Wettability of surfaces with oven heating ...... 99

5.2.1.2. Wear resistance using oven heating ...... 101

5.2.1.3. Coating thickness ...... 105

5.2.2. Coatings using different resins ...... 106

5.2.2.1. Wettability of surfaces with epoxy resin (EPON 1002F) and oven heating ...... 107

5.2.2.2. Wear resistance with epoxy resin (EPON 1002F) and oven heating ..... 108

5.2.3. Coatings with different surface functionalities ...... 109

5.2.3.1. Wettability of surfaces ...... 109

5.2.3.2. Wear resistance ...... 110

5.2.3.3. Oil–water separation ...... 111

5.2.3.4. Superhydrophilic and superoleophobic surface functionality using other resins ...... 114

5.3. Summary ...... 116

Chapter 6: Summary and future work ...... 118

Bibliography ...... 121

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List of Tables

Table 1: Contact angle and tilt angles for water and hexadecane droplets on flat and micropatterned PDMS with various coatings ...... 52

Table 2: Glass and thermoplastic materials and their industrial applications ...... 66

Table 3: Contact angle and tilt angles for water and hexadecane droplets on various substrates with the nanoparticle-binder and fluorosilane coating at p-b = 2.5 ...... 72

Table 4: Comparison of static CA and TA before and after finger touch tests ...... 87

Table 5: Estimated mean contact pressures in wear resistance tests using Hertz analysis 88

Table 6: Comparison of static CA and TA before and after finger touch tests with oven heating ...... 104

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List of Figures

Figure 1: Bioinspiration from lotus leaf for liquid-repellency. A lotus leaf (Nelumbo nucifera) is superhydrophobic, repelling water droplets with contact angles of ~164°. SEM micrographs (shown at three magnifications) consist of a microstructure formed by papillose epidermal cells covered with 3-D epicuticular wax tubules, which create nanostructure (adapted from Bhushan et al., 2009)...... 3

Figure 2: Schematic of a liquid droplet in contact with a flat, solid surface (CA, θ0) and with a rough solid surface (CA, θ) (adapted from Bhushan, 2016)...... 6

Figure 3: Schematic of an interface in the Wenzel wetting model (complete wetting) and the in the Cassie-Baxter wetting model (composite wetting) (adapted from Bhushan, 2016)...... 8

Figure 4: Surface tension of water, ethylene glycol, hexadecane, and octane at varying temperatures (adapted from Jasper, 1972). As temperature increases, surface tension decreases...... 9

Figure 5: Liquid interacting with various surface geometries showing θflat. (a) Non-re- entrant geometry showing wettability based on θflat relationship to 90°. (b) Re-entrant geometry showing wettability based on θflat + α relationship to 90° (adapted from Brown and Bhushan, 2016a). (c) Spherical re-entrant geometry showing curvature that can support various θflat angles of ≤90° (adapted from Brown and Bhushan, 2016a)...... 12

Figure 6: Top-view of Si master pattern with cylindrical pillars of 14 μm diameter, 30 μm height, and 126 μm pitch...... 18

Figure 7: Schematic of a liquid droplet on a surface showing contact angle (CA), contact angle hysteresis (CAH), and tilt angle (TA)...... 18

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Figure 8: Four-step soft lithography procedure to create micropatterned samples from silicon master patterns. First, impression material is applied to the silicon master patterns to create negative molds (step 1). Next, epoxy is poured into the impression material mold to create a positive mold (step 2). Afterwards, urethane is poured to create a negative mold from the positive epoxy mold (step 3). Lastly, PDMS is poured to create final samples from the negative urethane molds (step 4). This procedure is necessary to ensure proper de-molding and sample replication...... 19

Figure 9: Three nanoparticle-binder coating techniques for adaptable wettability. (a) Superhydrophobic and superoleophilic method using nanoparticles and binder. (b) Superhydrophilic and superoleophobic method created by adding ultraviolet-ozone treatment and fluorosurfactant to (a). (c) Superhydrophobic and superoleophobic method created by adding ultraviolet-ozone treatment and fluorosilane to (a)...... 22

Figure 10: Chemical composition for each step in the coating process. (a) Deposition of the nanoparticles and binder for superhydrophobic and superoleophilic properties. (b) Surface activation using ultraviolet-ozone treatment. (c) Deposition of fluorosurfactant where the hydrophilic head group of the fluorosurfactant is favorably attracted to the chemically activated surface resulting in the hydrophobic tail pointing away from the surface and superhydrophilic and superoleophobic properties. (d) Deposition of fluorosilane, which bonds to -OH groups for superhydrophobic and superoleophobic properties...... 26

Figure 11: Schematic of ultraviolet-ozone lamp setup using a U-shaped, ultraviolet lamp capable of ozone production...... 28

Figure 12: (a) Irradiation using 185 nm and 254 nm (UVO) light. A wavelength of 185 nm decomposes molecular oxygen into atomic oxygen and synthesizes ozone. In the presence of 254 nm ultraviolet light, ozone decomposes into atomic and molecular oxygen. (b) Comparison of ultraviolet and ultraviolet-ozone surface activation processes. Both methods lead to a chemically active, hydrophilic surface; however, UVO is faster and more intense...... 30

Figure 13: Setup for the fingerprint tests: (a) rubber finger tip test at 5 N and (b) an actual thumb impression test at > 100 N...... 37

Figure 14: Optical images of Si master pattern and micropatterned PDMS showing feature replication after the four-step soft lithography procedure. When the nano-scale roughness is deposited, the micropillars have a larger diameter due to the added nanoparticles and resin...... 49

Figure 15: Comparison of liquids on flat and micropatterned PDMS with various coatings. Contact angle and tilt angles for water and hexadecane droplets on: flat and micropatterned PDMS; with fluorosilane; with nanoparticle-binder coating; and with nanoparticle-binder and fluorosilane coating...... 51 xv

Figure 16: SEM images of the nanoparticle-binder and fluorosilane coating. (a) The top- down view shows agglomerates of nanoparticles and binder that form micron-sized structures. (b) The tilt view shows these structures have quasi-spherical re-entrant geometries that were found to be repellent to water and hexadecane. Arrows point to re- entrant overhangs...... 56

Figure 17: Concept for nanoparticle agglomeration and re-entrant geometry formation. Nanoparticles and binder form clusters in solution that when deposited on a sample have re-entrant curvature...... 58

Figure 18: Photographs showing transparency on flat PDMS, superhydrophobic samples from flat PDMS with nanoparticle-binder coating and micropatterned PDMS, and superhydrophobic and superoleophobic samples from flat and micropatterned PDMS with nanoparticle-binder and fluorosilane coating. The reduction in transparency in the coated samples is due to the SiO2 nanoparticles and binder. Edges of each sample are shown in dashed lines. On the micropattern samples, the micropattern occurs in the center of the sample and outlined in white dashed line...... 61

Figure 19: CA and TA measured using water and hexadecane droplets on glass with the superhydrophobic and superoleophobic coating using methylphenyl silicone resin as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.5 is optimal for repellency and durability. The CA curve fits are 4th order polynomials, and the TA curve fits are 5th order polynomials. Error bars are one standard deviation...... 69

Figure 20: Droplets deposited on various substrates to show repellency for the coating. (a) Using glass and polypropylene substrates, water and hexadecane droplets were deposited to show superliquiphobicity. (b) Images of hexadecane droplets on various substrates with the nanoparticle-binder and fluorosilane coating to show that hexadecane beads up on each substrate for the substrate-independent ability. CA and TA measurements on these substrates were within one standard deviation (±2° for CA and ±1° for TA)...... 71

Figure 21: SEM images of the nanoparticle-binder and fluorosilane coating on glass and PP substrates. The top-down view (shown at two magnifications) shows agglomerates of nanoparticles and binder that form micron-sized structures. The tilt view shows these structures have quasi-spherical re-entrant geometries that were found to be repellent to water and hexadecane. Arrows point to re-entrant overhangs...... 73

Figure 22: SEM images of the nanoparticle-binder and fluorosilane coating on PP substrates at p-b = 2.5. The circles in the top-down view (same image as in Figure 21) are agglomerations of nanoparticles and resin that are possible re-entrant geometries. The tilt view several re-entrant geometries within one image...... 74

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Figure 23: Photographs demonstrating repellency of surfactant-containing liquids by comparing shampoo and laundry detergent droplets deposited on untreated and nanoparticle-binder and fluorosilane surfaces using (a) glass substrate and (b) polypropylene substrate. On the untreated substrates, the droplet spreads out and adheres to the surface. On the coated substrates, the droplet rolls and slides off the surface without any residue remaining...... 77

Figure 24: Nanoparticle-binder and fluorosilane coating showing repellency after prolonged contact with shampoo. (a) The coating can be dipped into shampoo for one minute and withdrawn with minimal shampoo residue on the surface. (b) The coating can be repeatedly dipped into shampoo with minimal shampoo residue on the surface after 10 submerges. (c) Schematic of setup and meniscus profile on the uncoated and coated side of the substrate (side view)...... 79

Figure 25: Nanoparticle-binder and fluorosilane coating showing AFM wear durability. Sample surface profiles (indicated at the arrow) and surface heights maps are shown before and after the AFM wear test using a 30 μm diameter borosilicate ball mounted on a rectangular cantilever with a load of 10 μN...... 81

Figure 26: Tribometer wear experiments on glass for the nanoparticle-binder and fluorosilane coating with p-b = 2.5 and p-b = 4.0. As p-b ratio decreased, durability increased with minimal damage at lower p-b ratios...... 82

Figure 27: Images of hexadecane droplets before and after wear/scratching on coated glass. Droplets were dragged or tilted across the defect in direction of arrows. Before the wear test, droplets rolled off the surface at 2 ± 1° tilt angle. For the worn samples, droplets placed to the right of the wear track rolled over the defect at 5 ± 1° tilt angle, and droplets placed directly on the wear track rolled over the defect at 17 ± 2° tilt angle. Droplets on the scratched sample were pinned at the defect until 53 ± 4° tilt angle regardless of droplet starting position...... 83

Figure 28: Fingerprint test using rubber finger and thumb on the nanoparticle-binder and fluorosilane coatings on glass. With the rubber finger test, nib impressions can be seen with improvements at the lower p-b ratios. With the thumb impression, a change in light reflectivity can be seen on the p-b = 4.0 sample, but no change is seen on the p-b = 2.5 sample...... 86

Figure 29: Optical micrographs of contaminated coatings before and after self-cleaning test on untreated and nanoparticle-binder and fluorosilane samples using glass and polypropylene substrates. Image analysis shows >90% removal of particles on the techniques...... 89

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Figure 30: Optical micrographs of contaminated surface and oil-impregnated microfiber cloth before and after anti-smudge test on untreated and nanoparticle-binder and fluorosilane surfaces using glass and polypropylene substrates. Dark spots on coatings and cloth indicate silicon carbide particle contaminants. Untreated samples show oil transferred to the substrate and few contaminants removed via cloth. Coated samples show oil was not transferred to the substrate and that many contaminants were removed via the cloth...... 92

Figure 31: Percentage of silicon carbide contaminants removed from the surface during the anti-smudge test for glass and polypropylene substrates, either untreated or coated with nanoparticle-binder and fluorosilane. Image analysis shows >90% removal of particles for the coating compared to about 15-20% removal on the untreated substrates...... 93

Figure 32: Photographs showing transparency for untreated and nanoparticle-binder and fluorosilane on glass and PET substrates. The reduction in transparency in the coated samples is due to the SiO2 nanoparticles and binder. Edges of each sample are shown in dashed lines...... 94

Figure 33: Contact angles of water and hexadecane on nanoparticle-binder and fluorosilane coating on glass showing superhydrophobicity and superoleophobicity at droplets with temperatures up to 80°C. The coating repels water and hexadecane at lower surface tensions due to the higher temperatures as well as staying resistant to the higher temperatures...... 95

Figure 34: CA and TA measured using water and hexadecane droplets on glass with the superhydrophobic and superoleophobic coating using methylphenyl silicone resin with oven heating as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.0 is optimal for repellency and durability. The CA curve fits are 4th order polynomials, and the TA curve fits are 5th order polynomials. Error bars are one standard deviation...... 100

Figure 35: Nanoparticle-binder and fluorosilane coating with oven heating showing AFM wear durability. Sample surface profiles (indicated at the arrow) and surface heights maps are shown before and after the AFM wear test using a 30 μm diameter borosilicate ball mounted on a rectangular cantilever with a load of 10 μN...... 102

Figure 36: Tribometer wear experiments on glass for the nanoparticle-binder and fluorosilane coating and oven heating with p-b = 2.0...... 103

Figure 37: Fingerprint test using rubber finger and thumb on the nanoparticle-binder and fluorosilane coating with oven heating on glass. With the rubber finger test, no nib impressions can be seen. With the thumb impression, some fingerprint ridges can be seen on the 2.0 sample...... 104

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Figure 38: Coating thickness measurement with an average thickness of ~2 μm on the epoxy resin with oven heating sample at p-b = 2.0. The step created in the coating procedure was located 30 μm along the sample...... 105

Figure 39: Static contact angles on glass with different resins in the nanoparticle-binder and fluorosilane technique on glass to show that different resins can be used for similar coating properties of superhydrophobicity and superoleophobicity...... 106

Figure 40: CA and TA measured using water and hexadecane droplets on glass with the superhydrophobic and superoleophobic coating using epoxy resin with oven heating as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.0 is optimal for repellency and durability. The CA curve fits are 4th order polynomials, and the TA curve fits are 5th order polynomials. Error bars are one standard deviation...... 107

Figure 41: Wear experiments using approach with epoxy with oven heating coating at p-b = 2.0 on glass. (a) AFM wear experiments with significant wear in the wear region. (b) Tribometer wear experiment with a visible wear scar. Compared to similar wear experiments using methylphenyl silicone resin and oven heating at the same p-b ratio (shown in Figure 25 and Figure 26), the epoxy resin shows greater wear for the AFM and tribometer experiments...... 109

Figure 42: Other wettability states for the coating. With the nanoparticle-binder coating, superhydrophobicity and superoleophilicity was obtained. With the nanoparticle-binder and fluorosurfactant coating, superhydrophilicity and superoleophobicity was obtained...... 110

Figure 43: Photographs of hexadecane oil–water separation using the superhydrophobic/ superoleophilic and the superhydrophilic/ superoleophobic coating methods, both deposited on a stainless-steel mesh in a horizontal or tilted orientation...... 112

Figure 44: Photograph of oil–water separation using the superhydrophilic and superoleophobic technique with the agitated hexadecane oil–water mixture being poured over the coated mesh...... 113

Figure 45: Static contact angles on glass with different resins in the nanoparticle-binder and fluorosurfactant technique to show that different resins can be used for similar coating properties of superhydrophilicity and superoleophobicity...... 115

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Chapter 1: Introduction

1.1. Biomimetics

These objects have many applications that can aid humanity and can be of commercial interest. Nature has a limited material toolbox, but by incorporating synthetic materials and better manufacturing processes, the surface properties can be enhanced to save time, money, and energy. Mimicking these biological structures and using them for design inspirations is the field of biomimetics (Bhushan, 2016).

Nature has many functions that can serve as an inspiration for design. Some functions from flora and fauna include aerodynamic lift, structural coloration, anti- reflectivity, reversible adhesion, self-cleaning, and self-healing. Birds are an inspiration for aerodynamic lift and structural coloration, color produced by microstructured surfaces that interfere with visible light and not color produced by pigments (Bhushan, 2016). The eyes of moths are antireflective due to a nanostructured surface that eliminates reflections

(Genzer and Efimenko, 2006). The toes of a gecko exhibit reversible adhesion supporting a gecko as it climbs vertical surfaces. Gecko toes are covered in a hierarchical structure composed of many hairs that increase the van der Waals forces between the gecko and 1 the surface. The total attractive force is enough that a gecko can safely climb surfaces

(Autumn et al., 2000; Gao et al., 2005). Plants and insects such as rice leaves and butterfly wings exhibit self-cleaning to remove contaminants from their surfaces (Bixler and Bhushan, 2012b). Biological systems found in plants and animals exhibit self-healing in order to repair their structure (Diesendruck et al., 2015). Another function that can have biomimetic inspiration is liquid repellency.

Biomimetic inspiration for extreme liquid repellency, also known as superliquiphobicity, can be found on the lotus leaf (Nelumbo nucifera), shown in Figure

1. The lotus leaf is superhydrophobic (extreme water repellency) with water droplets remaining nearly spherical and easily rolling away. This superhydrophobicity originates from the hierarchical structure formed by the combination of papillose epidermal cells as the microstructure and 3-D epicuticular wax tubules covering these cells as the nanostructure. The papillae are hemispherical with a diameter of about 20 μm at the base.

The wax tubules are 1.5–2 μm in length with an outer diameter of 100 nm and a wall thickness of 30 nm. (Barthlott and Neinhuis, 1997; Burton and Bhushan, 2005; Jung and

Bhushan, 2006). Due to water droplets beading up and easily rolling off, the lotus leaf exhibits the self-cleaning effect. Contaminants on the surface of a lotus leaf are picked up by the rolling action of a droplet and are removed from the surface (Barthlott and

Neinhuis, 1997).

This hierarchical roughness and incorporation of wax in the form of nanotubules are important factors in the design of liquid repellency and can serve as the inspiration.

The hierarchical roughness features trap air pockets and therefore minimize solid surface

Figure 1: Bioinspiration from lotus leaf for liquid-repellency. A lotus leaf (Nelumbo

nucifera) is superhydrophobic, repelling water droplets with contact angles of

~164°. SEM micrographs (shown at three magnifications) consist of a

microstructure formed by papillose epidermal cells covered with 3-D epicuticular

wax tubules, which create nanostructure (adapted from Bhushan et al., 2009).

contact with a water droplet. This minimal contact helps repel water droplets and allow them to easily roll away. A lotus leaf surface is approximately five times as rough as a flat surface based on roughness factor, which is a ratio of a rough surface area to its projected surface area (Jung and Bhushan, 2006; Bhushan, 2016). The wax itself is slightly water repellent and when combined with the hierarchical roughness increases the water repellency (Bhushan, 2016). Models for liquid repellency and surface functionality are further described in Section 1.3.

1.2. Significance of the problem

Surface design is important for many industries as material properties, surface morphology, energy efficiency, and durability become more of a concern. One problem that these industries face is liquid repellency. Liquid repellency is the decreased interaction of a liquid with a surface.

This problem is found throughout commercial industries such as the automotive, aerospace, electronics, plastic packaging, and biomedical fields. Many surfaces could be designed to have high liquid repellency and the ability for liquids to easily roll off the surface, termed self-cleaning. Applications include touchscreens such as smart phones, computers, and automobile dashes; glass such as windows, windshields, and solar panels; and plastic packaging such as shampoo bottles and laundry detergent containers. On touchscreens, finger oils cause streaks and smears that decrease user experience. For glass, water and ice on windows and windshields decrease visibility and on solar panels decrease efficiency. In plastic packaging, shampoo and laundry detergents can remain in the container leading to wasted product, recycling difficulties, and decreased user experience.

Wettability problems are found on numerous surfaces. For automotive applications, materials include glass, PC, polydimethylsiloxane (PDMS), PET, nylon,

HDPE, polyurethane, polymethyl methacrylate (PMMA), PP, steel, aluminum, and leather. Materials of interest for smart screens in electronic displays include soda-lime silica, polyethylene terephthalate (PET), and polycarbonate (PC). For plastic bottles and caps, materials include high-density polyethylene (HDPE), PET and polypropylene (PP).

