Chapter 2 the Design and Fabrication

Abstract

In this thesis, easy, scalable methods, relying on conventional and unconventional techniques are presented to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provide anisotropic adhesion properties. In the first part of the study, the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function are measured. Consistent with the Peel Zone model, samples with lower tilt angles yielded larger adhesion forces. Contact mechanics of the synthetic array were highly anisotropic, consistent with the frictional adhesion model and gecko-like.

Based on the original design, a new design of gecko-like dry adhesives was developed which showed superior tribological properties and furthermore showed anisotropic adhesive properties without the need for tilt in the structures. These adhesives can be used to reversibly suspend weights from vertical surfaces (e.g., walls) and, for the first time to our knowledge, horizontal surfaces (e.g., ceilings) by simultaneously and judiciously activating anisotropic friction and adhesion forces.

Furthermore, adhesion properties between artificial gecko-inspired dry adhesives and rough substrates with varying roughness are studied. The results suggest that both adhesion and friction forces on a rough substrate depends significantly on the geometrical parameters of the substrate. The results in this study may be helpful for understanding how geckos overcome the influence of natural surface roughness. The novel designs of our dry adhesives open the way for new gecko-like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication.

I would like to give my deepest gratitude to my dissertation advisor Noshir. S.

Pesika. It was fortunate for me to be able to join his group and work with him during my

Ph.D. He provided me lots of guidance, encouragement, inspiration that led to ideas, and above all, plenty of freedom to conduct scientific researches. His unique way of teaching has a significant role in shaping my research philosophy and skills, from which I will gain tremendous benefit in my life.

I would like to thank Prof. Vijay T. John for his generosity in giving me access to his laboratory equipment. It was great experience to have discussions with him, since every time our talk was very helpful to me in elucidating the technique backgrounds. I also thank him for providing constructive suggestions in this dissertation.

I owe my gratitude to Prof. Lawrence R. Pratt, who gave me many good suggestions during my Ph.D. research, and always being patience when explaining to me the theories.

It was through his proficiency and insightfulness in his field that I learned how to build a professional career.

My thanks also go to Prof. Michael J. Moore, for being in my dissertation committee, and his kindness to arrange time for me to learn and use his laboratory equipment.

I would like to acknowledge research funding support from National Science

Foundation and Louisiana Board of Regent.

ii During my study at Tulane, I had a good time working together with my research group, including Rajesh Venkatasubramanian, Joseph C Cremaldi, Lakhinder S Kamboj,

Tushar Khosla, and Moses M Oguntoye. It was joyful to be among them chatting about life in graduate school, holding discussions about our research, and exchanging scientific thoughts and ideas.

Finally, I owe countless thanks to my mother, my father and my wife, whose support in life and heart gave me strength and motivation all these years. Without their deepest love, the completion of this dissertation and my doctoral degree wouldn’t be possible.

iii

Table of Contents

Acknowledgments...... ii

List of Figures ...... viii

Chapter 1 ...... 1

Introduction ...... 1

1.1 The hierarchal structure of gecko pad ...... 1

1.2 Van der Waals Forces ...... 4

1.3 Roughness Considerations ...... 5

1.4 Force Generation and the Autumn Experiment ...... 7

1.5 Comparison with Measurements of the Whole Pad and the Single Spatula ...... 9

1.6 Attachment and Detachment ...... 10

1.7 Self-cleaning Mechanism ...... 11

1.8 Non-adhesive unloaded State and Non-self-adherence ...... 14

1.9 Substrates and the Environment...... 14

1.10 Fabrication of gecko inspired adhesives ...... 17

1.11 CNT-based dry adhesives ...... 22

1.12 Polymer-based dry adhesives ...... 22

1.13 Summary and future work ...... 24

1.14 Aims and objectives ...... 25

Chapter 2 ...... 27

The Design and Fabrication of Gecko-inspired Adhesives ...... 27

2.1 Introduction ...... 27

2.2 Experimental ...... 30

2.2.1 Fabrication of patterned silicon mold ...... 30

2.2.2 Fabrication of polyurethane fibrillar dry adhesive ...... 32

2.2.3 Adhesion and friction measurements ...... 34

2.3 Results and Discussion ...... 36

2.3.1 Design of dry adhesives ...... 36

2.3.2 Characterization of tribological properties ...... 37

2.3.3 Tribological test of glass probe on flat PU surface ...... 40

2.3.4 Comparison of gecko-inspired adhesive prepared by polyurethane of different

stiffness ...... 41

2.3.5 Comparison of gecko-inspired adhesive to gecko setal array ...... 42

2.3.6 Anisotropic tribological properties of gecko-inspired adhesive ...... 44

2.3.7 Comparison to Peel Zone model ...... 45

2.4 Conclusion ...... 48

Chapter 3 ...... 50

Biomimetic switchable adhesive with prismatic pillar arrays ...... 50

3.1 Introduction ...... 50

3.2 Experimental ...... 53

3.2.1 Fabrication of patterned silicon mold ...... 53

3.2.2 Fabrication of prismatic dry adhesive...... 55

3.2.3 Control of tilt angles of fibrillar arrays using the shear device...... 57

3.2.4 Characterization of tribological properties...... 58

3.2.4 Setup of the video recorder ...... 60

3.3 Result and discussion ...... 61

3.3.1 Determine the displacement location for maximum pull-off force...... 61

3.3.2 Determine the displacement location for minimum pull-off force ...... 63

3.3.3 Anisotropic properties of prismatic pillar gecko-inspired dry adhesive ...... 65

3.3.4 Tribological properties of the judicious prismatic pillar gecko-inspired dry

adhesive...... 68

3.3.5 Comparison to Peel Zone model ...... 71

3.4 Conclusion ...... 74

Chapter 4 ...... 75

Adhesive properties of gecko-inspired artificial adhesives on rough surfaces ...... 75

4.1 Introduction ...... 75

4.2 Experimental methods ...... 76

4.2.2 Design of the photo mask ...... 77

4.2.2 Preparation of coarse rough surfaces ...... 77

4.2.3 Preparation of fine rough surfaces ...... 78

4.2.4 Sample fabrication ...... 80

4.2.5 Characterization of tribological properties ...... 81

4.3 Result and discussion ...... 82

4.3.1 Adhesive properties on fine rough surfaces ...... 82

4.3.1 Adhesive properties on coarse rough surfaces ...... 83

4.3.2 Two stages during the shearing of the fibrillar array ...... 94

4.4 Possible Future Research Direction ...... 95

4.4.1 Future design of dry adhesive array pillar ...... 95

4.4.2 Elimination of the thickness effect during tribological tests ...... 96

4.4.3 Investigation of effect of humidity on gecko-inspired fry adhesive performance

...... 97

4.5 Conclusion ...... 98

Chapter 5 ...... 100

Conclusion and future work ...... 100

List of References ...... 103

Biography ...... 116

vii

List of Figures

Figure 1.1 Hierarchal breakdown of the Tokay gecko foot (pad) ...... 3

Figure 1.2 The hierarchal ability of gecko pads to deal with roughness ...... 6

Figure 1.3 The various tests run in Autumn et. al. “Adhesive Force of Single Gecko Foot-

Hair” ...... 7

Figure 1.4 Digital hyperextension in a gecko foot ...... 10

Figure 1.5 Hierarchy of the gecko pad...... 11

Figure 1.6 Unbalanced force generation in the gecko’s ability to self-clean ...... 12

Figure 1.7 Test setup used to show the effects of dynamic self-cleaning mechanism by

Hu et al...... 13

Figure 2.1 SEM images of (A) the side view of a Tokay gecko setal array, (B) a

magnified view of the spatula pads, (C) the gecko-inspired adhesive, (D) a

magnified view of the pad-like structures...... 28

Figure 2.2 SEM images of silicon master wafers with designed fibrillar array.(A) the top

view of patterned silicon mold, (B) the side view of patterned silicon mold 31

Figure 2.3 Schematic illustration of the different steps involved in fabricating the gecko-

inspired adhesives...... 32

Figure 2.4 Optical images of PDMS mold.(A) the side view of tilt PDMS mold, (B) the

side view of vertical PDMS mold. (C) the top view PDMS mold ...... 33

Figure 2.5 Optical images of gecko-inspired adhesives composed of PU with pillars..... 34

viii

Figure 2.6 Setup of Adhesion and friction measurements ...... 35

Figure 2.7 Schematic illustration of the tribological measurements of the gecko-inspired

adhesives tested in either the gripping or releasing direction...... 36

Figure 2.8 Dynamic measurements of the (A) adhesion forces, and (B) friction forces

generated by a gecko-inspired adhesive sample with 70° tilt when sheared in

the gripping and releasing direction. The drag velocity is 10 μm/s...... 38

Figure 2.9 Static and dynamic adhesion and friction forces generated by the gecko-

inspired adhesives with different tilt angles...... 40

Figure 2.10 Adhesion and friction forces test of glass probe on flat PU surface ...... 41

Figure 2.11 Adhesion and friction forces test of glass probe on PU samples with

differnent stiffness ...... 42

Figure 2.12 Normalized normal force vs friction force plot for a Tokay gecko setal array

(black dots) and for the gecko-inspired adhesive with 70° tilt. The forces are

normalized to their maximum respective magnitudes...... 43

Figure 2.13 Static adhesion (grey curve) and friction forces (black curve) generated by

the gecko-inspired adhesive during 8 cycles, each consisting of shearing in

the gripping direction followed by the releasing direction...... 44

Figure 2.14 Graphic representation of the peel-zone model cases ...... 45

Figure 2.15 Static adhesion force generated by the gecko-inspired adhesives as a function

of tilt angle. The solid line is a fit of the Peel Zone model...... 48

Figure 3.1 Gecko-inspired switchable adhesive...... 52

Figure 3.2 Scanning electron microscope image of patterned silicon mold with desired

triangular “prismatic” pillars on the base and rectangular pad on the top. ... 53

Figure 3.3 Side view of scanning electron microscope image of fabricated prismatic

polyurethane dry adhesive...... 54

Figure 3.4 Top view of scanning electron microscope image of fabricated prismatic

polyurethane dry adhesive...... 55

Figure 3.5 SEM images of the 2nd generation gecko-inspired adhesives composed of

polyurethane with pillars ...... 56

Figure 3.6 Schematic illustration of the home-build shear device...... 57

Figure 3.7 Schematic illustration of the tribological measurements of the gecko-inspired

adhesives tested in the gripping direction. (a) apply preload, (b)start shearing,

Figure 3.8 Schematic illustration of the tribological measurements of the gecko-inspired

adhesives tested in the releasing direction...... 59

Figure 3.9 Optical microscope profile view image frames from a typical measurement . 60

Figure 3.10 Adhesive pull-off and friction force measurements of vertical prismatic

pillars in the gripping direction...... 61

Figure 3.11 Adhesive pull-off and friction force measurements of 63˚ tilt prismatic pillars

in the gripping direction...... 63

Figure 3.12 Adhesive pull-off force measurements of tilted prismatic pillars in the

releasing direction...... 64

Figure 3.13 Side-view SEM images of the prismatic gecko-inspired adhesive under shear.

...... 65

Figure 3.14 Adhesion and friction force measurements in the gripping direction ...... 67

Figure 3.15 Adhesion and friction force measurements in the releasing direction ...... 69

Figure 3.16 Static adhesion force generated by the gecko-inspired adhesives as a function

of tilt angle. The solid line is a fit of the Peel Zone model...... 70

Figure 3.17 Supporting weights from both vertical and inverted horizontal surfaces, i.e.,

walls and ceilings...... 73

Figure 4.1 design of the photomask for creating rough surfaces ...... 76

Figure 4.2 Schematic illustration of the different steps involved in fabricating rough

surfaces using photolithography technique...... 78

Figure 4.3 Schematic illustrations of the tribological measurements of the gecko-inspired

adhesives tested on surfaces ...... 81

Figure 4.4 Adhesion and friction force measurements on polyurethane with different

roughness created by electrodeposit technique...... 82

Figure 4.5 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with

height of 60 µm...... 84

Figure 4.6 Adhesion forces of the dry adhesive sample on SU-8 grits rough surfaces with

height of 60 µm...... 85

Figure 4.7 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with

height of 30 µm...... 87

Figure 4.8 Adhesion forces of the dry adhesive sample on SU-8 grits rough surfaces with

height of 30 µm...... 89

Figure 4.9 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with

height of 5 µm...... 90

Figure 4.10 Adhesion force of the dry adhesive sample on SU-8 grits rough surface with

height of 5 µm...... 91

Figure 4.11 Measurements of the dry adhesive sample on SU-8 grits rough surfaces with

varying height...... 92

Figure 4.12 Measurements of the dry adhesive sample on SU-8 grits rough surfaces with

varying height...... 93

Figure 4.13 Future design of gecko-inspired switchable adhesive pillar...... 95

Figure 4.14 Schematic illustration the thickness effect during tribological tests ...... 96

xii

Chapter 1

Introduction

1.1 The hierarchal structure of gecko pad

The ability of geckos to run along vertical and inverted surfaces has been a source of curiosity for scientists for a very long time. Over the years, people have theorized that this ability has been a product of various properties of a gecko’s foot, or pad. These theories have included the use of glue-like secretions, suction1, 2, microscale interlocking3, friction3, electrostatic4, capillary adhesion5, 6, and van der Waals forces7. As the capabilities of microscopy have increased along with lowered costs over the years, more labs acquired the resources to try and tackle this scientific curiosity. Likewise, the theorized explanations of the gecko’s adhesive pads have been slowly ruled out over the years due to various shortcomings; large adhesion ruled out the friction mechanism8, performance in a vacuum ruled out suction, lack of glands on the pads ruled out wet adhesion through capillary action and glues, irregular surface-scaling of the spatulae ruled out microscale interlocking9, and the non-effect of X-rays on the adhesive capability ruled out electrostatic interactions. Finally, in 2000, Keller Autumn and his team reported conclusively that van der Waals forces are the source of adhesion in their

2 article “Adhesive Force of a Single Gecko Foot-hair.” Along with this essential publication, scientists have been unraveling the complex mechanisms and structures that grant geckos their exceptional abilities.

