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 By 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. iii Dedication Dedicated to my parents iv 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. v 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. vi 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. vii 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 viii 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 ix 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.....................................................................