Plastic bottles are typically made of high density polyethylene (~70%), polyethylene terephthalate (~25%), and polypropylene (~5%). Bottle caps are typically made of polypropylene (Martin et al., 2017)

1.3. Overview of liquid repellency

A part of this research concerns surfaces that repel liquids and the application of surface depends on the wetting characteristics of a droplet on the surface. A droplet can be any liquid and is commonly denoted as hydro- for water or oleo- for oil. A suffix of - philic or -phobic is used when the droplet is attracted or repelled, respectively. Therefore, four states can be obtained: hydrophilic or oleophilic for wetting with a water or oil droplet and hydrophobic and oleophobic for repelling with a water or oil droplet, respectively. When the contact angle (CA) is less than 10° in a -philic state or greater than 150° in a -phobic state, the prefix super- is added to denote extreme attractancy or repellency. The ability to repel all liquids is termed superomniphobicity or superliquiphobicity.

1.3.1. Wettability

The wetting of a droplet on a solid surface is dependent upon surface chemistry and surface roughness. The effect of surface roughness on contact angle is shown in

Figure 2 where the CA on a flat surface is θ0. On a flat surface with a low surface energy, the maximum achievable CA with a water droplet is ~120° (Nishino et al., 1999). In order to increase the CA, roughness-induced superhydrophobicity can be utilized

(Nosonovsky and Bhushan, 2008; Bhushan, 2016).

Figure 2: Schematic of a liquid droplet in contact with a flat, solid surface (CA, θ0) and

with a rough solid surface (CA, θ) (adapted from Bhushan, 2016).

The CA on a flat surface can be determined using Young’s equation (also called

Young-Laplace or Young-Dupré equation) shown in Equation (1).

훾푆퐴 − 훾푆퐿 cos 휃0 = (1) 훾퐿퐴

In Equation (1), γSA, γSL, and γLA are the solid-air, solid-liquid, and liquid-air surface tensions, respectively. Surface molecules do not have similar molecules on all sides, and therefore they more strongly cohere to each other and this enhanced attraction is called surface tension (Bhushan, 2016; Prakash and Yeom, 2014).

For self-cleaning, anti-smudge, antifouling, and low-adhesion, another property of interest is contact angle hysteresis (CAH), which is the difference between advancing and receding contact angles. CAH can be shown to be related to the energy dissipation of a droplet flowing along a surface. Low CAH results in a droplet rolling along a surface at a very low tilt angle (TA), which facilitates particle removal for self-cleaning. Tilt angle is

6 the angle a surface needs to inclined for the droplet to roll off the surface (Miwa et al.,

2000; Bixler and Bhushan, 2012a; Bhushan, 2016).

On a rough surface, two wetting states have been observed: the Wenzel model and the Cassie-Baxter model (Figure 3). In the Wenzel model, liquid fully penetrates the roughness features, which creates complete wetting of the solid interface. The change in

CA in the Wenzel model can be determined using the Wenzel equation shown in

Equation (2) (Wenzel, 1936).

(2) cos 휃 = 푅푓 cos 휃0

The roughness factor Rf is a ratio of the rough surface area to its flat projected area. The roughness factor is greater than one because the rough surface area will be greater than the flat projected area. When the CA on a flat, solid surface is greater than 90°, the roughness factor increases repellency, further towards superliquiphobicity. Similarly, when the CA on a flat, solid surface is less than 90°, the roughness factor decreases repellency, further towards superliquiphilicity (Wenzel, 1936).

In the Cassie-Baxter model, the liquid droplet sits on top of the roughness asperities with air pockets trapped in between, which creates a composite solid-air-liquid interface. The Cassie-Baxter equation can be used to determine the CA on a composite interface containing solid-liquid and liquid-air interfaces shown in Equation (3) (Cassie and Baxter, 1944).

(3) cos 휃 = 푅푓 cos 휃0 − 푓퐿퐴(푅푓 cos 휃0 + 1)

Figure 3: Schematic of an interface in the Wenzel wetting model (complete wetting) and

the in the Cassie-Baxter wetting model (composite wetting) (adapted from

Bhushan, 2016).

The fraction of the composite interface that is the solid-liquid interface is fLA. This equation predicts that the CA on a surface will increase as the fractional liquid-air interface increases (Cassie and Baxter, 1944).

The wetting behavior in the Cassie-Baxter model is preferred for liquid-repellent surfaces due to droplets resting on surface asperities and air pockets. The high liquid-air fractional contact area also leads to low CAH, which is important for droplets to easily roll off a surface at a low TA, known as self-cleaning.

The wetting state is dependent on the surface tension of the liquid and surface roughness. At 25°C, water has a surface tension of 72.0 mN/m. Oils have lower surface tensions that commonly range from 21.1 mN/m for octane and 27.0 mN/m for hexadecane, but can extend up to 47.7 mN/m for ethylene glycol (Jasper, 1972; Haynes,

2014). Studies that solely investigate oil repellency using diiodomethane (50.80 mN/m) and/or ethylene glycol (47.70 mN/m) are less likely to be repellent to other lower 8

Figure 4: Surface tension of water, ethylene glycol, hexadecane, and octane at varying

temperatures (adapted from Jasper, 1972). As temperature increases, surface

tension decreases.

surface tension oils. A more appropriate choice for oil-repellency testing is hexadecane

(27 mN/m) (Martin et al., 2017).

As temperature increases, the surface tension decreases as shown in Figure 4.

Temperature is important because at higher temperatures, surface tensions can decrease and surface can become fouled. The interior of a car on a warm day can reach 80°C

(Manning and Ewing, 2009) and change the repellency of liquids on dash displays.

Surfactant-containing liquids, such as shampoo and laundry detergent bottles, are more difficult liquids to repel due to their low surface tension and highly active, polar head group that adheres to surfaces more strongly than oils. Surfactant-containing liquids have surface tensions of 25–60 mN/m, depending on concentration with higher surface

9 tensions in the product if the surfactant is heavily diluted (Ross and Epstein, 1958; Al-

Sabagh et al., 2011).

While superhydrophobicity is common in nature, superoleophobicity is uncommon and more difficult to accomplish, as the surface tensions of oils are much lower than that of water (Jasper, 1972; Haynes, 2014). Certain species of leaf-hopper display high CA for diiodomethane and ethylene glycol (Rakitov and Gorb, 2013) and springtails show repellency to olive oil (Hensel et al., 2013). However, surfaces from nature with repellency to liquids <30 mN/m have not yet been found.

In order to create superoleophobic surfaces, fluorinated components and re- entrant geometries are typically required (Nosonovsky and Bhushan, 2008). Fluorinated components help repel liquids with low surface tensions such as oils by creating a low surface energy material. Re-entrant geometries have overhang structures, such as spherical geometries, where surface features become narrower at the base (Nosonovsky and Bhushan, 2008; Brown and Bhushan, 2016a).

1.3.2. Fluorinated compounds

Fluorinated components help repel low surface tension liquids such as oils by reducing adhesion forces. Oil is non-polar and therefore only interacts with another molecule through a London dispersion force, which is a temporary attractive force due to a pair of induced dipoles. Polarizability depends on the mobility of electrons and therefore quantifies the ability of a molecule to form instantaneous dipoles. Fluorine is commonly chosen to create low surface energy materials due to its low polarizability. In addition, fluorine is highly electronegative, which measures the tendency of an atom to

10 attract bonding electrons to itself. Fluorine only requires one more electron in its 2p electron shell to create a stable electron configuration. The low polarizability and high electronegativity of fluorine leads to weak London dispersion, cohesive, and adhesive forces. Consequently, fluorinated materials are good choices for creating materials with a low surface energy (Brown and Bhushan, 2016a).

1.3.3. Re-entrant geometry

Fluorinated materials are combined with re-entrant geometries for improved repellency. Re-entrant geometries are shapes that have overhang structures where the surface features become narrower at the base. Re-entrant curvatures can be inverse trapezoidal, spherical, etc. and lead to higher CA than non-re-entrant geometries

(Nosonovsky and Bhushan, 2008; Brown and Bhushan, 2016a). These geometries are necessary for repelling low surface tension liquids such as oils and surfactant-containing liquids.

A schematic explaining liquid wettability with various surface geometry and θflat is shown in Figure 5. The θflat CA is the angle formed by the surface and the tangent of the solid-liquid interface. In the Cassie-Baxter model, previously explained, air pockets are formed due to surface roughness (Figure 3) and results in a higher CA than the complete wetting in the Wenzel state. For the geometry shown in Figure 3, the ability to achieve the Cassie-Baxter model relies on θflat ≥ 90°. As shown in Figure 5a, a non-re- entrant surface (α = 0°) has a flat CA of 70° or 110°. The solid-liquid surface tension

(shown with Fγ) force acts along the liquid surface in the direction of the solid surface.

When θflat is less than 90°, the direction will be downward toward the bulk surface

Figure 5: Liquid interacting with various surface geometries showing θflat. (a) Non-re-

entrant geometry showing wettability based on θflat relationship to 90°. (b) Re-

entrant geometry showing wettability based on θflat + α relationship to 90°

(adapted from Brown and Bhushan, 2016a). (c) Spherical re-entrant geometry

showing curvature that can support various θflat angles of ≤90° (adapted from

Brown and Bhushan, 2016a).

showing wetting; when θflat is greater than 90°, the direction will be upward away from the bulk surface showing non-wetting (Liu and Kim, 2014; Brown and Bhushan, 2016a).

However, a high CA can be achieved, even if the CA of a liquid on the flat surface is small, by creating re-entrant structures. For example, Figure 5b shows a re- entrant surface with inverse trapezoidal features where the combination of the re-entrant angle (α) and θflat controls the wetting behavior. If the combination of angles (α + θflat) is

<90°, then the liquid will wet the re-entrant geometry with a surface tension force pointed 12 downward. By choosing α + θflat ≥ 90°, liquid can be repelled even when θflat < 90°.

When the liquid is repelled, the surface tension force points upward along the trapezoidal features. Instead of the liquid fully wetting the surface, vapor pockets and a composite interface with a low liquid-solid contact fraction are formed (Brown and Bhushan,

2016a).

Structures with re-entrant curvature (spherical, cylindrical, oval, etc.) as shown in

Figure 5c can support high droplet CA for various liquids with flat CA <90° since it is possible to draw multiple tangents of a corresponding flat surface. Therefore, liquids with various flat CA can wet the re-entrant curvature to different extents to achieve a favorable liquid–vapor interface shape with the surface tension force directed upwards (Brown and

Bhushan, 2016a).

Re-entrant structures have been found in nature on leaf-hoppers and springtails.

Leafhoppers are plant-feeding insects that suck plant sap and are coated in brochosomes that help act as a liquid barrier to water, diiodomethane, and ethylene glycol.

Brochosomes are spherical honeycombs (200–700 nm diameter) with pentagonal and hexagonal openings that lead to a hollow core (Rakitov and Gorb, 2013). Springtails are wingless anthropods adapted for living in temporarily flooded environments. The entire outside of a springtail is covered in a hexagonally arranged honeycomb structure with granules located at the structure intersections. Cross-sections of the honeycomb show T- shaped overhangs (Hensel et al., 2013). These features on leaf-hoppers and springtails exhibit re-entrant geometries in nature.

Re-entrant geometries have been physically created using a positive-negative, photolithography, sacrificial, or crystallization fabrication techniques. Im et al. (2010) used a PDMS positive-negative replication process along with a 3D diffuser lithography technique to create inverse-trapezoidal microstructures (similar to Figure 8b). Kang et al.

(2012) created mushroom-like PDMS micropillars via photolithography and a molding procedure. Utech et al. (2016) used a sacrificial replication procedure to create spherical overhangs. Brown and Bhushan (2016b) used a solvent-induced phase transformation for polymer recrystallization that formed quasi-spherical re-entrant structures.

1.4. Objective and layout

In the following four chapters, original research and methodology on surfaces inspired by lotus leaves is presented. Chapter 2 presents the experimental procedure and sample characterization used for the lotus leaf research in the next three chapters. The surfaces in this research were characterized for liquid repellency using contact angle and tilt angle with water and hexadecane and, in some cases, using shampoo and laundry detergent. Various manufacturing methods including spray coating, vapor and spin coat deposition, and micropatterning were implemented and are discussed in this chapter.

Chapter 3 discusses lotus-leaf-inspired surfaces that were created using a combination of micropatterning and a spray coating deposition technique with nanoparticles and binder.

Chapter 4 discusses the superliquiphobic coating and the various properties it demonstrates. Some of these properties include wettability, surface morphology, wear resistance, self-cleaning and anti-smudge, transparency, and high temperature durability.

Chapter 5 discusses adaptations to the coating which include adding a heating step to the coating process, varying the resin type, and varying the surface functionality.

The objective of this work was to develop an adaptable, superliquiphobic coating that has an easily scalable manufacturing technique and through this work understand the effects of surface structure on wettability. It is hypothesized that re-entrant geometries can be created in scalable manufacturing technique and that various wettability can be obtained with the same roughness by changing the surface chemistry.

Chapter 6 concludes the work with a summary and a plan for future work. These studies provide discussion on liquid repellency as well as valuable insight into superliquiphobic surfaces. This design knowledge has applications in a wide variety of industries as surfaces with these properties continue to develop and the number of applications requiring these properties increase.

Chapter 2: Experimental procedure and sample characterization

In this chapter, substrates are described, fabrication procedures for generating coatings will be explained, and then methods of characterizing the samples will be presented. Tested substrates included flat and micropatterned surfaces. There were several fabrication methods for generating coatings that varied the attraction or repellency to water and hexadecane. Sample characterization covers all the tests that were performed on the samples.

2.1. Substrate descriptions

This section describes the various flat substrates that were used and the methodology for constructing micropatterned substrates.

2.1.1. Flat substrates

Various flat substrates were used including soda-lime glass (125495, Fisher

Scientific), polypropylene (PP, ASTM D4101-0112, SPI), poly(dimethylsiloxane)

(PDMS, Sylgard 184, Dow Corning), polyethylene terephthalate (PET, Dexerials

Corporation, Japan), nylon 6/6 (8539K232, McMaster-Carr), polyurethane (PU,

8716K34, McMaster-Carr), poly(methyl methacrylate) (PMMA, 8560K171, McMaster-

Carr), polyethylene (PE, 8657K111, McMaster-Carr), and polycarbonate (PC, Lexan

SLX 2271T, Sabic). Various properties for general polymers are described in Brydson

(1999), Mark (1999), Harper (2000), and Harper and Petrie (2003). The PU that was used 16 is specifically Sorbothane® which is a thermoset, polyether-based, polyurethane material; however, the composition is not 100% known. It is expected that fillers were added to the thermoplastics and that these substrates are not 100% the listed material.

The typical substrate size for the glass and polymer samples (except for PDMS) was 25 by 10 mm created by cutting microscope slides using a glass cutter or cutting polymer sheet using an X-acto blade to size. In addition, larger sizes of 304 stainless steel mesh (85385T117, McMaster-Carr) were used as substrates for oil–water separation testing.

A PDMS sheet was fabricated using Sylgard® 184 (Dow Corning) with a thickness of 5 mm. The base and curing agent was mixed at a 10:1 weight ratio. The mixed, uncured PDMS was then degassed in a vacuum chamber at 100 mTorr for 15 min at room temperature to remove entrained air bubbles. Then, the mixture was fully cured in a gravity convection oven (1300U, Sheldon Manufacturing, Inc.) operating at 70°C for

2 hrs. The PDMS sheet was cut using an X-acto blade to dimensions of 10 by 10 mm for use as substrates.

2.1.2. Micropatterned substrates

Micropatterned PDMS was created from a silicon (Si) master pattern with pillars of 14 μm diameter, 30 μm height, and 126 μm pitch, shown in Figure 6. The master pattern was fabricated from single-crystal silicon using photolithography (Jung and

Bhushan, 2007). The photolithography process transferred features with about 1 μm precision (Barbieri, 2006). Using optical images of the Si master pattern, the dimensions were 14 ± 2 μm diameter and 126 ± 3 μm pitch. These micropattern dimensions were

Figure 6: Top-view of Si master pattern with cylindrical pillars of 14 μm diameter, 30 μm

height, and 126 μm pitch.

Figure 7: Schematic of a liquid droplet on a surface showing contact angle (CA), contact

angle hysteresis (CAH), and tilt angle (TA).

chosen based on a study of the role of pitch for cylindrical pillars in Si micropatterned surfaces with a fluorosilane coating and given droplet radius on contact angle (CA), contact angle hysteresis (CAH), and tilt angle (TA), shown in Figure 7. A pitch of 126

μm was found to have the greatest CA and smallest CAH and TA (Jung and Bhushan,

2007). A surface with high CA and low TA was desired for repellency and therefore this pitch was chosen. When CAH is small, the value for TA is about the same as the value for CAH.

Figure 8: Four-step soft lithography procedure to create micropatterned samples from

silicon master patterns. First, impression material is applied to the silicon master

patterns to create negative molds (step 1). Next, epoxy is poured into the

impression material mold to create a positive mold (step 2). Afterwards, urethane

is poured to create a negative mold from the positive epoxy mold (step 3). Lastly,

PDMS is poured to create final samples from the negative urethane molds (step

4). This procedure is necessary to ensure proper de-molding and sample

replication.

Micropatterned PDMS samples were fabricated using a four-step soft lithography technique, shown in Figure 8. A four-step soft lithography technique was needed because

PDMS could not be de-molded from the impression material after curing, even with a vapor deposition of a fluorosilane on the impression material mold. The impression mold could not be chemically activated using ultraviolet light and therefore the fluorosilane may not have attached to the mold. The fluorosilane would be required for the low

19 surface energy needed to de-mold PDMS from the impression mold. Therefore, intermediate replication steps using epoxy and urethane materials were required.

In step 1, hydrophilic vinyl polysiloxane impression material (Take 1®, Kerr), commonly used in dental applications, was applied to the micropattern using a cartridge dispensing gun with an attached mixing nozzle and pressed at room temperature for 30 min to create a negative replica. The impression material was chosen because of its ease of molding and de-molding, and replication of pillar features. In step 2, a positive replica was made by pouring a liquid epoxy resin (Epoxydharz L, Nr. 236349, Conrad

Electronics, Hirschau, Germany) with hardener (Härter S, Nr. 236365, Conrad

Electronics, Hirschau, Germany) at a 60:40 weight ratio and allowing to cure for 24 hours at room temperature. In step 3, liquid urethane polymer (Smooth-Cast® 300, Smooth-On) was poured into the epoxy replica at a 1:1 weight ratio and allowing to cure for 30 min at room temperature in order to form a negative replica. In step 4, PDMS was prepared by mixing the base and curing agent at 10:1 weight ratio and then poured into the urethane polymer mold. Afterwards, it was degassed in a vacuum chamber at 100 mTorr for 15 min at room temperature and cured in a gravity convection oven (1300U, Sheldon

Manufacturing, Inc.) operating at 70°C for 2 hours. The micropatterned PDMS was removed from the urethane polymer and used as substrates. The same urethane polymer negative mold was used to create additional micropatterned PDMS substrates.

2.2. Fabrication methods

This section explains the fabrication methods for constructing the coatings using

SiO2 hydrophobic nanoparticles and various binders and is denoted as nanoparticle-

20 binder. First, the adaptability of the coatings for varied attraction or repellency to water and hexadecane is described. Next, the different resins used in the coating are described.

Lastly, the surface activation treatment used in intermediate fabrication steps is explained.