In addition to the straight forward adhesive abilities of the gecko, scientists have uncovered other properties that allow a gecko to so easily and successfully utilize this method of movement; anisotropic (directional) forces, force generation in relation to preloading, ease of detachment (peel-zone modeling), material-independence of attachment, non-self-adherence, and the self-cleaning mechanism. In order to accommodate these various properties, it has been found that several of the gecko’s biological systems have been incorporated into a complex and unique system10-12. These various biological systems include the integration of the integumentary, circulatory, musculoskeletal, sensory, and connective tissues13-17. For the purposes of this chapter’s discussion on the gecko, the mechanical properties of the gecko as a whole will be analyzed without going into the biological functions underlying those abilities.

In order to begin to understand all of these various abilities, one must first understand the basic makeup of a Tokay gecko’s foot. A gecko’s foot is often referred to as a pad, and will be so in the context of this text. The gecko pad is a hierarchal structure that exists on a macro-, meso-, micro-, and nano- scale18-21 as shown in Figure 1.1.

Figure 1.1 Hierarchal breakdown of the Tokay gecko foot (pad) A. Optical image of an inverted gecko showing the location of the gripping pad on a toe. B. Higher magnification image of the pad placement on the underside of the foot C. Lamellar ridges composed of a soft tissue that serve as a more elastic underlay to the gecko setae. D. Setae structures attached to the lamellar ridge. E. Setae tips. F. Spatulas.

Starting at the largest scale and working downwards we have the gecko in 1.1 A-F.

While 60% of the 1450 gecko species exhibit the ability to move on walls and inverted surfaces22, the model organism most used in gecko adhesion studies is the Tokay Gecko

(gekko gecko) due to its larger size and general abundance. A general comparison of contact areas between alternate geckos was completed by Irschick23 and displays the large relative contact area that Tokay gecko utilization provides. The Tokay Gecko is the second largest gecko species and ranges in length from 11 to 20 inches for males and

4 approximately 7-19 inches for females. Overall, the Tokay weighs between 150 to 400 grams. The macroscale features can be broken down quite easily by sight. Each gecko has four legs and each leg has five toes. At the mesoscale level, the underside of each toe has a series of approximately twenty ridges called lamellae. They are spaced approximately 1-2 mm apart and provide an elastic tissue under-layer. On the microscopic level, on each lamellar ridge exists a dense packing of β-keratin hair-like setae. Each seta is approximately 30 to 125 microns in length and 5 to 10 microns in diameter24, 25. The last, nano-scale tier of the structuring is the tips of the setae. The end of each setae splits into approximately 100-1000 smaller spatulas 50-250 nanometers in length. The triangular ends of the spatulas get larger and flatter, hence the name, and become about 10 nm thick and about 200 nm wide. These tiers result in hundreds of millions to billions of spatulas with a total macroscopic contact area of 200-250 mm2.26-28

It is worth noting that a recent discovery shows that phospholipids are deposited in the wake of gecko movement, leaving a hydrophobic footprint in their wake29. The release of these lipids does not act as glue in any way, but may play a yet unknown part in the mechanics of gecko adhesion.

1.2 Van der Waals Forces

With the hierarchal structure established we return to the source of adhesion, i.e., the relatively weak van der Waals force based on the attractive and repulsive forces resultant from the proximity of any two molecules. These forces occur between the permanent or induced (temporary) dipoles that exist in molecules: dipole-dipole, induced dipole-dipole, induced dipole-induced dipole. In addition to the general weakness of the force, it decreases rapidly at very small distances and is effective only in the atomic interaction

5 range of angstroms. Due to the weak interactions and miniscule distances necessary, in order for this force to be a factor in adhesion, large area of very intimate contact between two surfaces must be achieved. The van der Waals force per unit area between two flat, parallel surfaces can be expressed as:30

In the equation, is the van der Waals force, D is the gap distance between the two surfaces, and A is the material dependent Hamaker constant. Understanding this idea is a key to understanding the design and properties of the gecko’s foot and is the reason such detail has been spent on the underlying tiers of the gecko pad.

1.3 Roughness Considerations

When people envision intimate contact, they envision two very flat surfaces in contact with each other. Unfortunately, nature doesn’t subscribe to such a notion and most surfaces, even those that appear flat macroscopically, have a certain degree of roughness on the nano and micro-scales. It is straightforward to see how the bending of legs and toes allows the gecko to get good contact on a macroscopic level of centimeters

(grappling a branch, etc.). The unique design of the gecko, however, is where it deviates from most other organisms in that its toes continue to provide structural methods of dealing with roughness down to the nanoscale levels through the meso-, micro-, and nanoscale features26, 31-35. Figure 1.2 shows the actual roughness of three common surfaces and displays how the hierarchical structures effectively create intimate contact.

Figure 1.2 Hierarchal image of how the different levels (sizes) of the gecko pad can deal with roughness at different length scale. By matching smaller features with smaller roughness scales, the pad effectively flattens the surface.

In looking at the way various adhesives deal with roughness, Persson utilizes three categories; brittle fracture, solid substrate/flowing polymer, and solid substrate/fibrous material36. He provides a mathematical analysis to provide for an effective material modulus that is essential in dealing with rough surfaces. Additionally, the theory offers an explanation for the L (length)/R (width) ratio of the setae and spatulae that act as fibrous material in his analysis. Based on the material modulus of β-keratin, this ratio exists right at the limit where the setae and spatulae would prefer to stay separate rather than to bunch together due to interactions between the fibers, effectively showing how natural selection has optimized this ratio for gecko adhesion37-40.

1.4 Force Generation and the Autumn Experiment

Just how much adhesive (perpendicular to contact) and frictional (parallel to contact) forces does a gecko generate? The structure of the gecko pad had been determined over the course of the 20th century and measurements had been taken on the shear adhesion (friction force) capabilities of the entire gecko pad41-44. In 1996 Irschick et. al. 45reported a force of 20N for two feet of a Tokay Gecko; approximately 200-220 mm2 and three million setae. In 2001, however, the Autumn group determined the forces associated with a single seta confirmed the van der Waals mechanism of adhesion8, 46.

The experimental setup utilized a single seta attached to a micro-electromechanical system (MEMS) consisting of a piezo-resistant cantilever. The testing surface consisted of a flattened aluminum wire in which the displacement of the wire could be measured through compound optical microscopy and image analysis software.

The gecko creates forces by placing its foot on the substrate (preload) and then bringing the toes inwards toward the center of the foot (gripping direction) as seen in

1.3A.

Figure 1.3 A. Optical image of a gecko foot. The arrows point toward the gripping direction. B. Schematic illustration of the tribological measurements in the gripping direction. C. Schematic illustration of proving the effect of the spatulas in the force generation. D Schematic illustration of experiment used to determine the critical angle needed for adhesion (i.e, detachment)

The first test consisted of a parallel force (friction) measurement utilizing a known perpendicular preload. Once the preload was applied, the seta was move in the parallel gripping direction at constant velocity and the resulting friction force was measured.

This setup can be seen in 1.3B.23 Based on a general 100 mm2 contact area and the fact that there exist approximately 5,000 setae per mm2, a resultant force of 20 µN could be expected for each seta. Due to the fact that in most cases all of the setae did not contact the surface simultaneously, a slightly larger number may have been possible. However, the experimental results gave a value almost an order of magnitude higher at 200 µN of friction force and 40 µN of adhesive force. These results occurred when small preload was applied and the seta was allowed to slide over the surface a small distance on the order of 5 µm. It is important to remember that at 10 N per foot and four feet the gecko was expected to already support an adhesive weight of a little over 4 kg, already a high number to support an organism that weighs from 0.15 to 0.4 kg. The new results meant that at 100% contact, 100 mm2 of contact area could be expected to achieve 40 kg of friction force (referred to as shear adhesion in the referenced article).

The second test simply reversed the orientation of the seta so that the spatulas faced away from the substrate as and then repeated the first test with varying preload (1.3C).

The resulting forces showed that unless the spatulas contacted the substrate, the seta behaved in a similar fashion to any material with a coefficient of friction of ~0.25 and force generation varied linearly with the preload. This test served to confirm that the spatulas play the direct role in creating intimate contact and therefore the van der Waals forces.

The third and last test varied the angle θ at which the seta (and therefore spatulae) contacted and finally detached from the surface as can be seen in 1.3 D. The data showed that there exists a critical angle of approximately 30˚ above which the seta no longer causes adhesion to the substrate25. This test offered insight into the basic mechanism that would account for the ease of movement and ability to quickly detach their feet in approximately 15 ms, required for rapid locomotion.

1.5 Comparison with Measurements of the Whole Pad and the Single

Spatula

While Autumn’s tests provided direct evidence for the van der Waals mechanism of adhesion, tests had previously been completed on who foot-pad measurements. It has been mentioned previously that Irschick23 reported 20 N of force for two feet. This seemingly low number shows how efficiency plays into the total force generation of the gecko. While the gecko theoretically generates enough force to support its body mass

100-fold, the reverse statement shows that only 1% of the spatulae need to be in contact for this mechanism to work. This is the much more realistic case that would be present in nature during a gecko’s normal everyday movement over rough surfaces. Additionally, in the time since the reporting of this discovery, one research group found a method to take measurements on a single spatula. In 2005, Huber et. al.47, 48 reported tests on force measurements of a single spatula using an atomic for microscope (AFM) cantilever with advanced sample preparation techniques. There resulting force of 10 nN per spatula is approximately four times less than those found in the Autumn experiments outlined above. This discrepancy will be covered after a discussion of how the method of testing affected the two different results.

1.6 Attachment and Detachment

At this point a more detailed discussion of the setae positioning in relation to the surface as well as the mechanism by which a gecko moves it toes to attach and detach the setae is necessary. The gecko’s toes spread from the leg in five directions radially from the center. With each step, the gecko utilizes two of its opposite leg sets, either back right/forward left or back left/forward right, in order to pull inwards towards a the gecko’s center, known as the Y configuration49. With a small downward preloading force and the inward motion to create the parallel force from sliding in order to obtain the 5 µm displacement, the gecko attains the large holding forces associated with each foot.

In terms of detachment, Autumn’s group found that little to no measurable forces were recorded after the setal angle reached 30°15, 50, 51. The gecko toe goes through a digital hyperextension which causes the toes to roll upwards and backwards towards the legs.

Figure 1.4 A. Optical image showing a gecko’s foot attached to a substrate. B Optical image showing a gecko’s foot with the digits hyperextended. C. Schematic illustration of how the hyperextension motion introduces a mechanism by which the setae “roll” off the surface and create large peel angles

In doing so, the toes cause the setae to reach the 30° threshold at which the detachment forces become very small almost to the point of being negligible. The reason this occurs is that during this rolling motion, only a tiny fraction of the setae are being detached at any one time. The “rolling” motion of the toes allows for a modified force model based on peeling tape off a substrate (Figure 1.5).

Figure 1.5 (A.B.C) Hierarchy of the gecko pad. (D.E.F) SEM images of the setae and spatula. (H.I.) schematic illustration of the Peel-zone Model.

1.7 Self-cleaning Mechanism

One may wonder how a foot pad that can attach to almost any surface could possibly remain clean. This self-cleaning ability is due to an energetic imbalance of particulate that may attach to a gecko’s pad. Furthermore, experiments in which this hyperextension has been prevented during gecko movement show that this ability further amplifies the ability to self-clean by almost two-fold due to a dynamic dislodging of the spatulas during the rolling off mechanism52-54.

Without digital hyperextension, self-cleaning occurs with each step taken by the gecko. The reason is that the contact area between the spatulas and the particle are not equal to the forces being generated by the particle and the substrate (wall). Therefore, the particle will prefer to adhere to the wall in preference to the spatula55, 56 as can be seen in

Figure 1.6. This has been confirmed by Hu et al.57, 58 in an experiment in which movement-prohibiting boots were placed on the gecko’s feet to restrict movement

(Figure 1.7). They found that the mechanism of detachment changed from a rolling off peel-zone format to a crack propagation form moving from the proximal end of the toe to the distal end.

Figure 1.6 Schematic illustration of the unbalanced force generation favoring the substrate over the gecko pad, enabling the gecko’s ability to self-clean

Figure 1.7 The test setup used to change the gecko movement from igital hyperextension (DH) to a lift-off mechanism (LM) and change the direction of the crack propegation front. This change showed the effects of dynamic self-cleaning mechanism by Hu et al.

This is opposite to the normal distal rolling associated with digital hyperextension.

The gecko pads cleaned themselves regardless of the rolling motion. However, once they took off the prohibitive boots, they found that the time to achieve the same amount of pad cleansing was almost halved. Their explanation is twofold. First, using the rolling motion, the number of setae decreases as the detachment moves across the dirt particle. So, in effect the particle remains pinned to the substrate by an ever decreasing amount of setae.

In addition, Hu et al.59, 60 suggest that there exists a dynamic component to the mechanism that removes the particulate from the pads. While the spatulas are rolling during digital hyperextension, large inertial forces are generated at the tips during detachment61. In essence, a buildup of elastic energy in the setae and spatulas is suddenly released as they rotate upwards. When combined with the very small contact area, this large force is enough to propel the particulate off of the pad. These mechanisms allow

14 the gecko to generate large friction and adhesive forces to substrates on a large scale while simultaneously removing dirt from the pads during locomotion.

1.8 Non-adhesive unloaded State and Non-self-adherence

Another factor is the unloaded contact area, which has been shown to be approximately 6% of total area62. A side effect of this small contact area is that the gecko’s feet are superhydrophobic when unloaded due to the hydrophobicity of the β- keratin and the structural nature of the setal array. If the surface of the pad were to come into contact with water, the lotus effect would cause an easy washing mechanism to remove debris. The amount of time that a gecko spends in contact with water is unknown however. While mentioning the non-adhesive unloaded state of the gecko pads, another two straightforward properties can be quickly mentioned. First, gecko pads will not self- adhere due to the extremely small areas of contact between the two pads. Second, the spatulas will not come together because the modulus of the β-keratin is high. Essentially it amounts to trying to get two incredibly small springs to stay together when they are pulling apart and the van der Waals forces cannot overcome the desire of the spatulas to separate.

1.9 Substrates and the Environment

Now that the mechanism by which a gecko can adhere to and detach from surfaces has been discussed, it is time to discuss other relevant aspects pertinent to gecko adhesion, i.e. the surface being attached to and the surrounding environment. By utilizing van der

Waals forces, one might think that the mechanism is substrate independent. While geckos have shown the ability to attach to both hydrophilic and hydrophobic surfaces

15 with ease as would be expected, it has been widely shown that geckos cannot transverse a

Teflon surface. A similar environmental consideration has been researched the temperature and humidity effects on the adhesive and frictional capabilities of geckos.

Many studies have proven that the relative humidity and temperature play a large role in the effectiveness of the gecko’s adhesion abilities14, 63, 64. Results revolving around the use of both hydrophobic and hydrophilic surfaces have shown the results to be surface independent, meaning that the humidity and temperature play a role in the gecko itself.

Disregarding the effects of biological factors, β-keratin softens and becomes more pliable with the increase in humidity, thereby increasing the gecko pad’s ability to conform to the surface as a whole. Because the relative humidity is so closely tied to the temperature, most consider this to be the explanation for any variation in temperature testing. This recent development is continuing to be explored to explain different modes of dynamic friction (i.e. friction as a function of shearing) in the gecko pad. At low shearing velocities the friction force of the adhesive pads are very dependent on the material properties that can change with relative humidity. As the velocity increases, the friction force becomes material and humidity independent. At the extreme end of environmental testing are the capabilities of gecko pads to perform underwater on both hydrophobic and hydrophilic surfaces. Tests show that in underwater conditions, gecko pads retain similar characteristics to their dry counterparts when dealing with hydrophobic and weakly hydrophobic (weakly hydrophilic) surfaces. However, when dealing with hydrophilic surfaces, the submerged samples provide for a quarter to a third as much adhesion.

Additionally, when tested on a Teflon (PTFE) surface that traditionally is almost non- adhesive to the gecko, the submerged sample provided for four to five times greater

16 adhesion. Tests such as these show that there is still a great deal to learn about gecko adhesion and may offer insight into the role of the anti-wetting phospholipids mentioned previously that have been recently found27, 60, 65-67.

By utilizing a sophisticated hierarchal structuring, the gecko’s foot can conform to roughness down a nano-scale level. In doing so, the gecko achieves intimate contact with billions of spatulas, where the net force achieves a large overall adhesion and friction capabilities through van der Waals forces68-72. Being a recent discovery, more exact measurements and models are being developed constantly, with the peel-zone model a good current representation of the mechanism. By using the peel-zone model, the biological mechanism of hyperextension by which a gecko achieves the large peel-angles needed to create almost zero detachment forces has been well-documented. On the environmental side of the equation, the substrates and environmental effects are now being solidified. More particularly, the dual temperature/relative humidity effects on the materials are being researched and initial data shows that it increases compliance thereby further increasing the contact of the two surfaces. With the extraordinary abilities that the gecko’s adhesive pads exhibit, creating a synthetic version has become a booming research area. Several properties are advantageous over the typical pressure sensitive adhesives that are so common; anisotropy, self-cleaning abilities, and surface independence to name a few. Fabrication of gecko inspired adhesives with better adhesive properties has attracted more and more interests of scientists in the field of biology and material science.

1.10 Fabrication of gecko inspired adhesives

Using the gecko as inspiration, important principles of fibrilla adhesive mictrostructures are revealed for reliable maneuvering on vertical surfaces28, 59, 73-91.

There are four main structural features which are generally required for the fabrication of a high-performance artificial dry adhesive. i. Fabrication of structures with hierarchy.

A hierarchical structure with muti-scale components (i.e. base, tip) can better mimic gecko fibrilla arrays with compliance and adaptability of the structure against various rough surfaces. The flexible thin speculate head in a hierarchical structure enhances the adhesion by providing a larger contact area as compared to that of a simple pillar. ii. Fabrication of slanted microstructures.

A directional angle of a fibrillar array is another crucial factor that plays a key role in the anisotropic adhesion properties because slanted structure significantly lowers the effective modulus of surface, resulting in a soft, tacky surface even with a rigid material.

In addition, anisotropic adhesion properties with strong attachment yet easy detachment can be obtained with angled structures due to the directionally dependent loading of the dry adhesive. However, as we will demonstrate, it is possible to obtain these anisotropic properties by the clever design of the pillar tips and not requiring tilt in the structures. iii. Fabrication of high aspect ratio microstrucutres.

It is essential to fabricate high aspect ratio nanostructures for fibrilla adhesives.

Structures with high aspect ratios enhance the adhesive force due to higher elastic energy dissipation at pull-off, increased number of pillars at the time of contact, and a decrease of the effective modulus. Fibrilla with larger aspect ratios increase the density of foot hair

18 arrays, which is a necessary condition for artificial microstrucutres to achieve high adhesion performance through a full contact. iv. Fabrication of asymmetric microstrucutres.

Fibrilla arrays with cleverly designed asymmetric microstructures can enhance anisotropic behavior of fibrilla adhesives by creating a shape with larger contact areas in the attachment direction than in the detachment direction. This asymmetric design opens the way for new gecko-like adhesive surfaces to recover the anisotropic tribological properties of gecko adhesive pads without relying on intensive nanofabrication albeit with lower adhesive forces compared to geckos.

Recently, significant developments of chemical, physical and micro-fabrication technologies for fabricating gecko-inspired artificial dry adhesives have been proposed to prompt the progress of artificial dry adhesives28, 62, 92-106. These technologies can be further subdivided into two main streams: polymer-based dry adhesives and carbon nanotube (CNT)-based dry adhesives. These two kinds of adhesives have been developed independently by utilizing different fabrication principles. In general, the polymer-based artificial fibrilla array has an adjustable elastic modulus and moderate adhesion properties as well as a convenient vulcanization process and moderate mechanical properties82, 84, 90,

107-113. It offers a simple and scalable approach to fabricating gecko-mimicking nanohairs with tailored geometry (angle, radius, height, shape of tip and hierarchy) and tunable material properties (modulus, surface energy, etc.) in a fast and cost-effective manner.

However, the adhesion performance of the polymer-based artificial dry adhesive is relatively low compared to those of CNT-based artificial arrays.

On the other hand, the CNT-based dry adhesive has a rather high adhesion performance and strong mechanical properties, since the CNTs have superior structural features such as high aspect ratios, extremely small radii and high moduli28, 100, 114, 115.

However this kind of fibrillar array also has some weaknesses: a high preload is always required for high adhesion force, and the fibrillar array’s detachment is rather difficult.

Due to different process characteristics and materials properties used in the fabrication, the polymer-based and the CNT-based dry adhesives demonstrate different adhesion capability and parameters, which are summarized in table 1.1.

Table 1.Compilation of adhesion capability and parameters of synthetic dry adhesives8, 28, 48, 68, 74, 75, 77, 80-82, 84, 86, 88, 90, 98, 99, 101, 102, 108, 110, 112, 113, 116-134 Hard Polymer Arrays Pull- Shear Area Normal Effective off Product Material Modulus sq. (N) (N) preload Modulus mm. (N/sq.cm) (kPa) Natural Gecko, nil Autumn et (<0.01) beta keratin 2 GPa 1 10 100 100 al Nature (shear 2000 engaged) Gecko Lamellar prep, Lee et al beta keratin 1.5 GPa ~0 0.27 0.5 <0.05 100 JRS Interface 2008 Davies et polyimide al. 2 GPa 0.53 ~0 196 0.25 NM Int. J. Adh. on SEM tape Adh. 2008 Geim polyimide 3000 Nature on Scotch 3 Gpa 3 NM 100 50 (w/o Materials tape buckling) 2003 Lee et al. JRS polypropylene 1 GPa ~0 4 N 240 0.05 ~200 Interface 2008 Lee et al. polypropylene 1 GPa ~0 9N 200 <0.1 ~200

Table 1. (continued)

Schubert et al. J. Adhesion polypropylene 1 GPa ~0 1 N 1000 0.05 ~200 Sci. Tech 2007 Kustandi et al. parylene 2.8 GPa 0.7 NM 100 1 NM Adv. Funct. Mat. 2007

Jeong et al. PMMA 2.8 GPa ~0 3 100 <1 NM Coll. Surf. 2008 Carbon Nanotube Arrays Pull- Shear Area Normal Effective off Product Material Modulus sq. (N) (N) preload Modulus mm. (N/sq.cm) (kPa) carbon Ge et al nanotube 1000 0.8 1~6 16 25-50 ~200 on Scotch GPa PNAS 2007 tape carbon Qu and Dai nanotube 1000 Advanced ~5 ~2.5 16 125 NM GPa Materials on Si 2007

carbon Qu et al. nanotube 1000 ~3 ~16 16 125 NM GPa Science on Si Oct. 2008 multiwall Maeno and carbon 1000 Nakayama NM 45 100 50 NM nanotube on GPa APL 2009 polypropylene carbon Zhao et al nanotube 1000 0.5 0.6 8 >500 ~200 J. Vac. Sci. GPa on silicon Tech. 2006 Soft Polymer Fiber Arrays Pull- Shear Area Normal Effective off Product Material Modulus sq. (N) (N) preload Modulus mm. (N/sq.cm) (kPa)

Table 1. (continued)

Sameoto PDMS and Menon 1.8 MPa 9mN NM 1 2 ~400 JMM 2009 (Sylgard 184)

Yoon et al. polyurthane 19.8 NM 31 100 0.3 NM Nano acrylate + Pt MPa Today 2009

Jeong et al. polyurethane 19.8 15 78 300 0.3 ~26 acrylate MPa PNAS 2009 Kim and Sitti Applied polyurethane 3 MPa 0.07 NM 0.4 12 ~300 Physics Letters 2006 Murphy, Aksak, polyurethane 3 MPa 5 10 100 NM ~20 Sitti, Small 2008

Gorb et al

JRS PVS 3 MPa 0.4 NM 7 2 ~300 Interface, 2006

Parness et 1.75 al. PDMS 0.5 1.7 100 0.25 ~50 MPa JRSI 2009

Santos et al. polyurethane 0.3 MPa 1 ~1 390 0.25 ~100 J. Adh. Sci. Tech. 2007 Sitti and Fearing PDMS 0.5 MPa 0.003 NM 100 0.025 ~100 IEEE ICRA 2003 Davies et al. PDMS 0.6 MPa 1.2 N NM 123 0.25 ~100 Int. J. Adh. Adh. 2008 Ge et al Scotch brand 0.1 Mpa NM 6 16 NM <100 PNAS 2007 tape

Shan et al PDMS 2.5 MPa 2.01 NM 100 NM

1.11 CNT-based dry adhesives

Having an extraordinary high aspect ratio and exceptional mechanical strength, carbon nanotubes have shown great potential for dry adhesive applications. With higher aspect ratios enhancing the adhesion, CNT structures are one of the most suitable materials for gecko-inspired dry adhesives as they exhibit the more benefits of non self- adhesion, adjustable mechanical properties, strong adhesion performance, and large-scale production.

Several research groups have proposed their work for fabrication of high density artificial dry adhesives with CNT arrays. In general, CNT-based dry adhesives have higher strength than polymer-based adhesives due to outstanding structural properties based on the extremely high aspect ratios (>104, diameter around 10nm and height over

100µm). Recently, Qu and coworkers135, 136 employed a method of chemical vapor deposition to prepare gecko-inspired adhesive tape. It was made up of high-density CNT fibrilla, each of which has a diameter of 50-500 µm and length 200-500 µm. Each square centimeter of this tape could support 36 N friciton force, which is almost four times that of the gecko.

1.12 Polymer-based dry adhesives

The polymer materials and geometries of artificial dry adhesives synthesized by micro/nanocasting are different in almost every research group. The majority of molded dry adhesives have been prepared using only a few basic materials, primarily polydimethylsiloxane (PDMS) and polyurethanes (PU). PDMS-based dry adhesives are thermally cured directly from molds of silicon or photoresist created through single or double mask lithography processes. Due to its relatively simple synthesis process,

23 complicated PDMS-based fibrilla structures with the presence of a cap for significant normal adhesion have been created by many research groups. However, in many instances, the desire for directional behavior and rough surface compatibility outweighed the need for high normal adhesion. Additionally polyurethane structures have been created by other groups to synthesize mushroom-shape fibers, hierarchical fibrilla arrays, and directional structures through use of dipping techniques.

The most commonly used PDMS (Sylgard 184 for Dow Corning) and polyurethane

(ST-1060 from BJB Company) have similar stiffness despite their different physical properties and the curing processes78. Their stiffness (E ~ 2-3 MPa) is about three orders of magnitude lower than that of the gecko keratin. This low stiffness limits the maximum aspect ratio of individual fibers and makes engagement to larger surfaces more difficult33,

44, 137, 138. Several groups have reported using more rigid and brittle polyurethane acrylate

(PUA) to fabricate polymer-based dry adhesives137. The Young’s modulus of PUA is about 19.8 MPa, approximately ten times stiffer than Sylgard 184 or ST-1060. This material has been used successfully for nanoscale replication of high aspect ratio fibers and has demonstrated very good shear strength with the addition of a small cap on an angled fibrillar design.

Another attractive material for polymer-based molding technologies for artificial dry adhesive is polyvinylsiloxane139. Based on a commercial manufacturing process and using mushroom shaped structures as tips, a high performance bioinspired adhesive has been fabricated which can be applied under water and in oil solutions has been reported.

In general, the trends for molded dry adhesives are very good, with improved processes being developed for higher strength adhesives, better yields, and better

24 adhesion performance. A convergence of stiffer materials, combined with angles fibers and offset overhanging caps may be the best option to achieve anisotropic adhesive behavior for using on rough or dirty surfaces, but fabrication will be the greatest challenge in the foreseeable future as features sizes are reduced.

1.13 Summary and future work

Synthetic dry adhesives with superior, smart adhesion properties have improved greatly in recent years with the aid of existing nanofabrication techniques. Compared to earlier dry adhesives, recently developed dry adhesives can equal or even exceed the shear stress of a real gecko (~54 N cm2)140. Other crucial properties such as anisotropic behavior and rough surface compliance have also been demonstrated in recent publications, and a consensus on the basic rules for designing an effective biomimetic dry adhesive have been mostly reached. In order to achieve optimized adhesion performance with the dry adhesives, however, several challenges still remain. The primary challenges for researchers is to reduce structural features to the nanoscale regime for better adhesion force and to better adapt to rough surfaces. As is expected, the selection of a suitable polymeric material is essential when developing the properties for synthetic dry adhesives. Polymer-CNT composites combining the processibility of polymers and the superior mechanical strength of CNTs could be another possible solution. In this case, complicated processes for the CNT growth, small patterning areas and a high preload are the major concerns of the CNT-based dry adhesives. A secondary priority for researchers in the field of artificial dry adhesives should be the fabrication of multi-scale hierarchical structures. Although gecko-inspired hierarchical hairs have been reported recently, more

25 investigations on the effect of the hierarchical structures on adhesion capability against surfaces with varying degree of roughness are required112, 141.