2.2.1. Changing coating wettability

There are three variations of a similar nanoparticle-binder coating that each give different wettability characteristics to water and hexadecane. These three coatings are shown in Figure 9 and use 10 nm, hydrophobic SiO2 nanoparticles and a binder of methylphenyl silicone resin. The nanoparticles were selected because they have high hardness for wear-resistance and high visible transmittance for transparency (Bhushan,

2013). Methylphenyl silicone resin was selected because it is known to be durable and offer strong adhesion between nanoparticles and substrate (Ebert and Bhushan, 2012).

The superhydrophobic and superoleophilic method used the nanoparticles and resin, as shown in Figure 9a. In addition to the nanoparticles and resin, the superhydrophilic and superoleophobic method required ultraviolet-ozone (UVO) activation and deposition of fluorosurfactant, as shown in Figure 9b. Similarly, the superhydrophobic and superoleophobic method required UVO activation, but deposition of fluorosilane, as shown in Figure 9c. The UVO deposition will be described in Section

2.2.3.

The nanoparticles were hydrophobic fumed silica particles treated with hexamethyldisilazane (HMDS) to silylate the particle and were purchased from Aerosil®.

For the coating mixture, 150–750 mg of the nanoparticles (10 nm diameter, Aerosil

Figure 9: Three nanoparticle-binder coating techniques for adaptable wettability. (a)

Superhydrophobic and superoleophilic method using nanoparticles and binder. (b)

Superhydrophilic and superoleophobic method created by adding ultraviolet-

ozone treatment and fluorosurfactant to (a). (c) Superhydrophobic and

superoleophobic method created by adding ultraviolet-ozone treatment and

fluorosilane to (a).

RX300) were dispersed in 30 mL of 40% tetrahydrofuran (THF, HPLC Grade, 99.9% assay, Fisher Scientific) and 60% 2-propanol (IPA, Certified ACS, 99.9% assay, Fisher

Chemical) by volume. This ratio was chosen because THF is incompatible with some

22 polymers such as PC and can rapidly dissolve them (Ebert and Bhushan, 2012). By mixing in IPA, which has excellent compatibility with many polymers, this incompatibility is decreased. Pure IPA could not be chosen because the solvent would not evaporate quickly enough in ambient environments. This mixture was sonicated using an ultrasonic homogenizer (20 kHz frequency at 35% amplitude, Branson Sonifier 450A) for

15 min. Then, 150 mg of methylphenyl silicone resin (SR355S, Momentive Performance

Materials) was added for a particle-to-binder ratio of 1.0–5.0. A maximum ratio of 5.0 was used because a ratio of 5.0 was found to have poor adhesion to the substrate and lacked durability. By increasing the ratio further, even poorer durability would result. The mixture was then sonicated for an additional 15 min to form the final mixture. After a couple of minutes in the sonifier, the mixture appeared thoroughly mixed with the nanoparticles and resin distributed throughout; however, 15 min was chosen because it was the maximum timing length on the sonifier and would even further thoroughly mix the components.

Instead of using the THF-IPA mixture, additional solvents with a similar evaporation rate could be used. The evaporation rate of THF is 8.0 and IPA is 1.7 when compared to a baseline of n-butyl acetate at 1.0. Evaporation rate is compared to the rate of evaporation of a specific material and is therefore a ratio and unitless. Factors that affect evaporation rate include the ambient temperature, pressure, surface area, and intermolecular forces in the liquid or surface tension (Flick, 1991). Acetone (Certified

ACS, 99.7% assay, Fisher Chemical) with an evaporation rate of 7.7 (Flick, 1991) was tested as the solvent and was found to result in a coating with the same properties. Using

23 just acetone as the solvent may reduce complexity in scale-up. A chemical compatibility chart could be referenced when determining an appropriate solvent to minimize adverse interaction with the substrate; however, minimal time is required for solvent evaporation, and the solvent is unlikely to severely attack the substrate before it has evaporated.

The coating solution was deposited on the substrates via spray gun (VL1007

Paasche® with VLT-1 tip) from 10 cm away with compressed air at 210 kPa. Before use, the spray gun was cleaned with IPA and acetone to remove any leftover nanoparticle and binder residue. While in use, the spray gun was moved in a back and forth passing motion to thoroughly and evenly coat the sample. After coating, the sample was allowed to air dry or was transferred to a gravity convection oven (1300U, Sheldon Manufacturing,

Inc.) operating at 70°C for 5 min to remove the remaining solvent. These steps create the coating shown in Figure 9a and are the initial steps in the coatings shown in Figure 9b and Figure 9c.

For the samples shown schematically in Figure 9b and Figure 9c, the samples were irradiated using UVO treatment with the samples placed 2 cm underneath the lamp source for 60 min. This step chemically activated the surface for increased surface energy by adding highly active, polar surface groups such as hydroxyl groups and was required for fluorosurfactant or fluorosilane deposition. For the superhydrophilic and superoleophobic method (Figure 9b), 1 mL of a fluorosurfactant solution (Capstone FS-

50, DuPont) diluted with ethanol to an overall fluorosurfactant concentration of 45 mg/mL was spin coated or spray coated onto the sample. For the superhydrophobic and superoleophobic method (Figure 9c), one drop of trichloro(1H,1H,2H,2H-

24 perﬂuorooctyl)silane (448931, Sigma Aldrich) was vapor deposited on the sample using a closed container. The sample was attached to the top of the container via double-sided sticky tape with the surface facing down, and the drop was placed on the bottom of the container. This setup allowed the fluorosilane gas to uniformly coat the sample, and a vapor deposition time of 30 min was used. The vapor deposition time could vary from approximately 15 min up to 60 min with similar results. However, outside of these approximate limits, the fluorosilane would either not fully coat the sample or would be too thick. In both cases, repellency would be decreased. A similar procedure for depositing fluorosilane on flat PDMS was used except UVO treatment time was 90 min.

The additional time was required to modify the functionality of PDMS that is inherently hydrophobic to superhydrophilic, whereas only 60 min was required to get the nanoparticle and resin coating to be superhydrophilic.

The chemical structure for the coating fabrication steps is shown in Figure 10.

The chemical composition of the methylphenyl silicone resin and hydrophobic SiO2 nanoparticles is shown in Figure 10a. The surface activation step using UVO treatment is shown in Figure 10b. The deposition of a fluorosurfactant or fluorosilane is shown in

Figure 10c or Figure 10d, respectively. The orientation of the fluorosurfactant has the hydrophilic head group favorably attracted to the chemically activated surface and the hydrophobic tail pointing away from the surface.

Figure 10: Chemical composition for each step in the coating process. (a) Deposition of

the nanoparticles and binder for superhydrophobic and superoleophilic properties.

(b) Surface activation using ultraviolet-ozone treatment. (c) Deposition of

fluorosurfactant where the hydrophilic head group of the fluorosurfactant is

favorably attracted to the chemically activated surface resulting in the

hydrophobic tail pointing away from the surface and superhydrophilic and

superoleophobic properties. (d) Deposition of fluorosilane, which bonds to -OH

groups for superhydrophobic and superoleophobic properties.

2.2.2. Modifying coating resin and heating

In addition to methylphenyl silicone resin, four other resins were tested to see if similar properties could be obtained. These resins were two silicone resins (Dow Corning

220 and 217 flake resin) and two epoxy resins (EPON 1002F and 828). These resins were chosen to give a variety of properties related to hardness, shrinkage, melting point, and 26 chemical composition. The same procedure for creating the coating mixture and coating deposition as the methylphenyl silicone resin was followed.

When the samples were heated, coated samples using the methylphenyl silicone resin and the epoxy resin EPON 1002F were placed into a gravity convection oven

(1300U, Sheldon Manufacturing, Inc.) at 100°C for one hour. This temperature was above the softening point of the methylphenyl silicone resin (softening point of 93°C) and the melting point of epoxy resin EPON 1002F (melting point of 88°C). When the resins were in the oven for 10 min, they would flow and start to cover the entire substrate.

One hour was chosen during the heating process to allow them to flow throughout the entire sample. Once the samples were taken out of the oven, they were placed under

UVO treatment, and the rest of the procedure was followed. While silicones and epoxies are generally considered thermosets, these resins could be repeatedly heated and cooled to remold their shape and are therefore similar to thermoplastics.

2.2.3. Ultraviolet-ozone surface activation treatment

For coating application, the nanoparticle-binder coating surfaces needed to be chemically activated for increased surface energy by adding highly active, polar surface groups such as hydroxyl groups. This activation occurred through ultraviolet-ozone

(UVO) treatment as irradiation solely through ultraviolet light was not sufficient for chemical activation of PDMS or the nanoparticle-binder coating (Figure 9a). In this work,

UVO exposure was generated from a U-shaped, ozone-producing, ultraviolent lamp (18.4

W, Model G18T5VH-U, Atlantic Ultraviolet Co.). It is expected that this lamp outputs a total of 5.8 W of 254 nm light, 0.4 W of 185 nm light, and 1.6 g/hr of ozone in ambient

Figure 11: Schematic of ultraviolet-ozone lamp setup using a U-shaped, ultraviolet lamp

capable of ozone production.

conditions. The samples were placed 2 cm underneath the light source so that most of the

UV light was focused on the sample without the samples being too close to crack due to ozone cracking. The intensity of light varies in inverse proportion to the square of the distance from the light. The setup for UVO surface activation is shown in Figure 11. The lamp was placed in an enclosure with width, height, and length dimensions of 17 by 15 by 45 cm and was connected to an electronic ballast (120 v, Model 10-0137, Atlantic

Ultraviolet Co.) in order to provide the proper electrical conditions (Martin and Bhushan,

2017).

Ultraviolet (UV) and ultraviolet-ozone treatments are commonly used to activate surfaces and for completeness both processes are discussed and shown schematically in

Figure 12. These treatments are photo-sensitized oxidation processes in which organic

28 molecules in the surface layer are excited and/or dissociated by the absorption of UV radiation (Vig, 1985; Efimenko et al., 2002).

The UV method uses irradiation at 254 nm, whereas the UVO method uses a combination of 185 and 254 nm wavelengths. In the presence of 185 nm light, molecular oxygen can be dissociated into atomic oxygen, and atomic and molecular oxygen can be synthesized into ozone. In the presence of 254 nm light, ozone is decomposed into atomic and molecular oxygen (Vig, 1985; Bhurke et al., 2007; Park et al., 2009). These chemical reactions are shown in Figure 12a.

The UV and UVO surface activation processes are shown in Figure 12b. In the

UV process, 254 nm light breaks up organic bonds in the surface layer, which causes the surface to attempt to return to a stable condition. When these open bond sites react with oxygen, oxidation occurs and highly active, polar surface groups are formed. These groups are commonly hydroxyl, carbonyl, and carboxylic acids and lead to a chemically active, hydrophilic surface (Bhurke et al., 2007). In the UVO process, the combination of

185 and 254 nm light causes atomic oxygen to be continuously produced and ozone to be continuously synthesized and decomposed. The wavelengths of light excite and/or dissociate molecules at the surface and then react with atomic oxygen to form desorbing, volatile molecules, such as CO2, H2O, and N2. The desorbing molecules create adsorption sites for oxygen to form highly active, polar surface groups, which lead to a chemically active, hydrophilic surface. The UVO process is an order of magnitude faster than the UV process due to an extra wavelength that produces highly-reactive atomic oxygen and helps excite and/or dissociate molecules (Vig, 1985; Efimenko et al., 2002). In addition

Figure 12: (a) Irradiation using 185 nm and 254 nm (UVO) light. A wavelength of 185

nm decomposes molecular oxygen into atomic oxygen and synthesizes ozone. In

the presence of 254 nm ultraviolet light, ozone decomposes into atomic and

molecular oxygen. (b) Comparison of ultraviolet and ultraviolet-ozone surface

activation processes. Both methods lead to a chemically active, hydrophilic

surface; however, UVO is faster and more intense.

30 to being faster, higher overall surface energies were found with UVO treatment (72 mN/m) compared to UV treatment (26 mN/m) after PDMS was treated for 60 min with a starting surface energy of 19 mN/m (Efimenko et al., 2002).

Although surface energy was not measured, it was expected that after UVO treatment of 60 min, the surface energy of PDMS was on the order of 70 mN/m based on the study by Efimenko et al. (2002). When the nanoparticle-binder was activated with

UVO treatment, water droplets would fully wet the coating and so similar high surface energies are expected.

2.3. Sample characterization

In this section, characterization methods for the samples are described. These methods include contact and tilt angle measurements, scanning electron microscope imaging, coating thickness, repellency of surfactant-containing liquids, wear resistance, transparency, self-cleaning, anti-smudge, oil–water separation, and high temperature durability. Characterization is primarily presented on PDMS, glass, and PP substrates due to their numerous possible industrial applications. However, this technique is not limited to these substrates and similar coating properties were achieved on different substrates by following similar coating procedures. For the other substrates, only CA and TA characterization was completed to demonstrate that similar repellency was obtained. The substrates that were tested and their industrial applications can be found in Table 2.

2.3.1. Contact angle and tilt angle

Contact and tilt angle (refer to Figure 7) data were measured using a standard automated goniometer (Model 290, Ramé-Hart Inc.) using 5 μL deionized (DI) water and

31 hexadecane (99%, Sigma-Aldrich) droplets deposited onto the samples using a microsyringe. Hexadecane was chosen because it has a surface tension of 27 mN/m and is representative of alkane-based oils that typically have surface tensions of 20–30 mN/m.

The specification sheet for the goniometer reports a 0–180° CA range, 0.01° resolution, and ±0.10° accuracy. Contact angle was measured by taking a static profile image of the liquid-air interface and was analyzed using DROPimage software. All angles were averaged over at least five measurements on different areas of at least three samples and reported as ±σ for one standard deviation.

2.3.2. Scanning electron microscope (SEM) imaging

Top down, scanning electron microscope (FEI/Philips Sirion) images were taken to determine the topography of the PDMS, glass, and PP samples with the nanoparticle- binder and fluorosilane coating. To image the re-entrant geometry, SEM images were taken at a 70° angle. Samples were mounted with conductive tape and gold-coated prior to imaging using a Pelco sputter coater or a Leica ACE600 sputter coater. A 10 nm thick gold film was used for minimal charging effects.

2.3.3. Coating thickness

The coating thickness was measured using a step technique. One half of an untreated substrate was covered with a glass slide using double-sided sticky tape before coating and then removed after the coating procedure resulting in a step. A scanning area of 100 by 100 μm2 (maximum scan area size) with 512 ˣ 512 points including the step was imaged using a D3000 Atomic Force Microscopy (AFM) with a Nanoscope IV controller (Bruker Instruments) to obtain the coating thickness. A Si, n-type (Si3N4) tip

(resonant frequency f = 66 kHz, spring constant k = 3 N/m, AppNano) operating in contact mode was used. The coating thickness was averaged over the 100 by 100 μm2 scanning area.

The vertical AFM resolution is dependent on sensor response, mechanical vibrations, and electronic noise. Typical resolution for rough surfaces is as low as 1 nm, which is over 1000 times smaller than the coating thickness of the samples. For comparison, typical resolution for smooth surfaces is as low as 0.1 nm. The horizontal

AFM resolution is dependent on the square root of the radius of curvature (ROC) of the

AFM tip (Bhushan, 2013). The AFM tip was silicon, tetrahedral in shape with a ROC of

<10 nm and so the spatial resolution would be <3 nm, which is much smaller than the scan size of 100 by 100 μm2. The cantilever had dimensions of 100 μm in length, 35 μm in height, and 0.2 μm in thickness.

When imaging a surface using an AFM, several sources of errors can occur. Since the AFM tip has a finite radius, the surface profile cannot be exactly reproduced. Asperity peaks may be represented with a larger radius of curvature, and steep valleys may not be represented at all. Profiles containing peaks and valleys with ROC less than approximately 1 μm have greater chances of poor AFM representation. In addition, AFM tip kinematics and load can lead to error. A tip that moves across the sample too quickly may fail to remain in contact with the surface. A small load applied to a sharp tip leads to high local pressures that can cause significant local elastic or plastic deformation of the surface (Bhushan, 2013). In this work, a tip velocity of less than 1 Hz was used to reproduce the sample topography, and local deformation of the sample was not

33 experienced using this tip because the sample topography did not exhibit sample scratching.

2.3.4. Repellency of surfactant-containing liquids

To test the repellency of surfaces towards surfactant-containing liquids, droplets of shampoo (Head and Shoulders®, Procter and Gamble Co.) and laundry detergent

(Tide®, Procter and Gamble Co.) were placed onto surfaces tilted at ~25°. This angle was chosen to compare superliquiphobic results with other coatings (Brown and Bhushan,

2016c; 2017). The resulting surface–liquid interaction was photographed. For repellency after prolonged contact with shampoo, nanoparticle-binder and fluorosilane coated glass samples were submerged into a container of shampoo for various periods of time. Before testing, the back side of the glass was covered with dark construction paper so that when the coating was photographed the shampoo on the back side would not be seen. In one test, the sample was submerged for one minute and before, during, and after photographs were taken. In another test, the sample was dipped into shampoo 10 times for approximately three second contact with the shampoo each time. A photograph after each removal from the shampoo was taken.

The shampoo primarily contains sodium lauryl sulfate and sodium laureth sulfate surfactants, while the laundry detergent primarily contains sodium alcoholethoxy sulfate and sodium alkylbenzene sulfonate surfactants. At high concentrations, these surfactants typically have surface tensions on the order of 25 mN/m. In the products listed, depending upon concentration, the surface tensions will likely be in the range of 25–60

34 mN/m with values closer to the higher end (Ross and Epstein, 1958; Al-Sabagh et al.,

2011).

2.3.5. Wear resistance

The wear resistance of the coating was measured in several ways: using an AFM for microwear, a tribometer for macrowear, and a rubber finger or thumb for fingerprint resistance.

2.3.5.1. Microwear with AFM

Microscale wear was examined through an AFM wear experiment. The surface was worn using a 30 μm diameter borosilicate ball mounted on a rectangular cantilever with nominal spring constant of 7.4 N/m (resonant frequency f = 150 kHz, All-In-One).

Areas of 50 by 50 μm2 were worn for 1 cycle at a load of 10 μN so as to be later imaged within the scanning limits of the AFM. To analyze the change in morphology of the surface before and after the wear experiment, height scans of 100 by 100 μm2 in area

(maximum scan area size) with 512 ˣ 512 points were obtained using a Si, n-type (Si3N4) tip (resonant frequency f = 66 kHz, spring constant k = 3 N/m, AppNano) operating in contact mode. Resolution and sources of error of the AFM are discussed in section 2.3.3.

Root mean square (RMS) and peak-to-valley (P-V) roughness values before and after wear experiments were obtained. The standard deviation for the RMS and P-V distances were determined using five different areas of one sample and reported as ±σ for one standard deviation. In order to estimate the contact stress, an elastic modulus of 70 GPa and Poisson’s ratio of 0.2 for the borosilicate ball was used (Callister and Rethwisch,

2013). The nanoparticle-binder composite coating had an estimated elastic modulus of 32

GPa and Poisson’s ratio of 0.36 using rule of mixtures for composites where methylphenyl silicone resin has an elastic modulus of 3.5 MPa and Poisson’s ratio of 0.5

(Ellis and Smith, 2008) and the nanoparticles have elastic modulus of 70 GPa and

Poisson’s ratio of 0.2 (Callister and Rethwisch, 2013). The mean contact pressure was calculated as 115 MPa using Hertz analysis as described in Bhushan (2013).

2.3.5.2. Macrowear with tribometer

The mechanical durability of the surface was examined using an established macroscale wear test of a ball-on-flat tribometer (Bhushan, 2013). A 3 mm diameter sapphire ball was fixed in a stationary holder. A load of 10 mN was applied normal to the surface, and the tribometer was put into a reciprocating motion for 100 cycles. Stroke length was at least 6 mm with an average linear speed of 1 mm/s. The surface was imaged before and after the experiment using an optical microscope with camera (Nikon

Optihot-2).

To calculate the contact stress, an elastic modulus of 390 GPa and Poisson’s ratio of 0.23 were used for the sapphire ball (Bhushan and Gupta, 1991). The nanoparticle- binder composite coating had an estimated elastic modulus of 32 GPa and Poisson’s ratio of 0.36, which resulted in a mean contact pressure of 65 MPa.

2.3.5.3. Fingerprint resistance with rubber finger tip or actual thumb

The fingerprint resistance characteristics were examined using a rubber finger tip and an actual thumb. A schematic for these tests is shown in Figure 13. In the fingerprint test with a rubber finger tip (Figure 13a), the tip (098173, Cosco) was a nibbed rubber finger cover typically used to protect fingers from paper cuts and improved grip. The

36 rubber finger tip was cut with scissors to be flat, taped to a glass slide, and placed on top of the coating with a 5 N load. This 5 N load was chosen based on experiments for the force a single finger exhibits in a tapping mode on a standard telegraph key for college- aged subjects (Todor and Smiley-Oyen, 1987). Other single finger tapping experiments using college-aged subjects (Inui et al., 1998) and pianists (Aoki et al., 2005) showed around 0.5 N tapping force, and so the greater tapping force of 5 N was used for the test.

In comparison, typical computer keyboard activation force is 0.6–0.7 N. The CA and TA before and after each fingerprint test was measured over five measurements on different areas of one sample and reported as ±σ for one standard deviation.

In the fingerprint test with an actual thumb (Figure 13b), a thumb was pressed into the coating. The imprint force was estimated to be over 100 N based on thumb pushing experiments where adults pushed downwards with their thumb on a force plate and averaged over 100 N of force. With 21-30 year old males (N=10), the mean downward force with their thumb was 184.1 N, σ = 52.2 N, and a range of 109.3–290.1 N

(Peebles and Norris, 2003). The force was also checked by pushing with a thumb onto an

Figure 13: Setup for the fingerprint tests: (a) rubber finger tip test at 5 N and (b) an actual

thumb impression test at > 100 N.

37 analog scale showing forces that ranged 45–180 N with a mean force of 110 N. The fingerprint stresses were estimated to be 1 MPa using the rubber finger and 70 MPa with the actual thumb. The rubber used in the calculation was styrene-butadiene (synthetic rubber) with an elastic modulus of 2 MPa and Poisson’s ratio of 0.5 (James, 1999). It was estimated that the nub diameter was 3 mm and that 40 nubs were in contact with the surface based on imprint photographs. The actual thumb was estimated to have an elastic modulus of 70 MPa (Edwards and Marks, 1995), Poisson’s ratio of 0.5 (Bhushan, 2016), fingerprint ridge diameter of 0.2 mm (Stücker et al., 2001), a fingerprint contact area of

325 mm2, and a total fingerprint ridge line contact of 1 m.

In the fingerprint resistance tests using a rubber finger tip and an actual thumb, photographs before and after the test were taken to visually determine if the coating was damaged. Photographs were taken with fluorescent light as the light source. The primary change that was viewed was a change in reflectivity based on the tested and untested side of a sample. There are standard test methods for measuring reflectivity (ASTM E1331 and ASTM F1252); however, the equipment for these tests was not available, and so this qualitative test was performed.

2.3.6. Transparency

For a qualitative representation of coating transparency, text was placed behind the samples and a picture of the sample was taken. The images were taken with a camera and so incorporate a range of wavelengths in the visible spectrum. Pictures of untreated samples and coated samples were taken for comparison. There are ASTM standard tests to measure transparency (ASTM D1746) and haze and luminous transmittance (ASTM

D1003). The equipment to complete these tests was not available, and so a qualitative test of placing text behind the samples was used. This method gives a quick indication of the transparent properties. The light can be absorbed, reflected, or transmitted; however, taking an image cannot distinguish between each type.

2.3.7. Self-cleaning and anti-smudge

The self-cleaning characteristics were examined by first contaminating the sample with silicon carbide particles and then comparing the removal of particles by water droplets before and after the experiment. The silicon carbide (SiC, Sigma Aldrich) particles were approximately of size 10–15 μm and hydrophilic. These particles were chosen due to their size representative of small dirt particles. Smaller particles are more likely to become trapped in surface features and harder to remove, whereas larger particles are more likely to rest upon surface asperities and be easily removed. In addition, because the particles were hydrophilic, they would be more likely to remain on a surface. As described by Young’s equation, hydrophilic surfaces have CA less than 90° and therefore greater contact area with the surface. Hydrophobic particles would have reduced attraction between the particle and the surface the particle was resting on and decreased contact area with the surface; therefore, a hydrophobic particle would be easier to remove through self-cleaning.

Contaminants have a wide range of possible materials and possible chemistries.

Some possible contaminants include dirt, smog, mineral matter, organic matter, and organisms. A self-cleaning surface would ideally be able to remove these different contaminants, but would require further testing to demonstrate this ability. Silicon

39 carbide contaminants were used as a starting point to represent contamination and can give an indication of the self-cleaning ability of the surfaces.

The particles were dispersed in a glass chamber (0.3 m diameter and 0.6 m high) by blowing 1 g of SiC powder for 10 s at 300 kPa. After dispersion, the particles were allowed to settle on the sample mounted on a 45° tilted stage for 30 min. The contaminated sample was then secured to a 10° stage and water droplets (total volume of

5 mL) were dropped onto the surface from 1 cm in height. The removal of particles by the water droplets was compared before and after tests using optical microscope images and quantified using image analysis software (SPIP 5.1.11, Image Metrology A/S,

Horshølm, Denmark). Using image analysis software to detect the contaminants, the contaminated area of the sample was compared to the contaminated area of the sample after the self-cleaning test. The test was repeated over five samples and reported as ±σ for one standard deviation.

The anti-smudge characteristics were examined by contaminating the sample with silicon carbide as described for self-cleaning. Similar to self-cleaning, SiC was used as contamination and represents a starting point for anti-smudge contamination. The contaminated samples were then secured on a stage and a hexadecane-impregnated micro-fiber wiping cloth was attached to a horizontal glass rode (diameter of 5 mm) fixed on a cantilever above the sample. As the cloth was brought in contact with the sample, the microfiber cloth was set to rub the contaminated sample under a load of 5 g for 1.5 cm at a speed of about 0.2 mm/s. Images were taken using an optical microscope with a

CCD camera (Nikon, Optihot-2). The removal and transfer of particles by the cloth was

40 compared before and after the test. The test was repeated over three samples and reported as ±σ for one standard deviation.

2.3.8. Oil–water separation

The superhydrophobic and superoleophilic (Figure 9a) and the superhydrophilic and superoleophobic (Figure 9b) coatings were found to be suitable for oil–water separation. Stainless steel meshes (#400) were first cleaned with acetone and IPA until they were hydrophilic, then the coatings were deposited onto the meshes via spray coating. The coated meshes were placed on top of beakers, and agitated mixtures of water and hexadecane were poured onto the coated meshes. In separate experiments, the meshes were inclined at an angle and the oil–water mixtures were poured over them. To improve contrast, Oil Red O (a fat-soluble dye) and Blue 1 (a water-soluble dye) were used as oil and water dispersible dyes, respectively. The use of dyes was not found to have any effect on the performance of the coating.

2.3.9. High temperature durability

The high temperature durability characteristics of the coating were examined using a hot plate to heat up water and hexadecane to a maximum temperature of 80°C. A beaker containing water or hexadecane and a coated glass sample was placed on the hot plate. Temperatures were recorded using a digital thermometer (CP1, Habor) placed in the beaker. The temperature measurement range was -50°C to 300°C with a resolution of

0.1°C and an accuracy of ±1°C. The digital thermometer did not touch the sides of the beaker using a clamp to hold the thermometer, and the liquid in the beaker was constantly stirred. Using a dropper, liquid was taken from the beaker, and a droplet of approximately

10 μL in size was placed on the sample. An image of the droplet was taken and the resulting CA was recorded. The temperature of the droplet was verified with the digital thermometer and was found to be the same as the temperature in the beaker. All angles were averaged over at least five measurements on different areas of at least three samples and reported as ±σ for one standard deviation.

2.4. Summary

In this chapter, the experimental procedure and methods for sample characterization were explained. Substrates included flat and micropatterned samples.

Experimental procedure included methodology for generating the various coatings.

Characterization included CA/TA, SEM, coating thickness, repellency of surfactant- containing liquids, wear resistance, transparency, self-cleaning and anti-smudge, oil– water separation, and high temperature durability. These procedures and tests were used in the following research chapters.

Chapter 3: Lotus-leaf-inspired surfaces for superliquiphobicity using micro- and nano-scale roughness

3.1. Introduction

Surface microstructure influences the wetting properties of the surface. A liquid droplet can be attracted to a surface or repelled from a surface dependent on roughness and chemistry of a surface. Surfaces that are highly repellant to liquids, denoted superliquiphobic with contact angles greater than 150° and tilt angles less than 10°, have many applications in commercial industry, and so, have been generated and engineered in many ways.

The aim for this work was to engineer superliquiphobic surfaces (CA > 150° and

TA < 10° with water and oils) by mimicking the hierarchical roughness found on lotus leaves. The manufacturing method needed to be easily scalable from the lab setting to industrial manufacturing. It was desired for the coating to contain transparency properties with >90% optical transmittance for possible transparent applications such as windshields and electronic displays, and be wear resistant to contact stresses of 100 MPa so that the surface would not be destroyed after exposure to stress typical of real-world applications.

The substrate that was chosen to be made superliquiphobic was polydimethylsiloxane (PDMS) because it is a versatile material commonly used in many applications. It is used in biomedical devices such as contact lenses and micro- and

43 nanofluidics devices due to its great biocompatibility (Prakash and Yeom, 2014), chemical stability (Xia and Whitesides, 1998), transparency (Xia and Whitesides, 1998;

Prakash and Yeom, 2014), and mechanical elasticity (Xia and Whitesides, 1998; Prakash and Yeom, 2014). Additional applications for PDMS include catheters and drainage tubing (Rosato, 1983), micropumps (Johnston et al., 2005), and microvalves (Wu et al.,

2011), where liquid repellency and self-cleaning are of importance.

Since finite surface roughness is often needed to observe superliquiphobicity, various methods such as etching, micropatterning, and adhering nanoparticles to generate surface roughness have been attempted (Bhushan, 2016). Surface etching to generate roughness on various polymers including PDMS has been completed using plasma etching (Tserepi et al., 2006; Manca et al., 2008; Flamm and Auciello, 2012) and has been shown to achieve superhydrophobicity on PDMS (Tserepi et al., 2006; Manca et al.,

2008; Tropmann et al., 2012; Ellinas et al., 2014). Etching has also been demonstrated on metals such as aluminum, copper, steel, titanium, and zinc using chemical means to etch the surface including acid-etching (Qian and Shen, 2005; Qu et al., 2007; Song et al.,

2013).

Another method for fabricating surface roughness has including micropatterning because it allows for patterning features with complex and possible re-entrant geometries, usually by replicating a patterned structure via micromolding (Chen et al., 2005; Cortese et al., 2008; Yao et al., 2009) or additive micromachining processes (Madou, 2002;

Prakash and Yeom, 2014). It has been successfully used to construct flower-shaped structures (Ghosh et al., 2009), inverse-trapezoidal pillars (Im et al., 2010), and

44 mushroom-like pillars (Kang et al., 2012) on PDMS substrate. Ghosh et al. (2009) created PDMS replicas of the Colocasia leaf with subsequent modification with silica nanoparticles and fluorosilane to obtain water CA of 155° and glyceryl trioleate (surface tension of 35 mN/m) advancing CA of 120°. Im et al. (2010) used a PDMS replication process to create inverse-trapezoidal microstructures with a fluoropolymer coating for water CA of 153° and methanol (surface tension of 23 mN/m) CA of 135°. These techniques achieve superhydrophobic PDMS, but are unable to repel low surface tension oils for superoleophobicity. Kang et al. (2012) engineered superoleophobic PDMS by creating mushroom-like micropillars via replication and after subsequent fluoroplasma, demonstrated water CA of 170° and ethanol (surface tension of 22.3 mN/m) CA of 162°.

However, this process would not be practical for scaling up.

Coatings that adhere nanoparticles to a substrate for surface roughness have been used in various ways and on various substrates. Titanium oxide (TiO2) nanoparticles (20–

50 nm) and perfluoroalkyl methacrylic copolymer were spray coated on a Si wafer resulting in water CA of 164° and ethylene glycol CA of 144° (Hsieh et al., 2005). Zinc oxide (ZnO) nanoparticles (50 nm) and perfluoroalkyl methacrylic copolymer were spray coated on glass resulting in hexadecane CA of 154° (Steele et al., 2009). SiO2 nanoparticles have been spin coated on glass for ethylene glycol CA of 165° using 30–50 nm nanoparticles and perfluoroalkyl methacrylic copolymer (Hsieh et al., 2009), spray coated on glass for hexadecane CA of 147° using 55 nm nanoparticles and a fluorinated acrylic copolymer (Muthiah et al., 2013), and dip coated on polyethylene terephthalate

(PET) for hexadecane CA of 153° using 55 nm nanoparticles and a coating of

45 fluorosilane (Wang and Bhushan, 2015). However, adhering nanoparticles with a fluorinated component to a substrate for surface roughness needs development. By combining a fluoropolymer with the nanoparticles, durability remains a concern and limited wear resistance testing is performed. In addition, repellency of low surface tension liquids is not reported or not above 150° for superoleophobicity. Lastly, these methods have only demonstrated that the coating can be applied to select few substrates, and it is unknown if the coating can be applied to other substrates.

For commercial applications, nanoparticles such as TiO2, SiO2, and silver have been added to paints for various benefits such as anti-bacterial, self-cleaning, UV light resistance, and durability (Kaiser et al., 2013a,b). However, paint with nanoparticles has only obtained superhydrophobicity and not superoleophobicity. Superoleophobicity is more difficult to achieve. Surfaces that achieve superoleophobicity typically require lower surface energies by also including fluorinated components and re-entrant geometries that are not easily created by simply adding nanoparticles.

Etching, micropatterning, and adhering nanoparticles to generate surface roughness and therefore superliquiphobicity have problems. Etching is advantageous in that it typically treats an entire surface, regardless of geometries, and can be split into wet etching using liquid chemicals or dry etching using vapor or plasma etching. However, both methods of etching have some disadvantages. In wet etching, the liquid chemicals need to be consistently restored to maintain the same chemical concentration at the surface for a constant etchant rate. In addition, the liquid chemicals need to be disposed of properly. Wet etching introduces the variable of etch rate where faster etch rates

46 typically result in a rougher surface, which increases the complexity for creating roughness with this method. In both dry and wet etching, the etching chemistry is dependent on the substrate and may need to be modified in order to achieve superliquiphobicity on a different substrate (Madou, 2002; Prakash and Yeom, 2014).

Micropatterning requires several fabrication steps in order to make geometries with re-entrant geometries, and therefore micropatterns with re-entrant geometries would be unsuitable for scale-up. Coatings that adhere nanoparticles to a substrate show promise

(Hsieh et al., 2005; Steele et al., 2009; Hsieh et al., 2009), but require additional development so that they are substrate independent (coating repellency is not determined based on the substrate), attract or repel any liquid, transparent, and wear-resistant.

The objective of this work was to develop a coating that was superliquiphobic

(CA greater than 150° and TA less than 10° with water and oils). In addition, the coating needed a fabrication method that could be easily scalable from the lab setting to industrial manufacturing, contain transparency properties for possible transparent applications such as windshields and electronic displays, and be wear resistant so that the surface would not be destroyed after exposure to stress.

In this chapter, superliquiphobic surfaces were generated by mimicking the hierarchical roughness found on lotus leaves. These engineered surfaces contained a combination of micro- and nano-scale roughness. Micro-scale roughness was generated using a micropattern of cylindrical pillars, and nano-scale roughness was generated using a coating comprising of methylphenyl silicone resin and hydrophobic SiO2 nanoparticles.

In addition, a low surface energy coating using a fluorosilane was deposited to increase

47 repellency toward low surface tension liquids. By utilizing micropatterning (without a re- entrant geometry) and nanoparticles, this method of fabrication should be easily scalable for industrial manufacturing and commercial applications requiring superliquiphobic surfaces.

3.2. Results and discussion

The micropatterning, wettability, surface morphology, and transparency for the surfaces will now be reported and comparisons will be drawn between them.

3.2.1. Micropatterned substrates

Due to the numerous steps in the four-step soft lithography procedure (see Figure

8), optical images of the micropatterning were taken to verify that features were successfully replicated. Top-down optical images of the original Si master pattern and the micropatterned PDMS with and without the nanoparticle-binder and fluorosilane coating are shown in Figure 14. Using optical images of the micropattern, the dimensions of the micropillars are 15 ± 2 μm diameter and 126 ± 3 μm pitch showing good feature replication. The master pattern and micropatterned PDMS have the same pattern of micropillars. In the micropatterned PDMS, there are darker regions outside of the micropillars. These darker regions are likely due to unintended roughness introduced during the replication steps. The main potential cause of this roughness is an incomplete match of a mold interface at any one of the steps leading to air bubbles trapped at the mold interface. In the coated micropattern, the micropillars have a larger diameter due to the nanoparticle-binder coating collecting on the tops of the micropillars. The micropillars with the nanoparticle-binder coating have a diameter of 35 ± 5 μm. When the

Figure 14: Optical images of Si master pattern and micropatterned PDMS showing

feature replication after the four-step soft lithography procedure. When the nano-

scale roughness is deposited, the micropillars have a larger diameter due to the

added nanoparticles and resin.

coating was sprayed onto the substrate, it accumulated on the tops of the micropillars with less visible change in the valleys around the micropattern. The 30 μm height of the micropillars was outside the z-range limit of the AFM and so was unable to be imaged in order to determine the exact height. Other methods of determining height such as using a surface profiler or SEM were not used because of cost or broken instruments at the time of data collection. Micropillars were visually seen protruding out of the surface when viewed from the side.

The variation in micropillar diameter and pitch due to the replication process would not influence the wetting on the surface. The variation would have to be much larger before the wetting state would change. The diameter of the micropillars (14 μm) is less than 1% of the diameter of the droplet (2 mm) and so changing the micropillar diameter by a couple microns has little effect on wetting. For pitch, micropillars with pitch of 105 μm were still superhydrophobic, and it took micropillars of pitch 168 μm

49 before the repellency decreased to full wetting described by the Wenzel model (Jung and

Bhushan, 2007).

In addition, changing the diameter would not change the wetting based on the

Wenzel model. The roughness factor (Rf) in the Wenzel model does not change by increasing or decreasing the diameter. Increasing the micropillar diameter would decrease the solid-liquid composite interface fraction (fLA) and lead to decreased repellency in the

Cassie-Baxter model. Changing the pitch influences the wetting in the Wenzel and

Cassie-Baxter models. By increasing the pitch, the roughness factor in the Wenzel model decreases and the solid-liquid composite interface fraction in the Cassie-Baxter model decreases. The goal is to have extremely repellent surfaces with air pocket formation as in the Cassie-Baxter model so the greatest effect is increasing the pitch to decrease the solid-liquid composite interface fraction. Adding the nanoparticle-binder and fluorosilane coating increases the micropillar diameter. It should also decrease the solid-liquid composite interface fraction due to the tops of the micropillars having a nanostructured rough surface instead of a complete wetting on the tops of the micropatterned PDMS.