Despite these challenges, the performance of the artificial dry adhesives has been improved greatly in recent years74, 85, 88, 92, 114, 126, 140, 142-147. There is a great reason for optimism in the field of synthetic dry adhesives to fulfill their promise in the near future as industrially and commercially relevant adhesive products for novel and high-end applications. Stikybot61, which utilizes synthetic setal arrays for wall climbing, is designed to achieve controllable adhesion through hierarchical structure compliance, anisotropic friction, and adhesion as well as distributed force control similar to that of the natural gecko. Manufacturers often use glue, wedding, or soldering to form temporary or weak permanent bonds. By applying the characteristics of gecko inspired adhesives, manufacturers could expect to avoid these costly methods and use a more convenient method which would be reusable and leave no residue.

In conclusion, theoretical studies are contributing to the understanding of the fundamental processes underlying adhesion, friction, and peeling. In doing so, these studies provide biological inspiration for the design of novel adhesives and climbing robots148-154.

1.14 Aims and Objectives

In this current study, we aim to develop polymer-based synthetic dry adhesives that have similar properties as the natural gecko adhesive system. Specifically, we plan to incorporate the anisotropic adhesion and friction properties (i.e., strong adhesion and friction forces required for gripping, and small adhesion friction required for detachment).

Characterizations of the tribological properties and adaptability on surfaces with varying roughness of our synthetic dry adhesive will also be investigated.

In the first part of our study, we hypothesize that our array of compliant pillars containing a thinner regular pad-like structure at the tips will provide a better contact to surfaces, thus resulting in larger adhesion and friction forces. We also expect our gecko- like adhesives to provides larger adhesion and friction forces as the tilt angle of the structures decreases, consistent with the peel zone model of tape peeling.

In Chapter 3, we will introduce a new generation of polyurethane-based gecko-like structures consisting of two levels of hierarchy, an improvement from the first generation of our synthetic dry adhesive. By cleverly engineering the pillars and the location of the tips, we hypothesize that we will be able to not only recover the adhesion properties of natural geckos but we also expect that these structures will provide even larger adhesion and friction forces, and larger anisotropy even without the need of tilted structures.

In chapter 4, the effect of surface roughness on gecko-inspired adhesive properties will be investigated. By preparing surfaces with different roughness and measuring the adhesion and friction force between gecko-inspired adhesives and rough surfaces, performance of our synthetic dry adhesives will be clarified. We hypothesize that there is a critical surface roughness, which is on the order of the lengthscale of the pillar tips, whereby the adhesion and friction properties will drop significantly due to the inability of the pillar tips to make good contact to the rough surface.

Our long term objective is to improve adhesion of gecko-inspired adhesive to a variety of surfaces through fundamental studies and simplify the processing scheme to make the technology commercially viable.

Chapter 2

The Design and Fabrication of Gecko-inspired Adhesives

2.1 Introduction

There has been significant interest in developing dry adhesives mimicking the gecko adhesive system, which offers several advantages compared to conventional pressure sensitive adhesives. Specifically, gecko adhesive pads have anisotropic adhesion properties: the adhesive pads (spatulae) stick strongly when sheared in one direction but are non-adherent when sheared in the opposite direction. This anisotropy property is attributed to the complex topography of the array of fine tilted and curved columnar structures (setae) that bear the spatulae. In this chapter, we present an easy, scalable method, relying on conventional and unconventional techniques, to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provides anisotropic adhesion properties. We measured the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function. Consistent with the Peel Zone model, samples with lower tilt angles yielded larger adhesion forces. Contact mechanics of the synthetic array were highly anisotropic, consistent with the frictional adhesion model and

28 gecko-like. When a 60° tilt sample was actuated in the gripping direction, a static adhesion force of ~1.4 N/cm2 and a static friction force of ~5.4 N/cm2 were obtained. By contrast, when the dry adhesive was actuated in the releasing direction, we measured an initial repulsive normal force and negligible friction.

Figure 2.1 SEM images of (A) the side view of a Tokay gecko setal array, (B) a magnified view of the spatula pads, (C) the gecko-inspired adhesive, (D) a magnified view of the pad-like structures.

The superior adhesive properties of geckos, which allow them to repeatedly attach strongly and detach easily from a variety of surfaces of varying roughness, have inspired the fabrication of synthetic dry adhesives. However, an adhesive material that exhibits all the characteristics of gecko adhesion still remains elusive. These superior properties are attributed to the complex, hierarchical topography of fine tilted and curved columnar structures (setae; Fig 2.1a) each bearing multiple trapezoidal-shaped terminal pads

(spatula; Fig. 2.1b) that make up the adhesive arrays on the gecko toes. Natural gecko

29 setal array show strong adhesion/friction forces originating from van der Waals interactions when sheared in the “gripping direction” towards the axis of tilt, but show no adhesion and smaller friction when sheared in the opposite “releasing direction”. This novel mechanically-controllable adhesion anisotropy allows for easy and rapid locomotion on vertical and inverted surfaces.

Synthetic gecko adhesives, based on a variety of materials and fabrication schemes, have been developed to mimic gecko adhesion; e.g., the Dhinojwala group used chemical vapor deposition to create gecko-inspired adhesive tape based on high-density carbon nanotubes155, the Sitti group fabricated slanted polyurethane-based microstructures which showed anisotropic adhesion by soft lithography approaches156, the del Campo group used a combination of multi-step photolithography and soft lithography to generate hierarchical structures157, Jeong et al. fabricated micro/nanoscale hierarchical polymeric hairs using a two step UV-assisted capillary molding technique106, the Cutkosky group used microfabrication techniques to develop single level wedge-shaped dry adhesives with anisotropic properties158, and the Israelachvili group used photolithography and molding techniques to create microflaps composed of polydimethylsiloxane which also showed anisotropic tribological properties30. Although anisotropic soft polymer structures

(elastic modulus E~0.1-1 MPa) had been fabricated previously, in 2008, the Fearing group was the first to fabricate anisotropic gecko-like dry adhesives from a hard polymer, polypropylene (E~1.4 GPa)159. In addition, the Turner group reported that improved adhesion could be obtained with the integration of nano and microstructures due to enhanced structural compliance160. Although a major focus in fabricating gecko-like adhesives has been on the geometrical design of the structures, recent work has shown

30 that chemistry (i.e. surface functionality and presence of lipids) should also be considered in their future designs. The above synthetic dry adhesives either do not show anisotropic adhesion or rely on multi-step, expensive fabrication schemes, which are not easily scalable for hierarchical structures.

We present an easy approach to fabricate polymer-based gecko adhesives, using conventional as well as unconventional techniques, to incorporate tilt into the structures, which provides anisotropic adhesion and friction properties. A combination of microfabrication and molding techniques is used to fabricate tilted 2-level polyurethane

(PU) hierarchical structures, which mimic the setal shaft and the spatula pad (Fig. 2.1c,

2.1d) of geckos. Adhesion and friction forces are measured, using a universal materials tester, along the tilt direction (i.e. “gripping” direction) and against the tilt direction (i.e.

“releasing” direction). The gecko-inspired adhesives show anisotropic adhesion and friction forces depending upon the shear direction; i.e., generating strong adhesion and friction when sheared (or actuated) in the gripping direction and repulsive normal forces and weak friction when sheared in the releasing direction. The magnitude of the adhesion force is also consistent with a nanoscale tape-peeling model (i.e., Peel zone model); the adhesion force increases as the tilt angle decreases.

2.2 Experimental

2.2.1 Fabrication of patterned silicon mold

Silicon master wafers were fabricated using standard microfabrication techniques.

Briefly, four-inch silicon wafers (Test grade, University Wafers) were cleaned in an

31 oxygen plasma, after which a 600 nm oxide layer was grown using plasma-enhanced chemical vapor deposition. Next, positive photoresist was spun onto the wafers (Shipley

Figure 2.2 SEM images of silicon master wafers with designed fibrillar array.(A) the top view of patterned silicon mold, (B) the side view of patterned silicon mold

1813), patterned with the gecko pads, and developed. The oxide layer was then opened by reactive ion etching with CF4. The patterned oxide layer formed the bottom of the two-layer etch mask. The remaining photoresist was stripped off, and a new layer of

Shipley 1818 was spun onto the wafers. This new layer was patterned with the gecko base and developed. The photoresist layer formed the top of the two-layer etch mask.

Once the two-layer etch mask was in place, the hierarchical structure itself was formed using deep reactive ion etching. First, the wafer was etched to a depth of 60 microns. The

32 photoresist was then stripped from the wafer, revealing the bottom layer (oxide) etch mask. The wafer was then etched an additional 20 microns to form the final structure.

2.2.2 Fabrication of polyurethane fibrillar dry adhesive

Figure 2.3 Schematic illustration of the different steps involved in fabricating the gecko-inspired adhesives.

Figure 2.3 shows a schematic of the different steps in the fabrication of a 2-level hierarchical polyurethane-based (ST-1060 BJB Enterprise, Inc., Tustin, CA, USA) dry adhesive. In the first step, the silicon master wafer was used as a mold to create an inverse PDMS replica. Sylgard 184 PDMS (Dow Corning, Midland, MI) was mixed in a

10:1 (pre-polymer to cross-linker) ratio. After removal of air bubbles formed during mixing, the viscous liquid was poured onto the silicon master and cross-linked at 75˚C for 40 min. The partially cured PDMS replica was then laterally sheared in a home-built

33 shearing device by applying a predetermined shear distance to achieve the desired tilt angle (60˚-90˚) (figure 2.4). The PDMS replica was then fully cured in its sheared state at

75˚C for 24 hours resulting in the PDMS replica mold with tilted cylindrical holes and tips. Polyurethane was then poured onto the PDMS replica and allowed to cure at 75˚C for 72 hours. The final polyurethane-based dry adhesive was finally peeled off from the

PDMS replica (which could be re-used to create additional samples).

Figure 2.4 Optical images of PDMS mold.(A) the side view of tilt PDMS mold, (B) the side view of vertical PDMS mold. (C) the top view PDMS mold

Figure 2.5 Optical images of gecko-inspired adhesives composed of PU with pillars at (A) 90°, (B) 80°, (C) 70°, and (D) 60° tilt angles.

2.2.3 Adhesion and friction measurements

Adhesion and friction measurements were performed on a universal materials tester

(CETR Enterprise, Inc., Campbell, CA, USA). (Figure 2.6) Specimens with tilted fibrillar arrays were fixed on a glass slide using double-sided tape. A curve borosilicate surface, used as the probe (radius of curvature=3 cm, Anchor Optics, Barrington, NJ), attached to a force sensor (FL, CETR, Campbell, CA) with a cantilever (spring constant k=520 N/m), was then brought into contact with the bottom surface at a predetermined preload.

During a typical experiment, the probe was brought into contact with the sample with a predetermined preload of 2 mN (contact area ~ 0.78mm2). The probe was then dragged

35 in gripping direction (or releasing direction) for 1mm at a velocity of 10 μm/s. After dragging, the probe was pulled off at 90˚ from the sample. Four samples with tilt angles ranging from 90˚ (no tilt) to 60˚ with 10˚ tilt increments (Figure 2.5) were tested. Prior to each experiment, the probe was cleaned with ethanol and the samples were tested without further cleaning. A camera was used to digitally record a side view of the experiments and extract the dynamic (i.e., while shearing) tilt angles.

Figure 2.6 Setup of Adhesion and friction measurements

Figure 2.7 Schematic illustration of the tribological measurements of the gecko- inspired adhesives tested in either the gripping or releasing direction.

2.3 Results and Discussion

2.3.1 Design of dry adhesives

The adhesive pads of the well studied tokay gecko consist of hierarchical structures

(a dense array of cylindrical setal shafts grouped in tetrads in a square lattice (~100 µm in length and ~5 µm in diameter, spaced ~18 µm apart) which split into hundreds of spatula shafts each bearing multiple spatula pads (~200 nm in width and 5-10 nm in thickness).

Although the gecko pads consist of a relatively stiff material, -keratin (~1-4 GPa modulus), the hierarchical structures provide compliance, which aids in maximizing the true contact area and adhesion to rough surfaces. In addition, the setal shafts are tilted by

~45˚, which provides anisotropic tribological properties depending on the direction in which the adhesive surface is sheared.

Inspired by the gecko adhesive system, we incorporated two hierarchical levels in our synthetic gecko structures. The first level consisted of an array of cylindrical pillars

(60 µm in length, 20 µm in diameter), which provided an overall compliance to the dry adhesive mimicking the setal shaft. The second level was a thinner pad-like structure (20

µm in width, 7 µm in thickness) over each pillar, which provided the local compliance and maximized contact with other surfaces for adhesion. The cylindrical pillars were organized in a rectangular lattice (40 µm center-to-center distance in the tilt direction and

30 µm center-to-center distance along the other in-plane axis) to minimize contact with the neighboring pillars when tilted. The synthetic gecko adhesives were fabricated with polyurethane based on the work of Murphy et al. Different tilt angles were incorporated into our structures as described in the experimental section to provide the anisotropic tribological properties upon shearing.

2.3.2 Characterization of tribological properties

Figure 2.7 shows a schematic of the stages encountered when shearing the tilted samples (with an initial pillar tilt angle θ) either in the gripping or releasing direction.

Each experiment began with applying a 2 mN preload (sufficient to deform the tips but not the cylindrical pillars to a large extent as determined optically). When shearing in the gripping direction, an initial period of static adhesion and friction (i.e., the thin pads remain adhered to the probe) was achieved during which the adhesion force caused the probe to move closer to the sample (by a distance Δzs), and the pillar tilt angle decreased to θs from the initial tilt angle θ. At a critical shear force, the pads began to slide resulting in dynamic adhesion F^,d and friction forces F||,d , which were lower than their static counterparts, while the tilt angle increased slightly to θd. By contrast, in the

38 releasing direction, the sample initially exhibited an intermediate stage in which the pads change their orientation, resulting in a repulsive (i.e., negative adhesion) force pushing

Figure 2.8 Dynamic measurements of the (A) adhesion forces, and (B) friction forces generated by a gecko-inspired adhesive sample with 70° tilt when sheared in the gripping and releasing direction. The drag velocity is 10 μm/s.

the probe by a distance Δzi. Continued shearing past the intermediate stage resulted in the cylindrical pillars and pads re-orienting themselves to the releasing direction while still providing dynamic adhesion and friction forces.

The different stages involved in the adhesion and friction forces in the gripping and releasing direction were apparent in the tribological data. Figure 2.8a shows the adhesion force as a result of shearing a polyurethane-based dry adhesive with cylindrical pillars tilted at 70˚ (Figure 2.5c). In the gripping direction, a maximum static adhesion force

F^,s of ~5 mN was obtained and the dynamic adhesion force F^,d dropped to ~2 mN

/ during sliding. In the releasing direction, the adhesion force F^,i initially dropped ~-3mN

/ (i.e., repulsive force) before increasing to a steady dynamic adhesion F^,d of ~2 mN.

Figure 2.8b shows the corresponding friction forces from the same surface. Before

sliding began, the static friction F||,s force in the gripping direction was ~10 mN

/ compared to F||,s ~6 mN in the releasing direction. Therefore, the dry adhesive provides a strong adhesion and friction (before sliding) when sheared in the gripping direction, and provides an initial intermediate repulsive force and low friction (before entering the static regime).

The adhesion and friction forces (both static and dynamic) in the gripping direction as a function of the tilt angle are summarized in Figure 2.9. Both, the adhesion and friction forces, showed an increase as the tilt angle was decreased with the 60˚ tilt sample showing a larger increase in forces compared to the other samples. In all cases, the static forces were larger than the dynamic forces.

Figure 2.9 Static and dynamic adhesion and friction forces generated by the gecko- inspired adhesives with different tilt angles. 2.3.3 Tribological test of glass probe on flat PU surface

To provide a comparison to the structural PU samples, flat PU was also tested by the tribometer as a control sample. Figure 2.10 shows both friction force and adhesion force on flat PU surface. A relatively large friction force is detected due to the larger contact area generated by flat surface. The periodical pulsatile profile of the friction is due to the sudden release of the probe from the sample surface after reaching the maximium static friction. On the other hand, normal force is below zero during the entire test, meaning that no adhesion generated between the probe and flat surface. This is because the flat

(non-structural) sample does not contain a less dense layer (like the pillar arrays of the adhesive samples) to reduce the effective modulus.

Figure 2.10 Adhesion and friction forces test of glass probe on flat PU surface

2.3.4 Comparison of gecko-inspired adhesive prepared by polyurethane of different stiffness

Tribological tests were carried out on 70˚ gecko-inspired dry adhesive samples composed of three kinds of polyurethane with varying stiffness using the previous method to compare their adhesion and friction forces. Figure 2.11 shows the plot of the measured forces of three materials. The polyurethane 1060, which was least stiff, provided the largest forces of both adhesion and friction. The forces were two times greater than those from stiffest sample, polyurethane 1085. The reason that softer materials provide better adhesive properties is because of their lower effective modulus.

However, it is not true that a softer material always gives better adhesion. An extremely soft material will lower the fatigue resistance and make the structure easy to deform. In some cases, a fibrillar array cannot maintain its shape due the low modulus of the

42 material. Thus choosing materials with proper stiffness is an important issue for synthesis of gecko-inspired dry adhesives.

Figure 2.11 Adhesion and friction forces test of glass probe on PU samples with different stiffness

2.3.5 Comparison of gecko-inspired adhesive to gecko setal array

The adhesion and friction forces generated by the 60˚ tilt sample were compared to the tribological properties of a Tokay gecko setal array when sheared in the gripping direction (Figure 2.12). The forces were normalized to their corresponding maximum magnitudes to facilitate the comparison. A remarkable similarity was found in the tribological properties of our gecko-inspired adhesive to a Tokay gecko setal array. The dotted line (having a slope of -2 in dimensional form) is consistent with the frictional adhesion model proposed by Autumn et al. for shearing in the adhesive or gripping

43 direction, which predicts the ratio of the friction force to the adhesion force ought to be greater than 1.7 (based on a detachment angle of 30˚ for the Tokay gecko). A key difference in our gecko-inspired adhesive compared to a gecko setal array is the single stick-slip event which occurs in our samples but absent in the case of the gecko setal array. We attribute this phenomenon to the fact that a gecko setal array consists of millions of individual contact points, as opposed to the relatively fewer contact points in our samples (estimated number of contacts ~650 for our experimental technique). In the case of a gecko setal array, the resulting adhesion and friction forces obtained during shearing originate from the temporal fraction of the millions of spatula that are adhered to the surface. This phenomenon will be further explored by the Autumn group in future studies.

Figure 2.12 Normalized normal force vs friction force plot for a Tokay gecko setal array (black dots) and for the gecko-inspired adhesive with 70° tilt. The forces are normalized to their maximum respective magnitudes.

2.3.6 Anisotropic tribological properties of gecko-inspired adhesive

Figure 2.13 Static adhesion (grey curve) and friction forces (black curve) generated by the gecko-inspired adhesive during 8 cycles, each consisting of shearing in the gripping direction followed by the releasing direction.

Figure 2.13 shows the adhesion and friction forces generated by the gecko-inspired adhesive during 8 shearing cycles. Each cycle consisted of actuating the sample in the gripping direction until maximum static adhesion and friction were achieved (i.e. static adhesion regime) followed by actuating the sample in the releasing direction until the adhesion force was repulsive. The gradual decrease in the friction force generated in the gripping direction is attributed to material transfer (i.e. polyurethane) from the gecko- inspired adhesive to the glass probe with repeated use. We found that if the probe was

45 cleaned with ethanol, then we recovered maximum adhesion and friction forces. The adhesion force does not change significantly over the 8 cycles.

2.3.7 Comparison to Peel Zone model

The Peel-Zone Model49 utilizes a force balance around the setal front of the detachment zone. In this zone three forces are acting: a peel force acting in the direction of the peel, the adhesive force perpendicular to the surface, and the friction force acting parallel to the surface. The peel force is generated by the rolling action of the gecko toe and can be described mathematically. A model first introduced by Rivlin161 and later modified by Kendall to include the elastic energy of the tape substrate is utilized as a basis to derive the Peel zone Model123. The model aptly describes the detachment of elastically backed tapes to rigid surfaces, where the gecko detachment can be viewed as the elastic tape in this model (Figure 2.14).

Figure 2.14 Graphic representation of the peel-zone model cases

The adhesion data generated from the samples with different tilt angles was compared to the Peel zone Model, which was developed to model the adhesion force of pressure sensitive tapes as a function of the applied pulling force. Applying the Peel Zone model,

F 2 ,s  s , b  1 coss 

where F^,s is the static adhesion force, b is the total width of the peeling elements, γ is

the adhesion energy, and qs is the static peel angle. To our system, the model predicts that the adhesion force ought to increase as the peel angle is decreased. In our experiment, the peel angle is equivalent to the static tilt angle of the cylindrical pillars and pads during the static adhesion regime of our measurements in the gripping direction.

A camera was used to capture the side view of the samples while performing the tribological experiments and to extract the static tilt angle (Table 2.1) for each sample.

For example, the tilt angle of the initial 70˚ tilt sample reduced from θ=70˚ to θs=29˚ at the point of maximum static adhesion force (peak in adhesion force in Figure 2.7a) during the static regime of the adhesion measurement. The difference between this model and the Kendall model is that the increase in the length of the peel zone, S, is accounted for in this model, and shows that the forces generated by the Kendall model will always be too large based on the angle-dependent multiplier.

In relation to the actual gecko, this model takes into account the keratin’s large elastic modulus and typically small peel angles. The scale of the individual setae is particularly difficult to study, however, meaning that modeling the individual setae multiplies into large errors. This experiment on a single spatula has been conducted by

Huber et al.48 He found the maximum pull-off force to be one quarter of that found by

Autumn, or 10 nN per spatula. However, based on the overall friction and well documented number of spatula per seta one can deduce that this number is too low. This particular experiment is utilized because the slight change in methods between the two experiments offers a glimpse into the nanoscale mechanism of peel-off. Huber’s experiment measured the peel off force by pulling perpendicular to the surface, similar to

o the common T-peel test. This is analogous to keeping the peel angle ( ) constant at 90 .

Autumn’s test utilized a single seta utilizing shearing as well as perpendicular forces.

Now that the mechanism of peel-off has been explained, it is more clearly seen how the gecko can accomplish this biologically. By bending its toes away from the plane of contact, digital hyperextension, the spatulas are exposed to larger peel angles until the threshold is met and the adhesive forces per area drop to almost zero. This same hyperextension brings us to the next very important quality of a gecko’s movement that keeps it functional.

Table 2.1 Initial (or as fabricated) tilt angle of the pillars compared to their static tilt angles during shearing in the gripping direction.

Sample  s 1 90 70(1) 2 80(1) 43(2) 3 70(2) 29(2) 4 60(3) 12(4)

Figure 2.15 Static adhesion force generated by the gecko-inspired adhesives as a function of tilt angle. The solid line is a fit of the Peel Zone model.

Figure 2.15 shows a fit of the Peel Zone model to our data after accounting for the change in the tilt angle during the static regime of the adhesion force measurement. A good agreement was found between our data and the Peel Zone model inferring that our dry adhesive adhere to surfaces through a similar mechanism to adhesives tapes except that in our case, the adhesion energy is a result of van der Waals forces as opposed to tacky filaments found in adhesive tapes.

2.4 Conclusion

A novel, easy and scalable fabrication scheme was developed to incorporate tilt into polymer-based dry adhesives. A series of dry adhesives with different tilt angles were fabricated and the adhesion and friction forces were characterized using a universal materials tester. The samples with tilt angles θ<90˚ showed anisotropic adhesion and

49 friction data with the magnitude of the static adhesion and friction forces was increased as the tilt angle was decreased. The ratio of the friction force to the adhesion force of our samples when actuated in the gripping direction was in agreement with the frictional adhesion model and similar to the value obtained when shearing setal arrays of the tokay gecko in the gripping direction. The static adhesion data also showed good agreement to the Peel zone model for tape peeling after accounting for the decrease in the tilt angle of the structures during the static regime of the measurement.

Chapter 3

Biomimetic switchable adhesive with prismatic pillar arrays

3.1 Introduction

In this chapter, our second generation of dry adhesives inspired by geckos is introduced with unique anisotropic frictional-adhesive capabilities. These capabilities originate from complex hierarchical structures and just as importantly, the anisotropic articulation of the structures. By cleverly engineering asymmetric polymeric microstructures, we can fabricate a reusable and switchable gecko-like adhesive yielding

2 2 steady high adhesive ( ≈ 1.25 N/cm ) and friction ( ≈ 2.8 N/cm ) forces when actuated for the “gripping direction”. These same structures release easily with minimal

2 2 adhesion ( ≈ 0.34 N/cm ) and friction ( ≈ 0.38 N/cm ) forces during detachment in the “releasing direction”, these results are shown over multiple attachment/detachment cycles, with a relative small normal preload of 0.16 N/cm2 to initiate the adhesion. We also demonstrate how these adhesives can be used to reversibly suspend weights from vertical surfaces (e.g., walls) and, for the first time to our knowledge, horizontal surfaces

(e.g., ceilings) by simultaneously and judiciously activating anisotropic friction and

51 adhesion forces. This novel design opens the way for new gecko-like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication.

The complex anisotropic and hierarchical structures of the gecko provide a compliant surface with an effective modulus of ~100 kPa although the setal structures are composed of relatively hard β-keratin (~1-2 GPa). The resulting compliance allows for the thin terminal structures of the setae and spatula pads to come into intimate contact with opposing surfaces, which maximizes the weakly attractive, short-ranged van der

Waals interactions required for high adhesion (~1 N/cm2) and friction (~10 N/cm2) forces.

The anisotropic adhesion of gecko pads originates from the fact that the setae are tilted at an angle of ~45°. Several gecko-like adhesive materials have been developed over the last decade, yet a true mimic remains elusive because of limitations in the current surface nanofabrication techniques as well as limitations involved in incorporating gecko-like adhesives into devices that offer proper articulation. In 2003, Geim et al. were the first to microfabricate a gecko-like material out of a soft polymer, although the latter did not possess anisotropic adhesive properties and required relatively large preloads (i.e., the force applied after contact) to activate the adhesion. New developments in the fabrication of gecko-like adhesives included the addition of tilt to the structures, using carbon nanotubes, introducing angled mushroom tips, and functionalizing the gecko-like surface with mussel adhesive proteins. Recently, our group developed a general approach to easily incorporate a desired tilt angle into gecko-like fibrillar structures, and the resulting gecko-like surface was reminiscent of the tribological properties of gecko pads.

This chapter introduces a new generation of polyurethane-based gecko-like structures

(hereafter referred to as “prismatic” pillars), consisting of 2 levels of hierarchy (Fig. 3.1);

Figure 3.1 Gecko-inspired switchable adhesive. SEM image of a gecko-inspired adhesive consisting of tilted prismatic pillars with a first level of triangular prism structures and a second level of thinner rectangular adhesion pads biased towards one of the flat edges. The inset is a schematic illustration of the terminal end of a prismatic pillar.

a triangular prism base (30 µm x 25 µm x 25 µm, and 80 µm high) terminated by a rectangular tip (20 µm x 20 µm x 7 µm), arranged in a rectangular lattice (30 µm center- to-center in the tilt (x-axis) direction, and 37 µm center-to-center along the in-plane or y- axis). Through proper articulation (i.e., to activate either gripping or releasing) of the prismatic gecko-inspired adhesive, we obtain anisotropic frictional-adhesive properties, which allow for reversible attachment and detachment over multiple cycles.

3.2 Experimental

3.2.1 Fabrication of patterned silicon mold

Silicon master wafers were fabricated using standard microfabrication techniques.