3.2.2. Wettability of surfaces

The measured CA and TA values for water and hexadecane on flat and micropatterned PDMS with various coatings are shown in Figure 15 and also summarized in Table 1. The particle-to-binder (p-b) ratio used for the nanoparticle-binder coating was p-b = 4.0 and was chosen to create large roughness features. A discussion of p-b optimization with the methylphenyl silicone resin can be found in Section 4.2.1.

Untreated, flat PDMS is slightly hydrophobic with water CA of 113 ± 2° and oleophilic

Figure 15: Comparison of liquids on flat and micropatterned PDMS with various

coatings. Contact angle and tilt angles for water and hexadecane droplets on: flat

and micropatterned PDMS; with fluorosilane; with nanoparticle-binder coating;

and with nanoparticle-binder and fluorosilane coating.

with hexadecane CA of 52 ± 2°. By micropatterning the PDMS, roughness was introduced (Rf = 1.08 and RMS = 2.95 μm) and enhanced these hydrophobic and oleophilic properties, resulting in superhydrophobicity with water CA of 151 ± 3°

(Cassie-Baxter model) and superoleophilicity with wetting of hexadecane (Wenzel 51

Table 1: Contact angle and tilt angles for water and hexadecane droplets on flat and

micropatterned PDMS with various coatings

Contact angle (°) / Tilt angle (°)

DI water Hexadecane Micropatterned Micropatterned Surface Flat PDMS Flat PDMS PDMS PDMS 113 ± 2 / 151 ± 3 / 52 ± 2 / Wet / None n/a 7 ± 3 n/a n/a 119 ± 2 / 152 ± 2 / 69 ± 2 / 68 ± 2 / With fluorosilane n/a 5 ± 3 n/a n/a With nanoparticle- 158 ± 2 / 157 ± 2 / Wet / Wet / binder coating ≤ 1 ≤ 1 n/a n/a With nanoparticle- 158 ± 2 / 157 ± 2 / 156 ± 2 / 156 ± 2 / binder and ≤ 1 fluorosilane coating ≤ 1 2 ± 1 3 ± 1

model). Fluorination via vapor deposition of fluorosilane was added to the samples in order to decrease surface energy and improve oil repellency. On flat PDMS with fluorosilane, the water CA increased to 119 ± 2°, which is nearly the maximum achievable CA on a flat surface with a water droplet (Nishino et al., 1999), and the hexadecane CA increased to 69 ± 2°. By adding the fluorosilane, the surface tension at the solid-liquid interface decreased to increase the hexadecane CA, as described in the

Wenzel model. On fluorinated, micropatterned PDMS, the water CA remained approximately the same as micropatterned PDMS, but the hexadecane CA increased to

68 ± 2°. Similar to flat PDMS, adding fluorosilane to the micropatterned PDMS deceased

52 the surface tension at the solid-liquid interface to increase the hexadecane CA compared to the micropatterned PDMS without a coating.

Along with simply micropatterning, superhydrophobicity was obtained with the nanoparticle-binder coating. Water CA of 158 ± 2° and 157 ± 2° were demonstrated on flat and micropatterned PDMS, respectively. The flat and micropatterned PDMS had hexadecane CA under 90°, and so by adding nanoparticle roughness, it enhanced the oleophilic property resulting in hexadecane wetting on both surfaces. The Wenzel model shows that oleophilic surfaces become more oleophilic once roughness is introduced. In this case, the nanoparticle-binder coating become superoleophilic. In order to decrease the surface energy, the nanoparticle-binder coating was fluorinated. After fluorination, superhydrophobicity still remained with the same water CA as before; however, superoleophobicity with hexadecane CA of 156 ± 2° on the flat and micropatterned surfaces was also obtained. Superoleophobicity on flat PDMS with the nanoparticle- binder and fluorosilane coating was obtained because the coating introduces re-entrant geometries (shown in Section 3.2.3) with re-entrant angles in excess of 20°. This re- entrant angle would be added to the hexadecane CA on flat PDMS with fluorosilane in order to become oleophobic. By adding the roughness from the nanoparticle-binder coating, the roughness enhances the oleophobic properties to become superoleophobic.

Tilt angles decreased as the CA increased. Water droplets did not roll off the flat

PDMS, but at 7 ± 3° water droplets began to roll on the micropatterned surface. The water TA on the micropatterned surface decreased further once fluorination was added to a TA of 5 ± 3°. The nanoparticle-binder coating had the best water TA of less than 1°

53 regardless if the substrate was flat or micropatterned PDMS or whether the coating was fluorinated. Hexadecane droplets rolled off the nanoparticle-binder coating once fluorosilane was added. Hexadecane TA of 2 ± 1° and 3 ± 1° resulted on flat and micropatterned PDMS, respectively. The TA were lower on the nanoparticle-binder coating than on the micropatterned surface because there would be less surface area for the droplets to contact the coating surface assuming the droplets were only contacting coating asperities. The larger contact surface area on the micropattern would lead to pinning which can be seen with the water TA of 7° and 5° on the micropatterned PDMS and micropatterned PDMS with fluorosilane.

The micropatterning did not improve the repellency of water or hexadecane compared to the nanoparticle-binder coating (Figure 15). The same repellency to water was achieved with just the nanoparticle-binder coating regardless of the substrate roughness. When the nanoparticle-binder and fluorosilane coating was deposited, the same repellency to hexadecane was achieved. In addition, the TA decreases to less than

1° with water when the coating was deposited. The micropattern with coating did not improve repellency of just the coating because the coating generated micro- and nano- scale roughness with re-entrant geometry. The micropattern generated additional roughness; however, the coating was superliquiphobic and the additional roughness was not necessary. Further discussion of why the micropatterning did not help improve the repellency when the coating was deposited is presented in the next section (Section

3.2.3).

3.2.3. Surface morphology

In order to understand the surface morphology of the nanoparticle-binder and fluorosilane coating, SEM images of the flat PDMS sample with the nanoparticle-binder and fluorosilane coating were taken as shown in Figure 16. The micron-sized agglomerates of nanoparticles and binder are shown in Figure 16a in a top-down view at two different magnifications. A spray coat was used for deposition and so similar features would be deposited throughout the substrate. Similar roughness features were found throughout the surface and representative images are shown in Figure 16. Additional

SEM images with similar roughness features are shown later in Section 4.2.2 as Figure

21.

The first level hierarchy exhibited structures of various size and shape due to agglomeration of nanoparticles and resin. In order to view the re-entrant geometry of the structures, SEM images at a 70° from normal tilt angle were taken as shown in Figure

16b. The coating was found to contain hierarchical structures showing re-entrant geometry (see Figure 5). By adjusting the SEM focus and panning across the surface, numerous similar features were seen. Although the back side of the features were not imaged, it was expected that the features had similar features with re-entrant overhangs.

These re-entrant geometries found throughout the coating can help support oil droplets for superoleophobicity by introducing a re-entrant angle (see Figure 5). Re- entrant angles allow for repelling low surface tension liquids that have CA less than 90° on a flat surface. A CA with hexadecane on flat PDMS with fluorosilane was 69 ± 2°, shown in Figure 15 and Table 1. By introducing the nanoparticle-binder with

Figure 16: SEM images of the nanoparticle-binder and fluorosilane coating. (a) The top-

down view shows agglomerates of nanoparticles and binder that form micron-

sized structures. (b) The tilt view shows these structures have quasi-spherical re-

entrant geometries that were found to be repellent to water and hexadecane.

Arrows point to re-entrant overhangs.

fluorosilane, the nanoparticle agglomerations formed re-entrant angles that could be added to the inherent CA hexadecane exhibited on the surface. As shown in the idealized schematic of re-entrant geometries (Figure 5), spherical re-entrant structures introduce various re-entrant angles. In order to repel hexadecane, re-entrant angles on this surface needed to be greater than about 20° based on the hexadecane CA on flat PDMS. This cumulative CA led to an angle in excess of 90°, which directed the surface tension force away from the surface for decreased wetting.

These images in Figure 16b show that the nanoparticles and binder generally combined into quasi-spherical shapes, which is one form of the re-entrant geometry. In addition, the nanoparticle and binder formed a rough outer surface of the micron-sized agglomerates, which is a second level of hierarchy. A rough surface would increase the number of air pockets and therefore increase the air-liquid contact area for increased

56 liquid repellency. The surface was unable to be imaged for surface topography using

AFM because the surface roughness features were outside the z-limit of the AFM.

However, the surface roughness was determined in coatings where the particle-to-binder ratio was decreased and is shown in Section 4.2.4, 5.2.1.2, and 5.2.2.2. Based on the data in these sections, it was expected that the RMS surface roughness is approximately 1.0

μm.

As shown in Figure 16a, the surface morphology has roughness features that vary in size from the micro- to nano-scale. These hierarchical structured surface features are related to the hierarchy principle from the lotus leaf due to the combination of epidermal cells and nanotubules. Since the coating itself creates micron-sized surface features, the micropattern was not required for additional roughness. If a coating were deposited that only generated nano-scale roughness, then a micropattern would help in achieving hierarchical roughness. This hierarchical roughness would increase the roughness factor and decrease the solid-liquid composite interface fraction which increases liquid repellency for the goal of achieving superliquiphobicity. The liquid repellency would depend on the liquid CA on the flat surface and if it were less than 90°, re-entrant geometries would need to be created at either the micro- or nano-scale.

It is believed that the re-entrant structures are formed due to nanoparticle agglomeration as shown in Figure 17. The small diameter of a nanoparticle (10 nm) leads to a very high surface-to-volume ratio. Since surface energy is inversely proportional to particle diameter, surface energy is a significant contributor to nanoparticle properties.

The large surface energy would promote nanoparticle agglomeration and the resin would

Figure 17: Concept for nanoparticle agglomeration and re-entrant geometry formation.

Nanoparticles and binder form clusters in solution that when deposited on a

sample have re-entrant curvature.

help bind the structure together. In order to minimize the surface energy, these agglomerates would form tightly packed and quasi-spherical shapes (Baletto and

Ferrando, 2005). When these agglomerates are spray deposited, the resin binding and van der Waals attractive forces are great enough to resist the agglomerates breaking down due to the pressure of the spray gun and the collision energy from impacting the surface. In addition, smaller particles form agglomerates with a rougher surface that are more likely to give the re-entrant structure. A larger particle is less likely to form agglomerates and more likely to become partially embedded in the binder leading to hemispheres or spherical caps.

In order to scale this process for industry, the principle of creating micro- and nano-scale structures through random agglomeration and then spraying these agglomerations onto the surface would be implemented. By adjusting the p-b ratio 58

(described in Section 4.2.1), intensity of mixing action, and spraying parameters, the agglomeration feature sizes could be adjusted based on the application. Increasing the p-b ratio adds more nanoparticles for larger agglomerations. Increasing the intensity of the mixing action would limit the size of the nanoparticle agglomerations that would form.

Two spraying parameters that were found to affect the coating were the distance from the substrate and the pressure of the compressed air. By decreasing the spray gun distance from the substrate, a thicker coating was obtained and it was more likely that the solvent would not evaporate quickly enough. The solvent would accumulate on the surface leading to nonuniformity of the finished coating even after the solvent evaporated. If the pressure of the compressed air was not great enough, then the coating could not be sprayed through the nozzle leading to clogging.

In order to implement the coating in industry, the surface tension of the liquid to be repelled would need to be known. If the liquid had a surface tension greater than hexadecane (27 mN/m), the coating would be able to repel the liquid. A liquid with a surface tension higher than hexadecane would have greater CA on a flat surface. Adding the re-entrant angles to this greater CA angle would be greater than 90°, and therefore superliquiphobicity would be obtained. For liquids with surface tensions less than hexadecane, the CA on a fluorinated surface would need to be known. In addition, the re- entrant angles would need be designed so that the CA on the fluorinated surface and the re-entrant angle are greater than 90°. The combination of these two angles would then allow for the liquid to be repelled.

3.2.4. Transparency

Many applications of liquid-repellent surfaces require transparency of the coating.

When text is placed behind the coatings on flat and micropatterned PDMS, the text remains readable demonstrating transparency properties as shown in Figure 18. For applications sensitive to transparency such as windshields and electronic displays additional characterization using transparency standards should be performed. The edges of each sample are indicated with dashed lines. The micropatterned samples have the micropattern in the center with flat PDMS surrounding it and is shown with a white dashed line. PDMS is inherently transparent as shown with flat PDMS. For the superhydrophobic samples, a reduction in transparency is seen with improved transparency with the micropatterning rather than the nanoparticle-binder coating.

Although the thickness of the coating is less than the height of the micropatterning, the increased roughness of the coating leads to decreased transparency. The micropatterned

PDMS is primarily flat except for the small areas occupied by the micropillars. For the superoleophobic samples, transparency is similar to the superhydrophobic samples.

Compared to the nanoparticle-binder layer, the fluorination step only increases the thickness by a comparatively small amount (~40 nm), and therefore there is little reduction in transparency for the fluorination step. If these surfaces were tested for transmittance, it is expected that the transmittance would be about 65% for 400–700 nm wavelengths based on transmittance data for a coating using 7 nm nanoparticles (Brown and Bhushan, 2015).

Figure 18: Photographs showing transparency on flat PDMS, superhydrophobic samples

from flat PDMS with nanoparticle-binder coating and micropatterned PDMS, and

superhydrophobic and superoleophobic samples from flat and micropatterned

PDMS with nanoparticle-binder and fluorosilane coating. The reduction in

transparency in the coated samples is due to the SiO2 nanoparticles and binder.

Edges of each sample are shown in dashed lines. On the micropattern samples, the

micropattern occurs in the center of the sample and outlined in white dashed line.

The goal of greater than 90% optical transmittance was based on transmittance of windshields and current transparent coatings that have been created. Optical transmittance of various aircraft windscreens was measured for visible light (400–600 61 nm) and was reported as 82.8% ± 4.6% (Nakagawara et al., 2007). Current coatings have a transparency of around 60% (Deng et al., 2012; Lee et al., 2013; Brown and Bhushan,

2015) to around 80% (Im et al., 2010) for the visible light spectrum. Very transparent coatings have greater than 90% optical transmittance for the visible light spectrum (He et al., 2011; Ebert and Bhushan, 2012). Further improvement in transparency, potentially by decreasing the thickness of the nanoparticle layer, may be possible in the future. For applications sensitive to transparency such as windshields and electronic displays, additional characterization using transparency standards should be performed.

Figure 15 and Figure 18 show that the micropattern substrate had little benefit over a flat substrate in either repellency or transparency, respectively. Although the micropattern was superhydrophobic, it was oleophilic even with the fluorosilane treatment. The only way to achieve superoleophobicity was through the nanoparticle- binder and fluorosilane coating. The micropattern was unable to repel oil because it did not have re-entrant geometries. The re-entrant angle was required because the hexadecane

CA on the flat surface was oleophilic. When roughness was introduced, the surface properties were enhanced further towards oleophilicity (Wenzel model). Since a flat substrate and micropatterned substrate, both with the nanoparticle-binder coating, had the same properties, only further work on flat substrates with the nanoparticle-binder coating was performed. This additional work is discussed in Chapters 4-5.

3.3. Summary

The objective of this work in this chapter was to mimic the functionality that the lotus leaf exhibits in extreme repellency to water and self-cleaning of the surface. It was

62 the goal to improve upon the repellency the lotus leaf exhibits by also repelling lower surface tension liquids such as hexadecane. The inspiration of hierarchical roughness was used to design and manufacture superliquiphobic surfaces. The micro-scale roughness was created using a four-step soft lithography technique to end up with PDMS micropattern. The replication technique started with a silicon master pattern with 14 μm diameter, 30 μm height, and 126 μm pitch cylindrical pillars. The nano-scale roughness was created using spray deposition of a coating comprising of methylphenyl silicone resin and hydrophobic, 10 nm SiO2 nanoparticles and called nanoparticle-binder. The roughness of the coating was approximately 1.0 μm. In addition, a low surface energy coating using a fluorosilane was deposited via vapor deposition to increase repellency toward low surface tension liquids.

Using flat and micropattern PDMS, four different surfaces were compared: untreated, with fluorosilane, with nanoparticle-binder coating, and with nanoparticle- binder and fluorosilane coating. Flat PDMS had a CA of 113 ± 2°. In order to achieve superhydrophobicity with CA greater than 150°, micropatterned PDMS or flat PDMS with the nanoparticle-binder coating could be used. However, both samples were superoleophilic. In order to achieve superhydrophobicity and superoleophobicity, the nanoparticle-binder and fluorosilane coating needed to be deposited on either flat or micropatterned PDMS. These surfaces exhibited CA in excess of 155° with water and hexadecane and TA less than 3°.

Micropatterned surfaces were not required for superoleophobicity and using SEM images, the surface topology of the nanoparticle-binder and fluorosilane coating was

63 analyzed. Rough surfaces due to the nanoparticles and binder agglomerating were found on the surface with hierarchical roughness ranging from the micro- to nano-scale. In addition, the agglomerates formed re-entrant angles that helped repel low surface tension liquids.

By comparing the different surfaces, the repellency of a surface using an easily scalable method was determined and found that micropattern was unnecessary. By having hierarchical roughness formed through nanoparticle and binder agglomeration, superhydrophobicity as well as superoleophobicity could be achieved. This superliquiphobicity was achieved due to re-entrant geometries formed on the hierarchical roughness that allow for increasing the CA on the flat substrate. Increasing the CA above

90° allows for surface roughness to make the surface even more repellent (Wenzel model), and the additional roughness decreases the solid-liquid composite interface fraction which improves repellency (Cassie-Baxter model).

Chapter 4: Wear-resistant, substrate-independent coatings using hydrophobic nanoparticles for superliquiphobicity

4.1. Introduction

Superliquiphobic properties could be incorporated in many commercial industries such as the automotive, aerospace, electronics, plastic packaging, and biomedical fields for liquid repellency and self-cleaning. Many different materials are used in industry and some glass and thermoplastic materials and their applications are shown in Table 2.

These materials could use superliquiphobic properties to be liquid repellent and self- cleaning, and it is the goal to design a superliquiphobic coating that could be applied to these different substrates. Applications include touchscreens such as smart phones, computers, and automobile dashes; glass (both soda-lime and borosilicate) such as windows, windshields, and solar panels; and plastic packaging such as shampoo bottles and laundry detergent containers. On touchscreens, finger oils cause streaks and smears that decrease user experience. For glass, water and ice on windows and windshields decrease visibility and on solar panels decrease efficiency. In plastic packaging, shampoo and laundry detergents can remain in the container leading to wasted product, recycling difficulties, and decreased user experience.

In Chapter 3, a lotus-leaf-inspired, superliquiphobic surface was described using methylphenyl silicone resin and hydrophobic SiO2 nanoparticles. This coating could be

Table 2: Glass and thermoplastic materials and their industrial applications

Material and source Applications

Glass Windshield, window glass, side/rear view mirrors, camera lenses, smart screens in electronic displays, (125495, Fisher Scientific) solar panels PP Door liners, fabric, carpet, plastic caps, plastic (ASTM D4101-0112, SPI) bottles, plastic sheets PDMS Biomedical devices, catheters, drainage tubing, (Sylgard 184, Dow Corning) micropumps, microvalves, electronic circuits PET Plastic bottles, smart screens in electronic displays (Dexerials Corporation, Japan) Nylon 6/6 Automotive dash, instrument panel (8539K232, McMaster-Carr) PU Automotive dash, instrument panel, automotive seats (8716K34, McMaster-Carr) PMMA Smart screens for capacitive touch material in (8560K171, McMaster-Carr) electronic displays, headlights PE Plastic bottles (8657K111, McMaster-Carr) PC Smart screens in electronic displays, automotive (Lexan SLX 2271T, Sabic) dash, instrument panel

spray deposited on flat PDMS substrate and would create micro- and nano-scale roughness, reminiscent of the surface morphology of a lotus leaf. However, additional development needed to be completed for application in industry. The coating needed to be characterized for durability, whether it could be applied to different substrates and still be liquid repellent, the repellency of shampoo and laundry detergents which contain

66 surfactants, self-cleaning and anti-smudge, and repellency in different temperature environments.

In this chapter, a coating comprising of methylphenyl silicone resin and hydrophobic SiO2 nanoparticles was spray coated onto a variety of substrates to characterize the repellency of the coating on various substrates. In addition, repellency to surfactant-containing liquids such as shampoo and laundry detergent was characterized because of applications in plastic packaging where surfaces are exposed to these liquids.

In addition, the wear-resistance, transparency, and finger touch resistance properties of this coating are described for potential applications in electronic touch screens.

4.2. Results and discussion

The wettability, surface morphology, repellency of surfactant-containing liquids, wear resistance, self-cleaning, anti-smudge, transparency, and high temperature durability for the superliquiphobic (superhydrophobic and superoleophobic) coating will now be reported. In this chapter, data for the coating is primarily presented on soda-lime glass and PP substrates due to the numerous possible industrial applications for these substrates. However, this technique was not limited to these substrates and similar coating properties were achieved on different substrates by following similar coating procedures. For the other substrates, CA and TA characterization was completed in order to demonstrate that similar repellency was obtained. The substrates that were tested and their industrial applications can be found in Table 2.

4.2.1. Wettability of surfaces

A particle-to-binder (p-b) optimization was performed using CA and TA with water and hexadecane liquid because varying this ratio affects repellency and durability of the coating. At a low p-b ratio, liquid repellency is decreased and durability is increased because there is more resin to hold the particles together and to the substrate; however, there are not many nanoparticles to generate surface roughness. Oppositely, at a high p-b ratio, liquid repellency is increased and durability is decreased. A comparison of repellency at various p-b ratios is shown in this section, and a comparison of durability at two different p-b ratios is shown in Section 4.2.4.

Optimization using the nanoparticle-binder and fluorosilane coating on soda-lime glass is shown in Figure 19. This coating is the superhydrophobic and superoleophobic coatings as described in Figure 9c. A p-b range of 0.0–5.0 was tested in order to view a range of repellency properties. The CA curve fits are 4th order polynomials, and the TA curve fits are 5th order polynomials. The curve fits give an indication of the trend of the repellency going from low to high p-b ratios. Error bars show one standard deviation.

Since many of the error bars are ±2° or less, they overlap the data points themselves. In

Figure 19, at a p-b ratio less than 2.0, the surface is not repellent to both liquids and exhibits high TA. By further increasing the concentration of nanoparticles, a Cassie-

Baxter wetting interface rather than a Wenzel wetting interface was generated. At a p-b ratio greater than 2.0, the surface exhibited superhydrophobic and superoleophobic properties. The superliquiphobic properties would be generated due to incorporating air pockets into the nanoparticle-binder coating that decrease the solid-liquid composite

Figure 19: CA and TA measured using water and hexadecane droplets on glass with the

superhydrophobic and superoleophobic coating using methylphenyl silicone resin

as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.5 is optimal for

repellency and durability. The CA curve fits are 4th order polynomials, and the TA

curve fits are 5th order polynomials. Error bars are one standard deviation.

interface fraction. Jung and Bhushan (2007) created increasing air pocket size by increasing pitch on a micropattern. At a certain pitch, the air pockets become too large and there are not enough micropillars to repel the droplet which results in a sudden decrease in CA. Similarly by decreasing the p-b ratio, larger gaps between air pockets should be formed and increase the chance that a droplet would not be repelled.

An optimal p-b ratio of 2.5 was chosen based on the constraints of superliquiphobicity and high durability with the hexadecane data as the objective function. Hexadecane is more difficult to repel due to its lower surface tension; therefore, water CA and TA are greater than hexadecane CA and TA for the same p-b ratios. At a p- b ratio of 2.0, the CA for hexadecane is just barely above 150°, whereas a p-b ratio of 2.5 is clearly superoleophobic. In addition, the TA for hexadecane is beginning to show pinning because the TA is starting to increase. At p-b ratios greater than 2.5, similar CA and TA for hexadecane are shown; however, at these higher ratios, durability was decreased.

The optimum p-b ratio of 2.5 is expected to be a global optimum. At higher p-b ratios, the repellency of water and hexadecane using CA and TA was about the same. At p-b ratios even greater than 5.0, similar repellency is expected due to similar rough surface features that lead to repellency in the Cassie-Baxter model. However, these larger p-b ratios decrease durability and are therefore not optimal.

The measured CA and TA values for water and hexadecane on glass and PP with various coatings are shown in Figure 20a. Untreated glass has a water CA of 22 ± 2° and a hexadecane CA of 12 ± 2°, and untreated PP has a water CA of 96 ± 1° and a hexadecane CA of 35 ± 2°. By applying the nanoparticle-binder and fluorosilane coating to these substrates, superliquiphobicity was obtained as shown in Figure 20a. The nanoparticle-binder and fluorosilane coating results in water CA in excess of 164 ± 2°, water TA ≤ 1°, hexadecane CA in excess of 156 ± 2°, and hexadecane TA of 2 ± 1°.

Similar wettability can be obtained on other substrates as shown in Figure 20b where the

Figure 20: Droplets deposited on various substrates to show repellency for the coating.

(a) Using glass and polypropylene substrates, water and hexadecane droplets were

deposited to show superliquiphobicity. (b) Images of hexadecane droplets on

various substrates with the nanoparticle-binder and fluorosilane coating to show

that hexadecane beads up on each substrate for the substrate-independent ability.

CA and TA measurements on these substrates were within one standard deviation

(±2° for CA and ±1° for TA).

Table 3: Contact angle and tilt angles for water and hexadecane droplets on various

substrates with the nanoparticle-binder and fluorosilane coating at p-b = 2.5

DI Water Hexadecane Substrate Contact angle (°) Tilt angle (°) Contact angle (°) Tilt angle (°) Glass 165 ± 2 ≤ 1 157 ± 2 2 ± 1 PP 164 ± 2 ≤ 1 156 ± 2 2 ± 1 PDMS 164 ± 2 ≤ 1 156 ± 2 2 ± 1 PET 165 ± 3 ≤ 1 155 ± 2 2 ± 1 Nylon 163 ± 2 ≤ 1 156 ± 2 2 ± 1 PU 163 ± 3 ≤ 1 154 ± 3 3 ± 1 PMMA 164 ± 2 ≤ 1 156 ± 3 2 ± 1 PE 165 ± 2 ≤ 1 157 ± 2 2 ± 1 PC 164 ± 2 ≤ 1 154 ± 2 2 ± 1

nanoparticle-binder and fluorosilane coating was additionally deposited on PDMS, PET, nylon, PU, PMMA, PE, and PC. These images show superoleophobicity, due to the resulting spherical geometry, through a hexadecane droplet deposited on these surfaces.

Water droplets were not photographed because with a TA ≤ 1°, the water droplets would not remain stationary and would roll off the surface. These substrates are not 100% the listed composition and are expected to have fillers and impurities; however, even with these variations the coatings were still superliquiphobic. The CA and TA using water and hexadecane for the various substrates with the nanoparticle-binder and fluorosilane coating at p-b = 2.5 are shown in Table 3. With the claim of substrate independence, it is expected an optimization of p-b ratio on these additional substrates would result in curves similar to Figure 19. 72

4.2.2. Surface morphology

SEM images of the glass and PP substrates with the coating were taken as shown in Figure 21. Similar to SEM images on flat PDMS (Figure 16), the coating was found to contain hierarchical structures showing re-entrant geometry (see Figure 5). The glass sample is shown with a p-b ratio of 4.0, and the PP sample is shown with a p-b ratio of

2.5. In the top view images, this difference in p-b ratio shows how altering the concentration of particles and binder influences the coating. In this case, the glass sample

Figure 21: SEM images of the nanoparticle-binder and fluorosilane coating on glass and

PP substrates. The top-down view (shown at two magnifications) shows

agglomerates of nanoparticles and binder that form micron-sized structures. The

tilt view shows these structures have quasi-spherical re-entrant geometries that

were found to be repellent to water and hexadecane. Arrows point to re-entrant

overhangs.

Figure 22: SEM images of the nanoparticle-binder and fluorosilane coating on PP

substrates at p-b = 2.5. The circles in the top-down view (same image as in Figure

21) are agglomerations of nanoparticles and resin that are possible re-entrant

geometries. The tilt view several re-entrant geometries within one image.

contains much rougher structures than the PP sample due to the greater concentration of particles.

In order to determine how much of the coating was re-entrant geometry, agglomerations of nanoparticles and resin that are possible re-entrant geometries from the

PP substrate image in Figure 21 were circled. These circles are shown in the top view in

Figure 22. These potential re-entrant locations show that re-entrant geometry occurs throughout the coating. By comparing the area of the circled areas to the area of the image, approximately 5–10 % of the coating would be considered re-entrant geometry.

Although only a small amount of the surface contains re-entrant geometries, these features can be used to help repel liquids. Using a micropattern, the optimal pitch between micropillars is about 45 μm for 10 μm height pillars and about 125 μm for 30 74

μm height pillars (Jung and Bhushan, 2007). As shown in Figure 22, many of the possible re-entrant geometry locations are within 10 μm of another location. The areas without a re-entrant geometry have a diameter of about 25 μm. Optimal pitch distance is determined by the height of the surface features. The tilt view in Figure 22 shows several re-entrant structures in one area to support the idea that there are re-entrant structures throughout the coating. In addition, the re-entrant structures have heights less than 10 μm.

The shorter height of the surface features would require a smaller pitch distance so that the droplets do not touch the valleys between asperities due to the curvature of the droplet.

With rougher surfaces, such as the glass sample with p-b = 4.0, the distance between asperities decreased and the size of the asperities increased. With p-b = 4.0, decreasing the pitch distance creates a rougher surface that allows for more possible contacts with the droplet. It is the goal to decrease the number of contacts with the surface for a greater CA and so with taller surface features, the optimal pitch size increases (Jung and Bhushan, 2007). In this work, the difference between p-b = 2.5 and p-b = 4.0 for CA repellency was minimal as shown in Figure 19.

4.2.3. Repellency of surfactant-containing liquids

Oil repellency is now a common property to find in coatings and surface modification techniques, and there are many methods capable of producing superoleophobic surfaces. A new challenge is creating surfaces that repel surfactant- containing liquids. Surfactant-containing liquids are more difficult liquids to repel due to their low surface tension and highly active, polar head group that adheres to surfaces

75 more strongly than oils. Surfaces that repel these liquids could find use in consumer goods packaging, where repellency of surfactant-containing liquids is important in the packaging of shampoos and laundry detergents. Surfactant-containing liquids readily coat the inside of plastic bottles, leading to wasted product and issues with recyclability.

The repellency of surfactant-containing liquids is determined by the concentration of the surfactants in the liquid. Common surfactants in shampoo include sodium lauryl sulfate and sodium laureth sulfate, whereas common surfactants in liquid detergent include sodium alcoholethoxy sulfate and sodium alkylbenzene sulfonate. The concentration typically found in these applications leads to surface tensions in the range of 25–60 mN/m (Ross and Epstein, 1958; Al-Sabagh et al., 2011).

To determine the applicability of the nanoparticle-binder with fluorosilane coating in real world applications such as consumer packaging, its repellency against shampoo and laundry detergent was tested. Droplets of shampoo and laundry detergent were found to roll or slide from coated glass and PP surfaces with no noticeable contamination as shown in Figure 23. This contrasts with the untreated glass and PP substrates where both shampoo and laundry detergent were found to spread and foul the surface.

In the nanoparticle-binder and fluorosilane coating, surface topographic features trap air between the substrate and liquid, reducing the solid–liquid interface and therefore increasing droplet CA and reducing TA. However, when a droplet of surfactant- containing liquid such as shampoo or laundry detergent rests on these surfaces for a significant period of time, surfactant molecules may attach to the surface. These surfactant molecules may alter the surface chemistry of the solid surface, leading to a

Figure 23: Photographs demonstrating repellency of surfactant-containing liquids by

comparing shampoo and laundry detergent droplets deposited on untreated and

nanoparticle-binder and fluorosilane surfaces using (a) glass substrate and (b)

polypropylene substrate. On the untreated substrates, the droplet spreads out and

adheres to the surface. On the coated substrates, the droplet rolls and slides off the

surface without any residue remaining.

77 decrease in the CA and a collapsing of the composite interface between the droplet and the surface. The surfactant can also remain on the surface after the liquid is removed, which may lead to a permanent loss of repellency.

In order to characterize the nanoparticle-binder and fluorosilane coating for extended contact with surfactant-containing liquids, a coated glass sample was submerged in shampoo as shown in Figure 24. In the first test (Figure 24a), the sample was dipped in shampoo for one minute and then withdrawn. The sample shows minimal shampoo residue on the surface.

In the second test (Figure 24b), the sample was repeatedly dipped into shampoo.

Minimal shampoo residue remains on the surface after 10 submerges. This test shows the ability to repeatedly subject the coating to surfactant-containing liquids and repel these types of liquids. In both tests, some shampoo can be seen around the edges of the sample.

The edges of the glass substrate were not coated with the nanoparticle-binder and fluorosilane coating and likely functioned as an initial failure point. This point eventually allowed for the shampoo to adhere to the edges of the sample. A uniform application of the coating to the inside of a container or lid should eliminate failure points and therefore fully repel surfactant-containing liquids.

During the test, the shampoo did not become attached to the coating. If there was some thin film of shampoo that was attached, the low surface energy of the coating would become compromised. In the next dip, the shampoo could attach to the thin film of shampoo. Over addition dipping motions, more and more shampoo would accumulate on the surface.

Figure 24: Nanoparticle-binder and fluorosilane coating showing repellency after

prolonged contact with shampoo. (a) The coating can be dipped into shampoo for

one minute and withdrawn with minimal shampoo residue on the surface. (b) The

coating can be repeatedly dipped into shampoo with minimal shampoo residue on

the surface after 10 submerges. (c) Schematic of setup and meniscus profile on the

uncoated and coated side of the substrate (side view).

By modifying this experiment (Figure 24b) with a sensor to measure force required to retract the coating, a Wilhelmy plate experiment would be created. The force is normally measured using a tensiometer or microbalance. This experiment relates surface tension, the force required to remove the surface from the liquid (F), the

79 geometry of the surface, and the CA. The wetted perimeter of the surface is l. The relationship is given as the Wilhelmy equation.

퐹 훾 = (4) 푙 cos 휃

Using a Wilhelmy plate setup would also allow for determining how the CA changes as the number of dipping motions were performed. Based on Figure 23, the shampoo droplet would have a CA in excess of 150° due to the droplet beading up on the surface and rolling away. This setup was not performed because a sensor to determine force was unavailable.

During these tests, the shampoo formed a convex meniscus on the coated side and a concave meniscus on the untreated side (Figure 24c). In order to form a convex meniscus, the shampoo must be more strongly attracted to itself than the coated sample.

This shape is opposed to a concave meniscus commonly seen with water curving itself to climb the walls of a container.

4.2.4. Wear resistance

The mechanical durability of the nanoparticle-binder and fluorosilane coating was investigated through AFM wear, tribometer wear, and fingerprint resistance. The estimated contact stresses the coating would experience due to a hard fingerprint imprint was around 70 MPa (shown later in Table 5); therefore, it was the goal of the coating to be wear resistant for stresses of around 100 MPa. The AFM wear test creates a contact stress of about 115 MPa, and the tribometer wear test creates a contact stress of about 65

MPa.

Figure 25: Nanoparticle-binder and fluorosilane coating showing AFM wear durability.

Sample surface profiles (indicated at the arrow) and surface heights maps are

shown before and after the AFM wear test using a 30 μm diameter borosilicate

ball mounted on a rectangular cantilever with a load of 10 μN.

The AFM wear image, shown in Figure 25, shows a 100 by 100 μm2 area with the wear region covering an area of 50 by 50 μm2 area. For the nanoparticle-binder and fluorosilane approach at p-b = 2.5, some of the surface features are worn down or removed after wear; however, the majority of the coating was still seen in the wear region after the test. As well as wear being shown in the image, wear can be seen in the surface profile. In the after image surface profile, one of the peaks was taller than in the before image which suggests that part of the coating moved and remained at the top of the asperity.

The investigation using a tribometer wear experiment and the resulting optical images, showing a portion of the wear track, are shown in Figure 26. The wear test was carried out for 100 cycles at 10 mN and resulted in noticeable burnishing of the coating

Figure 26: Tribometer wear experiments on glass for the nanoparticle-binder and

fluorosilane coating with p-b = 2.5 and p-b = 4.0. As p-b ratio decreased,

durability increased with minimal damage at lower p-b ratios.

or coating removal. In the p-b ratio = 4.0 sample, significant wear can be seen with a thick wear scar. When the p-b ratio was decreased to 2.5, damage to the coatings was minimal. It is likely that the hard nanoparticles used in the coating help improve durability. The before images in Figure 26 show how the p-b ratio affects the coating morphology. In the p-b = 4.0 sample, many, large agglomerations of nanoparticles and resin were deposited. In the image with the decreased p-b ratio, the agglomerations decreased in size and there are fewer large agglomerations.

In the less durable case with p-b = 4.0, the coating was still able to repel hexadecane over the wear scar suggesting that the coating was not completely destroyed.

Hexadecane TA was recorded before and after the wear experiment as well as for a deliberately destroyed coating created through scratching with tweezers. The results for this investigation are shown in Figure 27.

Before the wear test, hexadecane droplets that were dragged across the surface were not obstructed in any way and rolled off the surface at a TA of 2 ± 1°. After the 82

Figure 27: Images of hexadecane droplets before and after wear/scratching on coated

glass. Droplets were dragged or tilted across the defect in direction of arrows.

Before the wear test, droplets rolled off the surface at 2 ± 1° tilt angle. For the

worn samples, droplets placed to the right of the wear track rolled over the defect

at 5 ± 1° tilt angle, and droplets placed directly on the wear track rolled over the

defect at 17 ± 2° tilt angle. Droplets on the scratched sample were pinned at the

defect until 53 ± 4° tilt angle regardless of droplet starting position.

wear test, hexadecane droplets became pinned at the defect when dragged over the surface and had an increased TA. At a TA of 5 ± 1°, the droplets rolled over the defect.