Four inch silicon wafers (test grade, University Wafers) were cleaned in an oxygen

Figure 3.2 Scanning electron microscope image of patterned silicon mold with desired triangular “prismatic” pillars on the base and rectangular pad on the top.

plasma, after which a 600 nm oxide layer was grown using plasma-enhanced chemical vapor deposition. Next, positive photoresist (Shipley 1813, MicroChem Corp.) was spun onto the wafers, patterned with rectangular structures mimicking the gecko spatulae, and developed. The oxide layer was then etched by reactive ion etching with CF4. The patterned oxide layer formed the bottom of the two-layer etch mask. The remaining

54 photoresist was stripped off, and a new layer of photoresist (Shipley 1818, MicroChem

Corp.) was spun onto the wafers. This new layer was patterned with the triangular prismatic structures mimicking the gecko setae, and developed. The photoresist layer formed the top of the two-layer etch mask. Once the two-layer etch mask was in place, the hierarchical structure itself was formed using deep reactive ion etching. First, the wafer was etched to a depth of 80 µm. The photoresist was then stripped from the wafer, revealing the bottom layer (oxide) etch mask. The wafer was then etched an additional 20

µm to form the final rectangular pad structure. (Figure 3.2)

Figure 3.3 Side view of scanning electron microscope image of fabricated prismatic polyurethane dry adhesive. Some of the triangular pillars (< 3%) lost pads on the top when the sample was peeled out of the PDMS mold.

Figure 3.4 Top view of scanning electron microscope image of fabricated prismatic polyurethane dry adhesive.

3.2.2 Fabrication of prismatic dry adhesive.

Two-level hierarchical polyurethane (PU) based (ST-1060 BJB Enterprise, Inc.,

Tustin, CA) dry adhesives were fabricated in a multi-step process as previously described.

In the first step, a silicon master wafer was used as a mold to create an inverse polydimethylsiloxane (PDMS) replica. To facilitate the peeling of the PDMS inverse mold from the silicon master, the latter was pretreated with a self-assembled monolayer of octadecyltrichlorosilane (OTS) coating. Sylgard 184 PDMS (Dow Corning, Midland,

MI) was mixed in a 10:1 (pre-polymer/cross-linker) ratio. After removal of air bubbles formed during mixing, the viscous liquid was poured onto the silicon master and cross- linked at 75 °C for 40 min. The partially cured PDMS replica was then laterally sheared

56 in a home-built shearing device by applying a predetermined shear distance to achieve the desired tilt angle (ranging from 60°to 90°). The PDMS replica was then fully cured in its sheared state at 75 °C for 24 h, resulting in the PDMS replica mold with tilted triangular holes and tips. PU was then poured onto the PDMS replica and allowed to cure at 75 °C for 72 h. The final PU-based dry adhesive was peeled off from the PDMS replica. Polyurethane based gecko-inspired dry adhesives were prepared using this protocol ranging from 60°to 90°. (Figure 3.3, 3.4, 3.5)

Figure 3.5 SEM images of the 2nd generation gecko-inspired adhesives composed of polyurethane with pillars at (A) 90°, (B) 80°, (C) 70°, and (D) 60° tilt angles.

3.2.3 Control of tilt angles of fibrillar arrays using the shear device

During the preparation of the PDMS replica, shear distance was predetermined to achieve the desired tilt angles. In the present work, four samples with tilt angles of 60°,

70°, 80° and 90° are fabricated for comparison of their adhesive properties (Figure 3.5).

For the sample with 90° tilt angle, the pillars are straight and require no shearing of the

PDMS mold. The partially cured PDMS was peeled off from the silicon master wafer and fully cured in the oven for the 24 hours as were the other sheared samples. To prepare the

80° tilt sample, a partially cured PDMS replica was put into the home-built shearing device.

Figure 3.6 Schematic illustration of the home-build shear device

A slight pressure was applied on the PDMS by rotate the top screws of the shearing device by half turn, following by rotating the side screws for 1 turn to apply a laterally shear. The sample was then fully cured in the oven to obtain 80° tilt PDMS replica. To prepare the 70° tilt sample, PDMS replica was put into the shearing device and applied pressure in the same way, the only deviation being 2.5 turns of the side screw (rather than

1) to apply a larger shearing distance to achieve a 70° tilt angle before being fully cured in the oven. For preparing a 60° tilt sample, a larger pressure was applied to the partially cured PDMS in the shearing device by rotating top screws 2 turns until a slight deformation of the PDMS was observed. The side screw was then rotated 2 turns followed by another turn of the top screws to apply more pressure. Lastly, the side screw were rotated 2 more turns to apply further laterally shear before the sample was fully cured in the oven.

3.2.4 Characterization of tribological properties.

Adhesion and friction measurements were performed on a Universal Materials

Tester (CETR Enterprise, Inc., Campbell, CA). A flat 2 cm x 2 cm borosilicate glass surface, attached to a force sensor (DFM, CETR Enterprise, Inc., Campbell, CA) with a cantilever (spring constant k ≈ 4000 N/m), was then brought into contact with the prismatic gecko-like surface at a predetermined preload. During a typical experiment, the glass surface was brought into contact with the sample with a preload of 50 mN. The glass surface was dragged in the gripping direction at a velocity of 10 µm/s after which it was pulled off at 90° from the sample at the same speed (Figure 3.7). For the detachment experiments, after shearing in the gripping direction (to activate the adhesion), the glass surface was then dragged in the opposite (releasing) direction and pulled off at 90° from

59 the sample (Figure 3.8). Samples with tilt angles θ of 90° (i.e., no tilt) and 63° were tested to study the influence of the tilt angle. Prior to each experiment, the glass surface was cleaned with ethanol while the samples were tested without pre-cleaning.

Figure 3.7 Schematic illustration of the tribological measurements of the gecko- inspired adhesives tested in the gripping direction. (a) apply preload, (b)start shearing, (c) reach static point, (d) pull off.

Figure 3.8 Schematic illustration of the tribological measurements of the gecko- inspired adhesives tested in the releasing direction. (a) apply preload, (b)start shearing, (c) reach static point, (d) pull off.

3.2.4 Setup of the video recorder

Figure 3.9 Optical microscope profile view image frames from a typical measurement showing polyurethane adhesive with 70˚ tilt fiber array with a line overlaid on the edge of the glass hemisphere for clarity: (a) before contact with the glass hemisphere, (b) fibers during compression with tip contact to the glass hemisphere, (c) fiber extension of the elastic polyurethane pillars, and (d) the fibers after detachment with the glass hemisphere.

During the measurements of adhesion and friction of the dry adhesive sample, a digital camera (OPTEM Zoom 70, Imagingsource, Germany) is set up to record the profile of the fibrillar array. A cross section of the fiber array area which contacts only half of the hemisphere is used for recording the side-view of the shearing experiment.

This allows the camera to record images and videos of the fiber deformation during

61 contact and retraction. Using this setup mode, it is possible to observe the compression, bending, buckling, and stretching behavior of the fibers during the testing as seen in

Figure 3.9.

3.3 Result and discussion

3.3.1 Determine the displacement location for maximum pull-off force.

Figure 3.10 Adhesive pull-off and friction force measurements of vertical prismatic pillars in the gripping direction.

To further investigate the adhesion and friction properties of the prismatic gecko- inspired adhesive, forces were measured as a function of displacement of the probe in the gripping direction after it engaged to the sample. Figure 3.10 shows the plot of the pull- off adhesion and friction forces generated by the 90˚ gecko-inspired adhesive (area

= 32 mm2) against a flat borosilicate glass surface as a function of the shear displacement.

The friction and adhesion forces for each data point were obtained after shearing the

62 surfaces for a predetermined distance and then followed by a pull-off step. The preload used for these tests was 50 mN. The shearing and pull-off velocities were both 10 µm/s.

The results show that the friction force increased as the displacement increased until 0.25 mm. During this first 0.25 shearing distance, the glass probe was dragged down by the thin pads of the prismatic pillar array to the glass probe. This process provided more contact area until the probe approached to the substrate of the sample. Both the friction force and the adhesion force reach the static maximum at this point. After this point, the probe started sliding in the gripping direction and pads on the prismatic pillars started detaching from the probe leading to a decrease of friction force. The reduction of contact area made the prismatic pillars unable to hold the probe in the normal direction, decreasing the adhesion force as well after the 0.25 mm displacement.

The plot of the pull-off adhesion and friction forces generated by the gecko- inspired adhesive (area = 32 mm2) against a flat borosilicate glass surface as a function of the shear displacement was also obtained for the 63° sample. The same setup was applied as the previous measurement. As shown in Figure 3.11, the maximum adhesive pull-off force (at a displacement ≈ 0.15 mm) for the tilted prismatic pillars does not correspond to the maximum friction force (which occurs at a displacement of ≈ 0.23 mm). This is because for the tilt structure, the effective modulus of the pillar array is smaller than that of straight ones. The probe was more easily dragged down in comparison to the 90° sample, making it easier to reach the static point for normal adhesion more quickly.

Maximum adhesion force occurred at a displacement of 0.15 mm, while the maximum friction force occurred with the displacement at 0.23 mm similar to that of the straight sample.

Figure 3.11 Adhesive pull-off and friction force measurements of 63˚ tilt prismatic pillars in the gripping direction.

3.3.2 Determine the displacement location for minimum pull-off force

Data was also collected for the pull-off adhesion force, generated by the 63° tilt gecko-inspired adhesive (area = 0.8 mm2) against a flat borosilicate glass surface as a function of the shear displacement in the releasing direction. The adhesion force for each data point was obtained after shearing the surfaces for a predetermined distance in the releasing direction, followed by a pull-off step. The preload was 20 mN during these tests.

The shearing and pull-off velocities were both 10 µm/s. Figure 3.12 shows that adhesion force decreased as the displacement increased until 0.05 mm. During this period, pillars

Figure 3.12 Adhesive pull-off force measurements of tilted prismatic pillars in the releasing direction.

and the tips had been pushed over as the probe was dragged in the releasing direction.

Once the probe reached the static point, the prismatic pillar provided greatest resistance to push away the probe from the substrate. After the static point, the probe started sliding to the releasing direction and the adhesion force maintained constant. Thus the minimum adhesion force occurred at the location with displacement of 0.05 mm.

Pull-off adhesion forces generated by the 90° gecko-inspired adhesive (area = 0.8 mm2) against a flat borosilicate glass surface as a function of the shear displacement were also measured in the releasing direction to investigate the detachment location for the 90° sample. In the case of the vertical fibrillar arrays, the minimum adhesion force occurred

65 at the location with displacement of 0.01 mm. The plot is not included here because the static point is too close to the 0 point.

Figure 3.13 Side-view SEM images of the prismatic gecko-inspired adhesive under shear. (A) In the gripping direction, a large contact area (red circle) is formed between the adhesive pads and the opposing surface. The apexes of the triangular prisms (blue circle) prevent large area contacts between neighboring pillars. (C) In the releasing direction, a smaller contact area is formed between the adhesive and the opposing surface (red circle). The apexes of the triangular prisms (blue circle) prevent large area contacts between the adhesive pads and opposing surface. (B, D) Optical microscope images of the gecko-inspired surfaces sheared against a transparent glass microscope slide in the gripping direction, and releasing direction, showing the very different anisotropic contact areas formed between the adhesive pads and the glass surface.

3.3.3 Anisotropic properties of prismatic pillar gecko-inspired dry adhesive

Scanning electron microscope (SEM) images of the prismatic surface sheared in the gripping and releasing directions demonstrate the anisotropic effects of the judicious

66 prismatic pillar design. In the gripping direction, a large contact area is formed between the adhesive pads and the opposing surface (red circle in Fig. 3.13A). In addition, the apex of the triangular prism only allows for a point contact between neighboring pillars

(blue circle in Fig. 3.13A) thus minimizing nearest neighbor adhesion. The latter is essential to allow the pillars to elastically regain their original configuration when they are unloaded. If the adhesive interactions between neighboring pillars cannot be overcome by the stored elastic energy (within the pillars), self-matting or clumping occurs (i.e., the pillars remain stuck to each other), which is undesirable.

In the releasing direction, the apex of each triangular prism (blue circle in Fig. 3.13C) now serves to prevent the adhesive pads from forming good contact with the opposing surface (red circle in Fig. 3.13C). Corresponding top-view optical microscope images of the prismatic pillars under shear against a transparent microscope glass slide in the gripping and the releasing directions (Fig. 3.13B, D) also confirm the anisotropic contact mechanics of our structures. The large contact area between the adhesive pads and the glass surface generated in the gripping direction (Fig. 3.13B) allows for both high friction and adhesion forces. The latter pin the adhesive pads to the opposing surface and upon further shearing leads to small peeling angles in the bifurcation region between the adhesive pads and the opposing surface, which enhances the adhesion forces. By contrast, in the releasing direction (Fig. 3.13D), a relatively small contact area is formed between the adhesive pads and the glass surface thus reducing both the friction and adhesion forces. Upon further shearing, the small friction forces are not sufficient to pin the adhesive pads, and sliding or detachment occurs.

Figure 3.14 Adhesion and friction force measurements in the gripping direction, i.e., during attachment, for the prismatic surface (area = 32 mm2) against a flat borosilicate glass surface. (A) Plot of the adhesion and friction forces generated during a typical measurement in the gripping direction. (B) Plot of the maximum friction force ( ) and maximum adhesion (pull-off) force ( ) over 30 cycles for vertical (θ=90°) and tilted (θ=63°) pillars. The preload was 50 mN. The shearing and pull-off velocities were both 10 µm/s.

3.3.4 Tribological properties of the judicious prismatic pillar gecko- inspired dry adhesive

Figure 3.14 shows the tribological properties of a tilted prismatic surface (tilt angle

θ=63° in the unloaded state) sheared in the gripping direction. In a typical experiment, a preload of 50 mN was applied for 5 s, followed by shearing of the opposing flat borosilicate glass surface. Upon shearing, the normal load entered the adhesive regime and the friction force increased. The glass surface was then retracted perpendicularly until it detached at the maximum adhesive (pull off) force of

. The relatively large ratio of the adhesion force to the applied preload force of

10 (i.e., 500/50) is particular attractive for climbing robots whereby low preloads allow for robust and efficient locomotion on vertical and inverted horizontal surfaces.[21, 22]

Data for the maximum adhesion and the maximum friction forces on pull-off were recorded for 30 cycles (Fig. 3.13B) for 2 different samples: one with a tilt angle of θ=63°, the other with no tilt (θ=90°). The results show the influence of the tilt angle θ on the adhesion and friction forces, and also demonstrate the reusability of this gecko-like adhesive structure.

According to the peel zone model for tape peeling, smaller peel angles between the adhesive pads and the opposing surface lead to larger adhesion and friction forces because of the larger interaction area in the bifurcation region of the peeling surfaces.