At a TA of 17 ± 2°, a droplet placed on the defect was able to roll away. The data suggest that damage to the surface increases pinning of droplets placed at the defect. However,

83 when the droplets are moving, they are still readily repelled. In contrast, on a deliberately destroyed coating (tweezers), a hexadecane droplet became highly pinned and required a

TA of 53 ± 4° for the droplet to roll off regardless of droplet starting position. The sample after the wear test still repelled hexadecane at a much smaller TA than the destroyed coating (tweezers) sample, and therefore showed that the coating had wear-resistant properties. It is expected that even better TA results will be obtained with a decreased p-b ratio due to the increased adhesion between binder and nanoparticles and binder and substrate. The increased adhesion will lead to a smaller wear scar (Figure 26) and less pinning of a droplet.

If a larger defect on the coating were created instead of a solitary wear scar, decreased repellency would result. The droplet would be more likely to become pinned at numerous locations and therefore decreased CA and increased TA would result. The droplet remains repelled over smaller defects because there are less attractive forces at small, solitary defects. The reduction in repellency for a wear scar is dependent on the thickness of the wear scar, size of the droplet, and interaction of the droplet with the substrate. The thickness of the wear scar and size of the droplet relate how much surface area is in contact with other. The surface area in the wear scar would have reduced repellency due to a loss of the low surface energy coating or a loss of roughness features.

Using Figure 26, the width of the wear scar for p-b = 4.0 was around 350 μm and a 5 μL droplet corresponds to a diameter of 2 mm, resulting in a wear scar width of about 18% of the diameter of the droplet. This percentage shows an increase in pinning of the droplet as shown in Figure 27. Wear scars would need to have width smaller than this ratio in

84 order to have better repellency such as with p-b = 2.5. In addition, if the substrate was superliquiphilic, then the droplet would be more attracted to the surface area at the wear scar and harder to repel.

The nanoparticle-binder and fluorosilane coating was investigated for fingerprint resistance through two tests (refer to 2.3.5.3): a rubber finger tip impression at 5 N and an actual thumb impression at >100 N as shown in Figure 28. For the coating at p-b = 4.0, rubber nib imprints can be seen from the rubber finger tip, and a minor change in light reflectivity on the right side from the thumb impression can be seen. This change in reflectivity occurs halfway across the sample. The rubber finger nub impressions leave visible round marks that can be seen under microscope as shown in the insert. An improvement in the coating durability was obtained for the p-b = 2.5 coating. The finger nub impressions are decreased and cannot be seen under microscope. Also, the thumb impression cannot be seen. For the thumb impression test, these samples are better than an untreated glass slide because after the test the oils from a finger are easily transferred to the slide and leave a visible fingerprint.

The CA and TA were measured before and after the finger touch tests to see if the coatings were still repellent. These measurements are shown in Table 4. After the rubber finger tip test, the coatings show excellent repellency to water by remaining superhydrophobic, but have a decrease in repellency for hexadecane. Similarly, after the thumb impression test, the coatings show excellent repellency to water, but have decreased repellency for hexadecane. The surface structures are damaged after the

Figure 28: Fingerprint test using rubber finger and thumb on the nanoparticle-binder and

fluorosilane coatings on glass. With the rubber finger test, nib impressions can be

seen with improvements at the lower p-b ratios. With the thumb impression, a

change in light reflectivity can be seen on the p-b = 4.0 sample, but no change is

seen on the p-b = 2.5 sample.

impression and allow for hexadecane to become pinned on the surface leading to the lower CA and higher TA.

The composition of fingerprints make fingerprint resistance more complex than comparison with a droplet of oil such as hexadecane. A fingerprint typically contains water, salts, long chain organic acids, and various oils and waxes. In addition, the 86

Table 4: Comparison of static CA and TA before and after finger touch tests

Contact angle (°) / Tilt angle (°) After rubber finger tip After thumb (> 100 N) Before test Technique (5 N) impression test impression test Water Hexadecane Water Hexadecane Water Hexadecane Nanoparticle- 158 ± 2 156 ± 2 158 ± 2 155 ± 2 156 ± 2 145 ± 4 binder with / 1 ± 1 / 1 ± 1 / 2 ± 1 p-b = 4.0 / 2 ± 1 / 18 ± 3 / 40 ± 3 Nanoparticle- 165 ± 2 157 ± 2 165 ± 2 157 ± 2 161 ± 3 152 ± 3 binder with / 1 ± 1 / 1 ± 1 / 2 ± 1 p-b = 2.5 / 2 ± 1 / 16 ± 3 / 38 ± 3

composition of a fingerprint varies from person-to-person as well as day-to-day (Lee and

Gaensslen, 2001). The varied composition of fingerprints creates a range of surface tensions that must be repelled.

The estimated mean contact pressures for the mechanical durability tests for the nanoparticle-binder and fluorosilane coating are presented in Table 5. Details on the experimental procedure and surface properties can be found in Section 2.3.5. The table helps explain contact stresses the coating would experience by comparing it to the real- world scenario of pressing hard on a surface with a thumb. This real-world scenario is similar to if the coating were on a smart screen or instrument panel. The estimated contact pressure by pushing hard on the surface with a thumb was 70 MPa. The estimated contact pressure with the AFM wear test was 115 MPa, which represented a single wear event creating stresses greater than the fingerprint test. The estimated contact pressure with the tribometer wear test was 65 MPa. This test represented repeated wear on the sample due to the 100 cycles at stresses similar to the fingerprint test. In comparison, a

Table 5: Estimated mean contact pressures in wear resistance tests using Hertz analysis

Fingerprint Fingerprint Macroscale Microscale resistance resistance wear with wear with AFM with rubber with actual tribometer finger thumb Load 10 µN 10 mN 5 N 100 N Borosilicate ball Sapphire ball Rubber nub Skin ridge Wear test E = 70 GPa E = 390 GPa E = 2 MPa E = 70 MPa properties ν = 0.2 ν = 0.23 ν = 0.5 ν = 0.5 d = 30 µm d = 3 mm d = 0.5 mm d = 0.2 mm Coating Nanoparticle-binder composite coating properties E = 32 GPa, ν = 0.36 Mean contact 115 MPa 65 MPa 1.0 MPa 70 MPa pressure

soft imprint was tested with a rubber finger for estimated contact pressures of 1 MPa to show coating changes at low stress.

4.2.5. Self-cleaning and anti-smudge

The self-cleaning properties of the coating were compared to the untreated substrate by first contaminating the surfaces with silicon carbide (SiC) and seeing how many contaminants could be removed by water droplets as shown in Figure 29. In the before images, the darker specs are SiC contaminants. On the coated samples, over 90% of the particles are removed due to the rolling action of water droplets collecting particles and removing them from the surface. This percentage was determined using optical microscope images and image analysis software to detect the contaminants. The contaminated area of the sample was compared to the contaminated area of the sample after the self-cleaning test. On the untreated glass and polypropylene samples, many

Figure 29: Optical micrographs of contaminated coatings before and after self-cleaning

test on untreated and nanoparticle-binder and fluorosilane samples using glass and

polypropylene substrates. Image analysis shows >90% removal of particles on the

techniques.

contaminants remain after the self-cleaning test. These coatings are self-cleaning due to their high water CA and low CAH. Water droplets deposited onto these samples roll over the coating with little impediment, collecting less hydrophobic contaminants as they go.

The self-cleaning action that has been described is due to water droplets beading up on a superhydrophobic surface and taking away contamination. However, self- cleaning can also occur with superhydrophilic surfaces or hydrophilic surfaces containing 89 titanium oxide (TiO2). On a superhydrophilic surface, water droplets wet the surface and go underneath contaminants. By wetting the surface, a thin film of liquid is formed that moves the contaminants off the surface as it spreads (Blossey, 2003; Bhushan, 2016).

This mechanism of self-cleaning is found in self-cleaning glass, and a similar mechanism can be seen in Figure 29 for untreated glass. On the untreated glass sample, water had a

CA of 22 ± 2° (Figure 20a) for hydrophilicity. The water droplets on this surface would wet the substrate lifting up the particles. If the contaminants were not removed under the water spreading action, they ended up clumped together as shown in Figure 29 for untreated glass. This clumping would be undesirable for self-cleaning and additional disadvantages include that the surface may remain wet for longer periods of time than superhydrophobic surfaces.

Surfaces that incorporate a TiO2 photocatalyst can break down organic materials when exposed to UV light. This mechanism occurs when UV exposure causes electrons from the TiO2 surface to break water molecules into hydroxyl radicals. Dirt particles react with these radicals and break into smaller particles that can easily be washed away by water (Fujishima and Honda., 1972; Wang et al., 1997; Zaleska, 2008).

One possible application for self-cleaning glass is in windows where several companies have created products incorporating TiO2. Pilkington Glass developed

Pilkington Activ™ and PPG has Sunclean™ and both products incorporate a thin layer of titanium dioxide (Parkin and Palgrave, 2005). The Pilkington Activ™ product contains a

20-30 nm layer of titanium dioxide deposited using chemical vapor deposition on soda- lime glass (Mills et al., 2003). Other commercial products include outdoor tiles and paint

90 by TOTO Ltd and branded Hydrotect™ (Parkin and Palgrave, 2005) and clay roof-tiles called ERLUS Lotus® (Gleiche et al., 2006). One downside of self-cleaning using TiO2 is that the surface must be exposed to UV light and therefore limits some applications of the surface. Further investigation needs to be performed in order to increase the mechanical and thermal stability of TiO2 self-cleaning surfaces (Liu and Jiang, 2012).

For anti-smudge, untreated and nanoparticle-binder surfaces were contaminated with silicon carbide contaminants and a hexadecane-soaked cloth was used to wipe the surfaces. The anti-smudge images are shown in Figure 30. For the oil-repellent surfaces, the particles were transferred to the cloth with no observable particles remaining on the surfaces. However, on the untreated samples, many particles remained on the sample and oil was transferred from the cloth to the sample. The anti-smudge property relies on a high CA and low CAH for the oil. The oil in the cloth is able to collect oleophilic contaminants from the surface of the coating without sticking to the surface. Using image analysis, the percentage of contaminants removed from the surfaces during the anti- smudge test was determined as shown in Figure 31. On the untreated surfaces, about 15-

20% of the contaminants were removed. On the surfaces with the nanoparticle-binder and fluorosilane coating, over 90% of the contaminants on the coated samples were removed.

Figure 30: Optical micrographs of contaminated surface and oil-impregnated microfiber

cloth before and after anti-smudge test on untreated and nanoparticle-binder and

fluorosilane surfaces using glass and polypropylene substrates. Dark spots on

coatings and cloth indicate silicon carbide particle contaminants. Untreated

samples show oil transferred to the substrate and few contaminants removed via

cloth. Coated samples show oil was not transferred to the substrate and that many

contaminants were removed via the cloth.

Figure 31: Percentage of silicon carbide contaminants removed from the surface during

the anti-smudge test for glass and polypropylene substrates, either untreated or

coated with nanoparticle-binder and fluorosilane. Image analysis shows >90%

removal of particles for the coating compared to about 15-20% removal on the

untreated substrates.

4.2.6. Transparency

Many applications of liquid-repellent surfaces require transparency of the coating.

When text is placed behind the coatings on glass and PET, the text remains readable suggesting that the coatings display characteristics of transparency as shown in Figure 32.

The edges of each sample are indicated with dashed lines. PP is an opaque polymer and so PET was chosen for transparency because PET is a transparent polymer. Further improvement in transparency, potentially by decreasing the thickness of the nanoparticle layer, may be possible in the future. For applications sensitive to transparency such as windshields and electronic displays, additional characterization using transparency standards should be performed.

Figure 32: Photographs showing transparency for untreated and nanoparticle-binder and

fluorosilane on glass and PET substrates. The reduction in transparency in the

coated samples is due to the SiO2 nanoparticles and binder. Edges of each sample

are shown in dashed lines.

4.2.7. High temperature durability

The nanoparticle-binder and fluorosilane coating on glass substrate was tested for hot environments by placing the sample on a hot plate. Water and hexadecane droplets of increasing temperature were placed onto the heated sample and the CA were recorded as shown in Figure 33. Droplets with temperatures up to 80°C were tested; however, it is expected that the substrate was subjected to even higher temperatures since it was in direct contact with the hot plate. Even at the max tested temperature, the coating exhibited superhydrophobicity and superoleophobicity. At 80°C, the water CA was 159 ±

3° and the hexadecane CA was 153 ± 2°. By testing hexadecane at a higher temperature, it shows that the coating repels lower surface tensions than hexadecane at room

Figure 33: Contact angles of water and hexadecane on nanoparticle-binder and

fluorosilane coating on glass showing superhydrophobicity and

superoleophobicity at droplets with temperatures up to 80°C. The coating repels

water and hexadecane at lower surface tensions due to the higher temperatures as

well as staying resistant to the higher temperatures.

temperature. Using Figure 4, the surface tension of hexadecane at 20°C is 27.5 mN/m, whereas at 80°C the surface tension is 22.4 mN/m.

4.3. Summary

In this chapter, the properties of a superliquiphobic coating comprised of hydrophobic, 10 nm SiO2 nanoparticles and methylphenyl silicone resin was characterized for wettability, surface morphology, repellency of surfactant-containing liquids, wear resistance, self-cleaning, anti-smudge, transparency, and high temperature durability. The coating was spray deposited on various substrates. In addition, a low surface energy coating using a fluorosilane was deposited via vapor deposition to increase repellency toward low surface tension liquids.

The coating exhibited excellent repellency to water and hexadecane with CA in excess of 164° for water and 156° for hexadecane and TA less than 1° for water and 2° for hexadecane. The coating successfully deposited on various substrates including glass,

PP, PDMS, PET, nylon, PU, PMMA, PE, and PC and still exhibited excellent repellency toward water and hexadecane. With the application to many different materials, this coating exhibited the property of substrate-independency because the repellency of the coating was not determined based on the substrate.

In addition to repellency to water and hexadecane, the coating exhibited repellency to surfactant-containing liquids such as shampoo and laundry detergent.

Droplets of these liquids were found to roll off the surface leaving no contamination and has potential applications in plastic packaging. Wear resistance was determined using tribometer, AFM, and fingerprint tests, and the coating was shown to exhibit resistance to various wear. The tribometer tested for macroscale wear; the AFM tested for microscale wear; and fingerprint tested for durability for applications where the coating would be touched such as in electronic touch screens. The coating exhibited additional properties such as self-cleaning, antis-smudge, transparency, and high temperature durability.

By understanding the various properties of the coating, it furthered the understanding of superliquiphobic coatings created by spray deposition of nanoparticles and binder. Numerous applications could implement superliquiphobicity and therefore an understanding of whether the coating could be applied to the substrate and the subsequent properties of the coating were important.

Chapter 5: Superliquiphobic coating adaptations using heating, different resins, and different surface functionalities

5.1. Introduction

In Chapter 4, various properties of a lotus-leaf-inspired, superliquiphobic surface were described. The coating consisted of methylphenyl silicone resin and hydrophobic

SiO2 nanoparticles with a subsequent coating of a fluorosilane for oil repellency.

Superliquiphobic coatings can be implemented in many different industries such as the automotive, aerospace, electronics, plastic packaging, and biomedical fields. However, understanding how the coating could be adapted to each application is valuable. The coating methodology had specific steps for procedure and components. If the coating procedure and components did not need to be as strict, it would improve the applicability of the coating for different applications (Bhushan, 2016; Martin et al., 2017).

The motivation for this work was to understand how the coating would respond to adding oven heating to the procedure, using different resins instead of the methylphenyl silicone resin, and the effects of changing the surface functionality of the coating. By adding oven heating to the coating after the coating had been deposited, it would help understand what happens to the durability and repellency of the coating after the resin softens and melts. By changing the methylphenyl silicone resin to other resins, it would determine if the coating can be used with other resins and the role of resin chemistry in

97 the coating. If different resins could be used, then coating properties could be adjusted by choosing a resin with appropriate properties. Properties that could be of interest include tensile strength, Young’s modulus of elasticity, melting point, glass transition temperature, and hardness. In addition, some applications could require resins that are environmentally-friendly, non-toxic, or widely available. Lastly, the coating has been tested with the fluorosilane surface functionality, but additional applications may be required that only repel one liquid and attract the other which would require different surface functionality. This adaptation helps understand how surface functionality affects repellency.

In this chapter, a coating comprised of various silicone or epoxy resins and hydrophobic SiO2 nanoparticles was spray coated onto a variety of substrates to characterize the repellency of the coating on various substrates. The effect of oven heating on the resin was characterized for wettability, durability, and coating thickness.

Different resins were used in place of the methylphenyl silicone resin and the wettability and wear resistance was characterized. The surface functionality of the coating was modified in two ways: by not depositing the fluorosilane coating and by replacing the fluorosilane coating with a fluorosurfactant coating. The wettability of these two new coatings and potential applications are tested.

5.2. Results and discussion

In this chapter, adaptations to the superliquiphobic (superhydrophobic and superoleophobic) coating are described. The superliquiphobic coating is shown in Figure

9c. These adaptations include adding oven heating to the methylphenyl silicone resin,

98 using other silicone and epoxy resins, and changing the surface functionality. Two other surface functionalities are demonstrated and are shown schematically in Figure 9a,b. The nanoparticle-binder technique results in superhydrophobicity and superoleophilicity, and the nanoparticle-binder and fluorosurfactant technique results in superhydrophilicity and superoleophobicity.

5.2.1. Coatings using oven heating

In this section, the adaptation to the superliquiphobic coating by adding oven heating to the methylphenyl silicone resin is described. The procedure for adding oven heating is described in Section 2.2.2 and used a gravity convection oven at 100°C for one hour. Oven heating above the softening point of the resin was used to allow the resin to flow to try to fill in voids and defects in the coating for improved durability. In addition, durability could be improved because the melted resin could reach the substrate and better bond the coating to the substrate. The wettability, wear resistance, and coating thickness for the superliquiphobic (superhydrophobic and superoleophobic) coating with oven heating will now be reported. In this chapter, data for the coating is presented on glass substrates due to its numerous possible industrial applications for this substrate.

5.2.1.1. Wettability of surfaces with oven heating

A particle-to-binder (p-b) optimization was performed using CA and TA with water and hexadecane liquid because varying this ratio affects repellency and durability of the coating. This optimization using the nanoparticle-binder and fluorosilane coating with oven heating is shown in Figure 34. A p-b range of 0.0–5.0 was tested in order to view a range of repellency properties. In Figure 19 using the coating with methylphenyl

Figure 34: CA and TA measured using water and hexadecane droplets on glass with the

superhydrophobic and superoleophobic coating using methylphenyl silicone resin

with oven heating as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.0

is optimal for repellency and durability. The CA curve fits are 4th order

polynomials, and the TA curve fits are 5th order polynomials. Error bars are one

standard deviation.

silicone resin without oven heating, an optimal p-b ratio of 2.5 was chosen based on the constraints of superliquiphobicity and durability. In Figure 34, the optimal p-b ratio was chosen to be 2.0 based on the same criteria (described in Section 4.2.1) and was lower than the approach without heating. The decreased optimal p-b ratio is likely because the higher temperature melts the resin and allows it to flow, which fills in voids and defects 100 in the coating. Similarly to Figure 19, Figure 34 shows that at a p-b ratio less than 2.0, the surface is not repellent to both liquids and exhibits high TA. By further increasing the concentration of nanoparticles, a Cassie-Baxter wetting interface rather than a Wenzel wetting interface was generated.

At p-b = 2.0, the CA and TA for water and hexadecane are nearly the same showing that the different surface tensions and liquid chemistries had little effect on the

CA and TA. The idealized re-entrant schematic (shown in Figure 5) shows that spherical structures give a range of re-entrant angles. Various re-entrant angles could be used to help repel the lower surface tension of hexadecane to a similar CA and TA of the higher surface tension of water. As shown in Figure 15, water droplets on a fluorinated surface result in CA of about 120°, whereas hexadecane droplets on the same surface result in

CA of about 70°. In order to have hexadecane droplets appear to have the same CA as water droplets, a re-entrant angle of 50° is required and can be generated with spherical re-entrant geometries. Eventually with a liquid with a low enough surface tension, the re- entrant angle would not be great enough and there would be a significant difference in

CA and TA between water and this liquid.

5.2.1.2. Wear resistance using oven heating

The mechanical durability of the nanoparticle-binder and fluorosilane coating with oven heating was investigated through AFM wear, tribometer wear, and fingerprint resistance. The AFM wear image, shown in Figure 35, shows a 100 by 100 μm2 area with the wear region covering an area of 50 by 50 μm2 area. For the nanoparticle-binder and fluorosilane approach with oven heating at p-b = 2.0, several surface features are worn

101

Figure 35: Nanoparticle-binder and fluorosilane coating with oven heating showing AFM

wear durability. Sample surface profiles (indicated at the arrow) and surface

heights maps are shown before and after the AFM wear test using a 30 μm

diameter borosilicate ball mounted on a rectangular cantilever with a load of 10

μN.

down or removed after wear; however, the majority of the coating was still seen in the wear region after the test. Compared to Figure 25 where AFM wear was performed on the same coating without oven heating, more surface wear was seen with the oven heating technique.

The investigation using a tribometer wear experiment and the resulting optical images, showing a portion of the wear track, are shown in Figure 36. The wear test was carried out for 100 cycles at 10 mN and resulted in a thin wear scar. When compared to

Figure 26 where the tribometer test was performed on coatings without oven heating,

Figure 36 showed improved wear resistance compared to the p-b = 4.0 sample and similar wear resistance compared to the p-b = 2.5 sample. Very few particle

102

Figure 36: Tribometer wear experiments on glass for the nanoparticle-binder and

fluorosilane coating and oven heating with p-b = 2.0.

agglomerations are shown in the before image due to the smaller p-b ratio than the before images shown in Figure 26.

The nanoparticle-binder and fluorosilane coating using oven heating on glass was investigated for fingerprint resistance through two tests (refer to Figure 13): a rubber finger tip impression at 5 N and an actual thumb impression at >100 N as shown in

Figure 37. For the rubber finger tip impression test, no nib imprints were seen, but with the thumb impression test, some fingerprint ridges were seen and are located below the arrow.

The CA and TA were measured before and after the finger touch tests to see if the coatings were still repellent. These measurements are shown in Table 6. After the rubber finger tip test, the coatings show excellent repellency to water by remaining superhydrophobic, but have a decrease in repellency for hexadecane. Similarly, after the thumb impression test, the coatings show excellent repellency to water, but have decreased repellency for hexadecane. The surface structures are damaged after the

103 impression and allow for hexadecane to become pinned on the surface leading to the lower CA and higher TA.

Figure 37: Fingerprint test using rubber finger and thumb on the nanoparticle-binder and

fluorosilane coating with oven heating on glass. With the rubber finger test, no

nib impressions can be seen. With the thumb impression, some fingerprint ridges

can be seen on the 2.0 sample.

Table 6: Comparison of static CA and TA before and after finger touch tests with oven

heating

Contact angle (°) / Tilt angle (°) After rubber finger tip After thumb (> 100 N) Before test Technique (5 N) impression test impression test Water Hexadecane Water Hexadecane Water Hexadecane Nanoparticle- binder with 167 ± 2 165 ± 2 167 ± 2 160 ± 2 167 ± 2 153 ± 2 heating in / 1 ± 1 / 1 ± 1 / 1 ± 1 oven and p-b / 2 ± 1 / 16 ± 3 / 31 ± 3 = 2.0

104

5.2.1.3. Coating thickness

Coating thickness was measured using an AFM step technique. Coating thickness of the epoxy resin with oven heating sample at p-b = 2.0 is shown in Figure 38. By keeping the coating solution volume constant at 1 mL, the coating thickness was only dependent on p-b ratio and whether oven heating was implemented. Using a p-b ratio of

4.0, the coating thickness was ~4 μm; using a p-b ratio of 2.5, the coating thickness was

~3.0 μm. Using the optimal p-b ratio of 2.0 with oven heating, the coating thickness decreased to ~2.5 μm. As the p-b ratio decreases, there are fewer nanoparticles in the solution leading to a less thick coating. For the coatings, the peak-to-valley distance was

5.5 ± 0.2 μm for the p-b = 2.5 methylphenyl silicone resin coating; 5.3 ± 0.2 μm for the p-b = 2.0 oven heated methylphenyl silicone resin coating; and 4.8 ± 0.6 μm for the p-b =

2.0 oven heated epoxy resin coating. These coatings all had RMS values of 1.0 ± 0.1 μm.

Figure 38: Coating thickness measurement with an average thickness of ~2 μm on the

epoxy resin with oven heating sample at p-b = 2.0. The step created in the coating

procedure was located 30 μm along the sample.

105

5.2.2. Coatings using different resins

The superliquiphobic coating was tested with additional resins to see if the coating would still be repellent to water and hexadecane (refer to Section 2.2.2) With each resin and an application of fluorosilane, the coatings resulted in superhydrophobic and superoleophobic properties as shown in Figure 39. With these results, it is believed that any resin could be used, and further testing was then completed with the epoxy resin

(EPON 1002F). The mixing action due to the sonifier would help combine the nanoparticles and resin into the agglomerations needed for re-entrant geometry. In addition, activation using UVO allows for fluorosilane to attach to the different resins to

Figure 39: Static contact angles on glass with different resins in the nanoparticle-binder

and fluorosilane technique on glass to show that different resins can be used for

similar coating properties of superhydrophobicity and superoleophobicity.

106 give it a low surface energy. The droplet is primarily interacting with the fluorosilane and does not contact the resin in the coating.

5.2.2.1. Wettability of surfaces with epoxy resin (EPON 1002F) and oven

heating

Figure 40: CA and TA measured using water and hexadecane droplets on glass with the

superhydrophobic and superoleophobic coating using epoxy resin with oven

heating as a function of particle-to-binder (p-b) ratio. A p-b ratio of 2.0 is optimal

for repellency and durability. The CA curve fits are 4th order polynomials, and the

TA curve fits are 5th order polynomials. Error bars are one standard deviation.

107

Particle-to-binder optimization was carried out with the epoxy resin and oven heating as shown in Figure 40. Oven heating was included because oven heating with the methylphenyl silicone resin decreased the optimal p-b ratio and decreased p-b ratio should result in durability improvements. These improvements would be desired in any coating with a different resin. The optimal p-b ratio was found to be at 2.0, which is the same as the methylphenyl silicone resin with oven heating (shown in Figure 34). Since the optimal p-b ratios were the same, it was believed that any similar resin could be used with oven heating, and the optimal p-b ratio would be 2.0 based on superliquiphobicity and durability (optimization described in Section 4.2.1).

5.2.2.2. Wear resistance with epoxy resin (EPON 1002F) and oven heating

The wear resistance using the approach with epoxy resin and oven heating was examined through AFM and tribometer wear experiments as shown in Figure 41. The

AFM wear region was a 50 by 50 μm2 area and after the wear, the entire coating in this region was removed as shown in Figure 41a. In addition, the tribometer wear experiment shows a thicker wear scar and more coating removal as shown in Figure 41b. These wear experiments show that the epoxy resin with oven heating did not improve durability over the coatings using different resins and without oven heating. The process of heating the epoxy resin increases hardening and therefore brittleness. A resin that is more brittle is more likely to fracture under stress than deform. However, the methylphenyl silicone resin is more compliant and elastically deforms under stress and therefore does not fracture.

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Figure 41: Wear experiments using approach with epoxy with oven heating coating at p-b

= 2.0 on glass. (a) AFM wear experiments with significant wear in the wear

region. (b) Tribometer wear experiment with a visible wear scar. Compared to

similar wear experiments using methylphenyl silicone resin and oven heating at

the same p-b ratio (shown in Figure 25 and Figure 26), the epoxy resin shows

greater wear for the AFM and tribometer experiments.

5.2.3. Coatings with different surface functionalities

In this section, the two other coating methods are described. These coatings are shown schematically Figure 9a,b. The nanoparticle-binder technique results in superhydrophobicity and superoleophilicity; the nanoparticle-binder and fluorosurfactant technique results in superhydrophilicity and superoleophobicity.

5.2.3.1. Wettability of surfaces

The measured CA and TA values for water and hexadecane on glass and PP with the other two coating adaptations are shown in Figure 42. From Figure 20a, untreated glass has a water CA of 22 ± 2° and a hexadecane CA of 12 ± 2°, and untreated PP has a 109

Figure 42: Other wettability states for the coating. With the nanoparticle-binder coating,

superhydrophobicity and superoleophilicity was obtained. With the nanoparticle-

binder and fluorosurfactant coating, superhydrophilicity and superoleophobicity

was obtained.

water CA of 96 ± 1° and a hexadecane CA of 35 ± 2°. By applying the coatings to these substrates, superhydrophobic/philic and superoleophobic/philic properties are obtained.

The nanoparticle-binder coating results in water CA in excess of 164 ± 2°, water TA ≤ 1°, and wetting with hexadecane. The nanoparticle-binder and fluorosurfactant coating results in wetting with water, hexadecane CA in excess of 156 ± 2°, and hexadecane TA of 2 ± 1°.

5.2.3.2. Wear resistance

The superhydrophobic and superoleophobic coating as opposed to the two approaches presented in this section was used in wear resistance testing in order to 110 compare repellency with water and hexadecane after the wear tests. It is expected that similar durability with these two other approaches would be obtained because only the surface treatment was modified. The primary component of the nanoparticle-binder layer is the same between all three of the coatings.

5.2.3.3. Oil–water separation

With water and oil, coatings that attract one liquid (-philic) and repel the other liquid (-phobic) are suitable for oil–water separation. The nanoparticle-binder coating exhibits superhydrophobic and superoleophilic properties, whereas the nanoparticle- binder with fluorosurfactant coating exhibits superhydrophilic and superoleophobic properties. Due to their different responses to water and hexadecane oil, these coatings can function as oil–water separators as shown in Figure 43. Agitated hexadecane oil– water mixtures were poured onto coated meshes suspended horizontally over beakers. In both cases, the -philic component quickly passed through the mesh, while the -phobic component remained on top of the mesh. When the mesh was tilted, the -phobic component rolled across the top of the mesh and was collected in another beaker. A droplet of the -phobic component placed on top of the mesh was repelled with CA over

150° and so re-entrant geometries were generated on the mesh. The agitated oil–water mixture is shown being poured over the superhydrophilic and superoleophobic coating and being separated in Figure 44. The mixture is known to be agitated because water and hexadecane liquid are shown separated in different beakers. Hexadecane oil floats on water and the photograph shows oil being poured over the mesh. For water to already

111

Figure 43: Photographs of hexadecane oil–water separation using the superhydrophobic/

superoleophilic and the superhydrophilic/ superoleophobic coating methods, both

deposited on a stainless-steel mesh in a horizontal or tilted orientation.

112

Figure 44: Photograph of oil–water separation using the superhydrophilic and

superoleophobic technique with the agitated hexadecane oil–water mixture being

poured over the coated mesh.

have been separated, some water must have been poured out with the hexadecane and would have occurred if the mixture was agitated.

The oil repellency of the superhydrophilic/superoleophobic coating, in addition to wetting by water, is due to the fluorosurfactant containing a low surface tension fluorinated tail and a high surface tension head group complexed with a hydrophilic polyelectrolyte. During spray coating, the polar head group forms an electrostatic complex with the polyelectrolyte layer below and the fluorinated tails orient themselves at the air interface. Large, bulky oil molecules are trapped at this fluorinated interface while smaller water molecules can more easily penetrate down through the thin layer to

113 the hydrophilic region. The result is a “flip-flop” of surface properties and a coating that repels oils, but is wet by water (Brown and Bhushan, 2015).

The benefit of using the fluorosurfactant in this coating rather than the coating in

Brown and Bhushan (2015) is that it requires fewer manufacturing steps. The UVO activation process allows for the fluorosurfactant to attach to the coating without depending on the electrostatic interactions of the components. In addition, the UVO process allows for other resins to be used (shown later in Section 5.2.3.4).

The use of hydrophilic/oleophobic coatings is preferable over alternative configurations where the surface is hydrophobic/oleophilic, because surface contamination by oil and other oil-based contaminants is common, and the porous material must then be cleaned or replaced, resulting in a drop in the separation efficiency.

It is therefore preferable for the water phase to be allowed to pass through the mesh and the oil phase be repelled. Additionally, water is denser than oil and tends to sink to the bottom of a mixture meaning hydrophobic/oleophilic materials are not suitable for certain applications, such as gravity-driven separation.

5.2.3.4. Superhydrophilic and superoleophobic surface functionality using

other resins

In order to determine how well additional resins work with the hydrophilic/oleophobic coating, silicone and epoxy resins were substituted for the methylphenyl silicone resin. The resins were tested with the coating with fluorosurfactant because it is preferred for the water phase to pass through the mesh and oil to be repelled

(discussed in Section 5.2.3.3). The purpose of using other resins was to see if the coating

114

Figure 45: Static contact angles on glass with different resins in the nanoparticle-binder

and fluorosurfactant technique to show that different resins can be used for similar

coating properties of superhydrophilicity and superoleophobicity.

was dependent on methylphenyl silicone resin or if the coating could be used with other resins. This information is helpful for commercial applications because certain resins can be more cost-effective, easier to acquire, and more environmentally friendly. In addition, if other resins can be used, then the properties of an oil–water separation coating can be modified by choosing a resin with appropriate properties. Figure 45 shows water and hexadecane droplets on nanoparticle-binder and fluorosurfactant coatings with these resins. The droplets show excellent oil–water separation properties by having water wetting and oil repelled with high CA and low TA. Since these four resins as well as the methylphenyl silicone resin (shown in Figure 42) can be used to create this coating, it is believed that any similar resin could be used. 115

5.3. Summary

The objective of this work was to understand how the superliquiphobic coating could be adapted for other properties and potential applications. These adaptations included adding oven heating, using different resins, and changing the surface functionality. By changing these components and comparing the coating differences, coatings created with nanoparticles and binder are better understood.

By adding oven heating to the coating process, the optimal p-b ratio decreased from 2.5 to 2.0. The mechanism that oven heating introduces is softening or melting of the resin which allows it to flow and fill in voids or defects in the coating. In addition, it should help improve the durability of the coating by improving adhesion to a substrate.

After completing the wear experiments using tribometer, AFM, and fingerprint resistance, similar durability to the unheated samples was found. The primary benefit of introducing oven heating was decreased p-b ratio. This reduction increases the ratio of binder to nanoparticles to better hold the coating together and to the substrate.

Four new resins were tested in place of the methylphenyl silicone resin. These resins were two additional silicone resins and two additional epoxy resins. By switching out the resins, it could be determined if the coating was dependent on the specific resin of methylphenyl silicone resin or if other resins could be used. By using these new resins in the superliquiphobic coating, similar properties in repelling water and hexadecane droplets were determined. This repellency shows that additional resins could be used in place of methylphenyl silicone resin and by choosing appropriate properties in the resin, the properties in the coating could be modified.

116

Lastly, the surface functionality of the coating was modified by removing the fluorosilane or replacing the fluorosilane with fluorosurfactant. When the fluorosilane component was removed, the coating was superhydrophobic and superoleophilic. When the fluorosurfactant was implemented, the coating was superhydrophilic and superoleophobic. This flip-flop of properties where one liquid is attracted and one liquid is repelled has applications in oil–water separation. Furthermore, the four other resins were found to also work with the flip-flop of properties.

By understanding the adaptability of the coating, it furthered the ability for coatings with nanoparticle and binder to be used in various applications. Adaptations involving heating, different resins, and modified surface functionality can be applied to different industries for various properties and applications.

117

Chapter 6: Summary and future work

Nature can be used as an inspiration for engineering design to solve scientific difficulties. Through evolution, nature has produced resourceful features that can be implemented for human usage to save time, money, and energy. One problem that surfaces in nature have helped solve includes liquid wettability. Liquid wettability is the interaction of a liquid with a surface and can be attracted or repelled from the surface.

Liquid repellency, also known as superliquiphobicity, is commonly desired on surfaces so that liquid droplets can easily roll away and not foul a surface. Some applications for liquid repellency include windows, solar panels, plastic packaging, and automobiles where self-cleaning and antifouling are desired.

Inspiration for an extremely liquid repellent surface can be found on lotus leaves where water droplets bead up and easily roll away from the surface. The unique surface features can be studied and improved for superliquiphobic properties. This work was motivated by wettability problems that people and industry faces every day. The objective of studying these surfaces was to understand their underlying principles and provide insight for improved surface design in superliquiphobic applications.

In this work, the hierarchical roughness found on lotus leaves was used as an inspiration for generating a superliquiphobic surface. Roughness was created through soft lithography of a 14 μm diameter, 30 μm height, and 126 μm pitch cylindrical pillars 118 master mold and a superliquiphobic coating comprised of various silicone and epoxy resins and hydrophobic SiO2 nanoparticles. In addition, a low surface energy coating using a fluorosilane was deposited via vapor deposition to increase repellency toward low surface tension liquids. By comparing the repellency of water and hexadecane droplets on various surfaces, it was determined that the micropattern was not necessary for superliquiphobicity. Only the coating was required to repel these liquids due to the nanoparticles and binder forming micron-sized, quasi-spherical agglomerates with a rough outer surface. These agglomerates contained re-entrant structures that help repel low surface tension liquids.

Further characterization of the superliquiphobic coating was completed to further understand its properties. This characterization included wettability, surface morphology, repellency of surfactant-containing liquids, wear resistance, self-cleaning, anti-smudge, transparency, and high temperature durability. The coating exhibited excellent repellency to water and hexadecane with CA in excess of 164° for water and 156° for hexadecane and TA less than 1° for water and 2° for hexadecane. The coating was deposited on various substrates including glass, PP, PDMS, PET, nylon, PU, PMMA, PE, and PC and showed excellent repellency in each case. With the application to many different materials, this coating exhibited the property of substrate-independency because the repellency of the coating was not determined based on the substrate composition.

Adaptations to the coating were tested by adding oven heating, using different resins, and changing the surface functionality. By introducing oven heating, the ratio of particle-to-binder could be decreased and retain the same repellency as the original ratio

119 of a coating without oven heating. The resin in the coating was changed to various silicone or epoxy resins, and the same properties as the original methylphenyl silicone resin were obtained, which showed that the coating was not dependent on the original resin. By modifying the surface functionality to achieve water attraction or repellency and hexadecane attraction or repellency, flip-flop of properties could be achieved where one liquid was attracted and the other liquid was repelled. This property has applications in oil–water separation where one liquid could be collected separately and the other liquid allowed to remain.

More investigation is needed for superliquiphobic surfaces before these surfaces can be widely used in industry. For example, durability can be further improved with the coating able to withstand greater stresses. Superliquiphobic roughness features are re- entrant, which are fragile and contain stress concentrators. With excess stresses, these features are susceptible to failure and can lead to pinning and wetting of a droplet.

Eventually, the repellency of the surface will be lost.

The knowledge and insights gained from this work in this thesis will further aid in understanding of surfaces for superliquiphobicity. The implementation of superliquiphobic surfaces will continue to grow as interest in surfaces with these properties increase and applications requiring these surfaces increase.

120

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