Our data is consistent with the peel zone model: pillars that had no tilt (θ=90°) produced smaller adhesion forces compared to the pillars with tilt. We also found that the maximum friction force was larger for the pillars with tilt compared to those without tilt, again consistent with the peel zone model. The lower friction forces recorded for the

Figure 3.15 Adhesion and friction force measurements in the releasing direction, i.e., during detachment, for the prismatic surface (area = 32 mm2) against a flat borosilicate glass surface. (A) Plot of the adhesion and friction forces generated during a typical measurement in the releasing direction. Each measurement cycle consisted of first shearing in the gripping direction followed by shearing in the releasing direction, before detachment. (B) Plot of the maximum friction force ( ) and maximum adhesive pull-off force ( ) over 30 cycles for vertical (θ=90°) and tilted (θ=63°) prismatic pillars. The preload was 50 mN. The shearing and pull-off velocities were both 10 µm/s.

70 pillars with tilt in Fig. 3.14B is because in these experiments the surfaces were sheared only to the point of the maximum adhesion (pull-off) force which did not coincide with the maximum friction force. We also found a monotonic decrease in both the adhesion and friction forces within the first 15 cycles. We attribute this decrease to material (i.e., polymer) transfer from the prismatic surface to the glass probe.

Similarly, the tribological properties of the prismatic surface were measured while shearing in the gripping direction, followed by shearing in the releasing direction, and finally pulling off to simulate the detachment process (Fig. 3.15). The maximum adhesion force in the releasing direction was lower than in the gripping direction. The magnitude of the friction force in the releasing direction was also found to be considerably lower than in the gripping direction (cf. ~0.9 N in the gripping direction versus ~0.13 N in the releasing direction).

Figure 3.16 Static adhesion force generated by the gecko-inspired adhesives as a function of tilt angle. The solid line is a fit of the Peel Zone model.

3.3.5 Comparison to Peel Zone model

The adhesion data generated from the samples with different tilt angles was compared to the Peel zone model, which was introduced previously in Chapter 2. The static tilt angles of the gecko inspired prismatic dry adhesives (Generation 2) were captured using the same method by using a camera capturing the side view of the samples while performing the tribological experiments (Table 3.1).

Figure 3.16 shows a fit of the Peel Zone model to the data of prismatic dry adhesives after accounting for the change in the tilt angle during the static regime of the adhesion force measurement. Compared to the cylindrical dry adhesives (Generation 1), prismatic dry adhesives yield smaller θdrag. This is because the apexes of the trianglar prisms prevent the pillars from falling to the substrate during the dragging. On the other hand, prismatic dry adhesives with smaller θdrag provide larger static adhesion than that of cylindrical dry adhesives. This further proved that the smart design of the prismatic structures exhibit better adhesive properties.

Good agreement of the data for both the cylindrical dry adhesives and prismatic dry adhesives was found between using the Peel Zone model inferring that our dry adhesive adheres to surfaces through a similar mechanism to adhesives tapes. The study of this agreement of artificial adhesives with the Peel Zone model opens a theoretical method for the future investigation of traditional adhesives tapes and synthetic fibrillar adhesives.

Table 3.1. Initial (or as fabricated) tilt angle of the pillars compared to their static tilt angles during shearing in the gripping direction.

Sample  s 1 90 78(1) 2 80(1) 63(2) 3 70(2) 48(2) 4 60(3) 27(4)

To demonstrate the practical applications of the prismatic surface, two configurations were tested to suspend weights on vertical and horizontal surfaces, thus simulating walls and ceilings. Fig. 3.17A shows the first configuration, which exploits primarily the friction forces generated by the prismatic surface to hold a 450 g weight against a vertical glass slide. An upward stroke of the prismatic surface (relative to the glass surface) would cause detachment.

In the horizontal configuration (Fig. 3.17B), a sliding stage was fabricated to apply and maintain a lateral shear force on the prismatic surface in the gripping direction allowing for a large adhesion force to be generated in the vertical direction. Reversing the lateral shear force would cause detachment. To our knowledge, this is the first demonstration of using a gecko-like adhesive to hold weights from inverted (ceiling-like) horizontal surfaces. In both cases, the weights have remained suspended for over 4 weeks.

A potential application for these switchable gecko-like adhesives involves moving delicate parts (e.g., electronic parts or computer chips) in assembly lines without using mechanical gripping or leaving a residue on the parts. To demonstrate the proof of

Figure 3.17 Supporting weights from both vertical and inverted horizontal surfaces, i.e., walls and ceilings. (A) Optical image of a 450 g weight supported from a vertical glass microscope slide using a prismatic surface of area ≈ 2 cm2. Friction forces ( ) are predominantly used in this configuration. An upward stroke of the prismatic surface would cause the weight to detach. (B) Optical image of a 50 g supported from a horizontal silicon wafer using a prismatic surface of area ≈ 1.6 cm2. Corresponding schematic illustrations are shown below each image. In the horizontal gripping configuration, (B), the shear motion of the slider (red arrow) was used to generate the friction forces ( ) that activated the 2 adhesion forces ( ) to hold the weight. Reversing the shear direction would cause the weight to detach. The weights have been hanging for several weeks.

concept for such an application, we have used the friction forces to pick up, translate and then drop a 100 g weight repeatedly. In particular, we note the minimal preload required

74 to activate the adhesion in the gripping direction, and instantaneous detachment of the weight once the shear force is relaxed. The video also shows, for the first time to our knowledge, how it is possible to exploit the adhesion forces to pick up, translate, and drop a 50 g weight repeatedly. In this configuration, a lateral shear force in the gripping direction was required to activate the adhesion while a shear force in the opposite, releasing direction caused instantaneous detachment of the weight.

3.4 Conclusion

Our current generation of gecko-like adhesive surfaces (i) possess required anisotropic tribological properties, (ii) provide relatively large adhesion and fiction forces in the gripping direction, (iii) require relatively small preloads to activate the strong adhesion, (iv) require weak opposing forces or motions to detach, and (v) are reusable over multiple cycles. Nevertheless, additional studies and improvements are needed to develop a true gecko adhesive mimic. Typical natural or synthetic surfaces are rarely smooth and clean (such as the silicon wafers or microscope glass slides used in our studies), and ambient contamination may be present. We are currently studying the influence of surface roughness on the performance of our prismatic surfaces, and although they do not possess a self-cleaning property, we are able to recover the initial high adhesion and friction by rinsing the prismatic surface with deionized water and allowing it to dry. Our new design of prismatic structures opens the way for new gecko- like adhesive surfaces that do not rely on intensive nanofabrication but rather on judicious engineering of surface microstructures and proper articulation of the surfaces.

Chapter 4

Adhesive properties of gecko-inspired artificial adhesives on rough surfaces

4.1 Introduction

76 examine the behavior of these structures under more realistic environmental conditions.

This chapter aims to clarify the effect of surface roughness on the adhesion of gecko- inspired adhesives by measuring the adhesion and friction force between gecko-inspired adhesives and surfaces with varying roughness.

4.2 Experimental methods

Figure 4.1 Design of the photomask for creating rough surfaces

4.2.2 Design of the photo mask

Photomasks were designed with patterns of circles to create controlled rough surface zones with different sizes and grit densities. Nine different grit zones with varying diameters and spacing were used for testing. (Figure 4.1)

4.2.2 Preparation of coarse rough surfaces

Coarse rough surfaces with different asperities were prepared by conventional photolithography. The experiment protocol used to create the rough surfaces by this method is briefly described below.

First, spin coat a 100 m thick layer of SU8 2075 negative photoresist on the commercial 5 inch silicon wafer (one side polished test wafer, University Wafer, Boston,

MA) using spin coater (WS 400B-6NPP/LITE, Laurell Technology Corporation, North

Wales, PA) under the spin speed of 500 rpm for 30 seconds, and then 2000 rpm for 1 min. A soft bake period of 3 min at 65 ˚C followed by 6 min at 95˚C was then applied to the coated wafer followed by UV-exposure for 30 seconds. After the exposure, the wafer was post baked at 95˚C for 10 min. The development was carried out by immersing the wafer in Microchem’s SU8 developer for 9 min. Finally the wafer was rinsed with a spray/wash with isopropyl alcohol and dried by filtered, pressurized nitrogen (figure 4.2).

By using the photomask, nine zones with different grit sizes and spacings are obtained to be used as rough surfaces. Three plates of rough surfaces with height of 5 m, 30 m and

60 m were prepared to test how the height of the rough grits affect the adhesive property of gecko-inspired adhesives. The heights of the rough surfaces were controlled through the spin coating speeds. The baking and exposure times based on the lithography process described above changed accordingly with the expected height of the coating.

Figure 4.2 Schematic illustration of the different steps involved in fabricating rough surfaces using photolithography technique.

4.2.3 Preparation of fine rough surfaces

Fine nano-scale rough surfaces were prepared by electrochemically deposited silver crystals on a silicon wafer. By tuning the electrochemical deposition potential for silver onto an electrode, the island nucleation density was systematically varied resulting in thin films of different roughness. These fine rough surfaces were then embossed and molded using polyurethane in order to measure the adhesive properties. i. Electrodeposition.

A three electrode cell was used to electrochemically deposit silver from a 20mM aqueous solution of a silver electrolyte consisting of 20mM potassium silver cyanide, 0.25M

79 sodium carbonate, and 0.1M sodium hydroxide. No additives such as brighteners or levelers were used in the electrolyte recipe since surface roughness was desirable. A

Ag/AgCl (3 M NaCl) (0.22 V vs SHE) and a platinum wire mesh (99.9% purity, Aldrich) were used as reference and counter electrodes, respectively. A 100 nm evaporated silver film (deposition rate = 0.5 Å/s) on a silicon wafer (Test grade, University Wafer) with a 1 nm chromium adhesion layer (deposition rate = 0.1 Å/s) was used as a working electrode surface. Potentiostatic electrodeposition and cyclic voltammetry (CV) were carried out using a potentiostat (263A, Princeton Applied Research). The working electrode area was

0.64 cm2 and a scan rate of 5mV/s was used for CV experiments. The as-evaporated silver surface had an rms roughness of less than 3.5 nm over an area of 5μm x 5μm. In all experiments, deionized water (Direct-Q purification system, Millipore) with a resistivity of 18.0MΩ cm was used, and all materials were purchased from Sigma-Aldrich and used as received unless noted otherwise. ii. Embossing and Molding.

A self-assembled monolayer (SAM) was formed on the rough electrodeposited silver surface to form a low energy surface by immersing it into a 2mM ethanoic octadecanethiol solution. The SAM-modified rough silver surface was then placed on a hot plate held at 150 ˚C over which a thin sheet of polystyrene (0.23 mm) was held in place using a flat stainless steel sheet as a weight. The latter ensured intimate contact was made between the polystyrene and the silver surface, thereby imprinting the roughness.

After 30 min, the surfaces were allowed to cool and the polystyrene was then peeled off from the silver surface. The polystyrene was then used as a reverse mold to replicate the

80 roughness onto polyurethane (ST-1085, BJB enterprises, Inc.) by curing the latter at 75

˚C for 48 h in an oven. iii. Surface Roughness Gradient.

Silver was electrochemically deposited from a 20 mM aqueous solution of silver cyanide.

A syringe pump (Harvard Apparatus) was used to control the rate (0.1 mm/min) at which the working electrode (10 mm in width by 15 mm in length) was extracted from the silver electrolyte solution. While the working electrode was being continuously extracted from the electrolyte, a sequential series of potentiostatic experiments were carried out at varying predetermined potentials with 0.5 C of charge in each case to obtain surfaces with different roughness. After the experiment, the surface was rinsed with DI water, dried with nitrogen, and stored for characterization. iv. Surface Roughness Characterization.

The roughness of the surfaces, including silver films, polystyrene, and polyurethane, were characterized using atomic force microscopy (PicoSPM, Agilent Technologies) by measuring at least 3 different regions within the same sample. Gwyddion software was used to analyze the AFM images. A polynomial background subtraction was applied to all AFM images before obtaining the rms roughness values.

4.2.4 Sample fabrication

Prismatic gecko-inspired adhesives were prepared using the same method introduced previously in Chapter 3. Samples are cut into 5mm x 5mm squares before peel-off from PDMS mold after curing. This is to protect the edge of the fibrillar array from being destroyed during the peel-off step.

4.2.5 Characterization of tribological properties

A 5mm x 5mm gecko-inspired adhesive sample was cured onto a 2cm x 2cm glass slide using polyurethane as a mode of attachment. The glass was glued horizontally onto the force sensor of the tribometer via a metal probe with a cantilever (spring constant k ≈

4000 N/m). During the test, the sample was brought into contact with the rough surface with a preload of 50 mN. The sample was dragged in the gripping direction at a velocity of 10 µm/s after which it was pulled off at 90° from the sample at the same speed. In this study, only straight prismatic gecko-inspired adhesive was used to test the effect of surface roughness on adhesive properties. Prior to each experiment, the glass surface was cleaned with ethanol while the samples were tested without pre-cleaning. Figure 4.3 shows two possible configurations that occur when the fibrillar tips engage the rough surface during the test depending on the asperity of the grits.

Figure 4.3 Schematic illustrations of the tribological measurements of the gecko- inspired adhesives tested on surfaces with varying roughness. (a) Measurement on a rough surface with large asperity. (b) Measurement on a rough surface with small asperity.

4.3 Result and discussion

4.3.1 Adhesive properties on fine rough surfaces

Figure 4.4 Adhesion and friction force measurements on polyurethane with different roughness created by electrodeposition technique. (area = 4 mm2)

To investigate the adhesive properties of gecko-inspired dry adhesive, the adhesion and friction forces were measured on the rough surfaces created using electrodeposition.

(Figure 4.4) From the results, both adhesion and friction forces decreased as the RMS increased. These trends show that when the RMS increased, rougher surfaces provided less contact area to the artificial dry adhesive. The tips of the dry adhesive could not go into the gaps of the rough surface due to the small scale of the roughness. However, although the adhesive properties dropped when tested on these fine rough surfaces, they

83 still maintained relatively high. More than 80% of the dynamic forces and around 80% of the static forces remained in comparison to those obtained from the control (flat) surface.

These results suggest that the nano-scale roughness does not affect the adhesive properties to a large extent.

4.3.1 Adhesive properties on coarse rough surfaces

To further understand the effect of surface roughness on gecko-inspired adhesive performance, force properties on different rough surfaces with the grit sizes and spacing dimensions close to dimensions of the adhesive tips were measured using a tribometer.

These surfaces were coarse in respect to the electrodeposited surface’s nanoscale roughness. In Figure 4.5, friction force data of the dry adhesive sample on rough surfaces with 60 µm height SU-8 grit was plotted versus grit size and grit spacing, respectively.

When increasing the grit size while keeping spacing constant, all tests showed an increasing trend because due to larger contact area with the larger grit surface. On the other hand, friction force decreased when grit spacing increased. Obviously, increasing the spacing between grits increased the gap area to which the sample would not be able to touch and thus reduced the contact area.

Similarly, adhesion force showed the same trend as the friction force (Figure 4.6).

With fixed grit spacing, the force increased when the grit size increased; while with a fixed grit size, the force decreased when the grit spacing increased. The standard deviation for the adhesion force was large with the 10 µm and 20 µm grit spacing because the sample tips kept penetrating into the grit array and being clipped by neighboring grits during the test, resulting in a deviation of the normal force.

Figure 4.5 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with height of 60 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Figure 4.6 Adhesion forces of the dry adhesive sample on SU-8 grits rough surfaces with height of 60 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Adhesive properties on rough surfaces with grit height of 30 µm were also measured using the same method. For friction force, the same trend was shown in Figure 4.7.

Because of the smaller height of the grits with this configuration, the sample tips were possibly touching the bottom of the gaps (the wafer surface) between grits, leading to even larger uncertainty. Thus, standard deviation of the 30 µm height grit rough surface was larger than those of 60 µm grit rough surface. For adhesion forces (Figure 4.8), fixed grit size with increased grit spacing shows a general trend of decrease forces. However, the interlocking effect that caused by adhesion from the sides and bottom of the grit array made the force trend very unpredictable when keeping the grit spacing fixed and changed the grit size. This side and bottom adhesion will be discussed more specifically later in this chapter.

Figure 4.7 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with height of 30 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Measurements of rough surfaces with very small height of 5 µm grit, were also investigated. Figure 4.9 shows a plot of friction force that was generated from the contact of the dry adhesive sample and the 5 µm rough surface. Surprisingly, the force trend was different from the previous rough surfaces with the same geometry but the different grit height. This deviation can be explained by the deformation of the polyurethane tips and pillars going into the gaps of the uneven surface. Since the height of the grit is small in comparison to the height of the pillars, the fibrillar array was able to penetrate and fully cover the surface. For the rough surfaces with denser grit arrays, the side adhesion had more effect on the friction force. This caused a larger friction from the rough surface with smaller grit size. In addition, with the grit sizes fixed, a larger spacing of the grit caused

88 the tips to touch the bottom of the gaps; a short gap with smaller spacing might be difficult for the polyurethane to penetrate and adhere to. The adhesion force also failed to give a clear trend, which may have been caused by the extremely short grit making the surface similar to a flat surface. Thus, the difference in the adhesion measurements was not obvious. (Figure 4.10)

Figure 4.11A shows the comparison of adhesion and friction forces generated between the gecko-inspired adhesive and rough surfaces of varying grit heights. When the spacing of the grit was small (10 µm), a large friction force was created by the rough surface with a height of 30 µm. This result implied that the taller grit along with small spacing clipped the tips of the adhesive fibrillar array, causing a larger friction during the dragging. Grit with height of 30 µm showed larger friction forces than those of 60 µm height, possibly because extra contact happened at the bottom, causing enhanced friction for the 30 µm test. For the surface with spacing of 50 µm, larger friction than the other two heights was observed due to the adhesion to the bottom of the rough surface because of the elasticity of the polyurethane.

Figure 4.11B shows the adhesion force provided by rough surfaces with different heights of the grit. In all cases of decreased spacing, the adhesion decreased when the height of the grits increased. This trend occurs because the higher the grit is, the more difficult it is for the elastic polyurethane to touch to the bottom of the substrate. The standard deviations for the measurements are the largest with the rough surfaces of 30

µm height. This offers additional proof that the side and bottom adhesion significantly affect the adhesive properties of the fibrillar polyurethane based structures.

Figure 4.8 Adhesion forces of the dry adhesive sample on SU-8 grits rough surfaces with height of 30 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Figure 4.9 Friction forces of the dry adhesive sample on SU-8 grits rough surfaces with height of 5 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Figure 4.10 Adhesion force of the dry adhesive sample on SU-8 grits rough surface with height of 5 µm. (A) Plot of forces with fixed grit spacing, (B) Plot of forces with fixed grit size.

Figure 4.11 Measurements of the dry adhesive sample on SU-8 grits rough surfaces

with varying height. (A) Friction force, (B) adhesion force.

Figure 4.12 Measurements of the dry adhesive sample on SU-8 grits rough surfaces with varying height. (A) Friction force. (B) adhesion force.

4.3.2 Two stages during the shearing of the fibrillar array

Figure 4.12 shows the adhesion and friction profiles of a typical shearing cycle. In both figures two independent slopes of the force acceleration were observed. These two slopes represented the two stages of the attachment between the gecko-inspired sample array and the rough surface. At the beginning of shearing, the fibrillar array started contacting to the surface gradually. During this period, the adhesion and friction forces increased slowly until the polyurethane pillar bent to a certain point where the pillar fell down to the largest extent without deformation. After this point the pillar was not able to move further, and the elastic polyurethane started stretching. Because of the elastic deformation, friction against the shearing direction increased rapidly, causing the pillar to become further pinned on the surface and leading to a larger adhesion force. This stage lasted until the shearing reached the static point of elastic limitation. After this point, the polyurethane pillar could not stretch anymore and started detaching from the surface.

When all the pillars detached from the surface, the sample started to slide on the rough surface.

4.4 Possible Future Research Direction

4.4.1 Future design of dry adhesive array pillar

Figure 4.13 Future design of gecko-inspired switchable adhesive pillar. (a) side- view when the pillar is not engaged to the surface, (b) top-view when the pillar is not engaged to the surface, and (c) top-view when the pillar is engaged to the surface.

To further improve the adhesive properties of our synthetic gecko-inspired dry adhesive, new configurations of complex hierarchical anisotropic polyurethane structures can be designed and fabricated as the next generation in the future research. Figure 4.13 shows the concept for a new design of the adhesive fibrillar array, which can provide larger contact area between dry adhesive and surface by adding two wing-like pads on the sides of the prismatic pillar. The wing-like pads are designed to wrap up towards the back side of the pillar and provide no contact when the pillar is dragged in the releasing direction. In the gripping direction, the two pads are able to touch the surface and stretch

96 to generate extra contact area when compared to the 2nd generation design. The fibrillar array should be designed with proper spacing between pillars in order to make sure they cover the most area on the surface without adhering to neighboring pillars. In addition, this complex design increases the steric volume of the individual pillars, which prevents the fibrillar arrays from aggregating when rinsed by polar solvent (i.e. water and ethanol).

This makes the new design easier to be cleaned and reused. One factor that should be taken into consideration is that due to the larger volume, the new arrays probably incur difficulties when engaging fine rough surfaces. The relatively larger tips will not penetrate the gaps of the rough surface that have smaller sizes than the tips. In this case, arrays with smaller dimensions are suggested based on this issue.

4.4.2 Elimination of the thickness effect during tribological tests

Figure 4.14 Schematic illustration the thickness effect during tribological tests (a) a test setup with sample deformation, (b) a test setup without sample deformation.

While characterizing the tribological properties of the gecko-inspired dry adhesives, a main source of error was the deformation of the polyurethane sample (Figure 4.14a).

Since the polyurethane is elastic, part of the friction forces generated during the test were stored as elastic energy by the sample’s bulk substrate. This affected the measurement of both adhesive properties and the displacement of the fibrillar arrays. Theoretically, the thicker the sample substrate, the more deformation occurs during the test and the more error that is generated. The common way to solve this problem is to make the sample as thin as possible to reduce the deformation. However, during some tests, the large friction force would destroy the sample which could be frail due to the thin substrate. Thus part of the future work for precisely measuring the adhesive properties of the dry adhesive sample is to design a new setup which can avoid the deformation of the sample substrate.

Figure 4.14b shows an idea to potentially solve this problem. By wrapping the extra length of sample on the probe glass, the front (leading test) edge of the sample would be prevented from rolling backwards onto itself, keeping deformation errors small. With this setup, more accurate experimental data would be expected to be collected when investigating our PU based gecko-inspired adhesives.

4.4.3 Investigation of effect of humidity on gecko-inspired fry adhesive performance

Gecko adhesion principles are primarily dependent on the geometry of the fibers.

Thus gecko-inspired synthetic adhesives can resist extreme conditions such as high humidity or under water environments. Although extensive empirical and theory-based literature on the mechanics of gecko adhesion has developed over the last 10 years, relatively little is known about how common environmental factors affect the adhesive

98 capabilities of geckos moving in their natural habitats. Geckos that live in tropical environments may encounter additional variables such as wet surfaces and high or variable humidity, yet the way in which surface water and humidity affect the adhesion of geckos is still poorly understood. In the previous research of our group, Pesika et al. tested the adhesive force of gecko setal arrays in wet condition. They found that the adhesion forces increased with humidity due to an increase in the surface energy and decreased when immersed under water due to a decrease in the van der Waals interactions compared to dry conditions. In the future, properties of synthetic gecko- inspired dry adhesives in wet conditions are expected to be investigated to further understand the mechanism of gecko adhesives. Eventually water resistant artificial gecko-inspired dry adhesives are expected to be synthesized as a substitution to traditional adhesive tapes.

4.5 Conclusion

In this chapter, we provided test results from our synthetic gecko-inspired dry adhesives on surfaces with roughness from the nano-scale to micron-scale. Our findings showed that the dry adhesive fibrillar arrays did not make full surface contact to the rough surfaces. For the rough surfaces with nano-scale asperities, the adhesive properties deceased as the roughness of the asperities increased, suggesting that the tip elements were unable to make contact to the surface by bending themselves. For the surfaces with micro scale roughness, a small grit height provided a larger friction compared to other rough surfaces with larger grit heights, suggesting that the tips of the fibrillar arrays were able to be clipped by the grit during shearing.

On the surfaces with larger grit spacing, fibrillar tips were found interlocking with the grit, thereby providing larger friction forces. This is consistent with our expectations when designing the rough surfaces. A similar increase in friction for micro-rough surfaces has been noted by previous authors.

By comparing the rough surfaces with different asperities of the grit, we draw the conclusion that grit with asperities close to the dimensions of the fibrillar tips provides larger adhesion and friction forces than those with larger asperities due to the interlocking effect with both of the side of the grit and the bottom of the substrates.

In recent years many significant advances have been made in the field of biomimetic adhesives and climbing robots. However, the performance of these systems has so far been mainly tested on dry smooth substrates. In the future, new sophisticated designs of the fibrillar arrays and more accurate experimental setups, along with a combination of adhesive and interlocking devices will be essential for a robot to be able to climb a wide variety of surfaces in both dry and wet conditions.

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Chapter 5

Conclusion and future work

In this study, novel, easy and scalable fabrication schemes were developed to incorporate tilt into polymer-based dry adhesives. Two generations of synthetic dry adhesives with different fibrillar structures and tilt angles were fabricated. The adhesion and friction forces were characterized using a universal materials tester.

The samples with tilt angles θ<90˚ showed anisotropic adhesion and friction data with the magnitude of the static adhesion and friction forces was increased as the tilt angle was decreased. By cleverly engineering asymmetric polymeric microstructures, the second generation of our reusable and switchable gecko-like adhesive can yield steady

2 2 high adhesive ( ≈ 1.25 N/cm ) and friction ( ≈ 2.8 N/cm ) forces when actuated for the “gripping direction”. These same structures release easily with minimal adhesion (

2 2 ≈ 0.34 N/cm ) and friction ( ≈ 0.38 N/cm ) forces during detachment in the “releasing direction”. These results are shown over multiple attachment/detachment cycles, with a relative small normal preload of 0.16 N/cm2 to initiate the adhesion. The static adhesion data also showed good agreement to the Peel zone model for tape peeling after accounting for the decrease in the tilt angle of the structures during the static regime of the measurement.

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We also demonstrate how these adhesives can be used to reversibly suspend weights from vertical surfaces (e.g., walls) and, for the first time to our knowledge, horizontal surfaces (e.g., ceilings) by simultaneously and judiciously activating anisotropic friction and adhesion forces. This novel design opens the way for new gecko-like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication.

Furthermore, we provided test results from our synthetic gecko-inspired dry adhesives on surfaces with roughness from the nano-scale to micron-scale. Our findings showed that the dry adhesive fibrillar arrays did not make full surface contact to the rough surfaces. For the rough surfaces with nano-scale asperities, the adhesive properties deceased as the roughness of the asperities increased, suggesting that the tip elements were unable to make contact to the surface by bending themselves. For the surfaces with micro scale roughness, a small grit height provided a larger friction compared to other rough surfaces with larger grit heights, suggesting that the tips of the fibrillar arrays were able to be clipped by the grit during shearing. On the surfaces with larger grit spacing, fibrillar tips were found interlocking with the grit, thereby providing larger friction forces.

A similar increase in friction for micro-rough surfaces has been noted by previous authors.

By comparing the rough surfaces with different asperities of the grit, we found that grits with asperities close to the dimensions of the fibrillar tips provides larger adhesion and friction forces than those with larger asperities due to the interlocking effect with both, the side of the grit and the bottom of the substrates.

Our current generation of gecko-like adhesive surfaces (i) possess the required anisotropic tribological properties, (ii) provide relatively large adhesion and fiction forces

102 in the gripping direction, (iii) require relatively small preloads to activate the strong adhesion, (iv) require weak opposing forces or motion to detach, and (v) are reusable over multiple cycles. Nevertheless, additional studies and improvements are needed to develop a true gecko adhesive mimic.

Future work in this project will include additional improvements on the current design (e.g., adding wing-like structures to the pillar to enhance contact) as well as identifying polymers with the appropriate surface chemistry for underwater adhesion.

These gecko-like adhesive has the potential to replace conventional pressure sensitive adhesives and would be functional in wet environments, e.g., within the body for mucosal adhesion.

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Biography

Kejia Jin was born in Shenyang city of Liaoning province in China on October 15,

1982. In September 1998 he attended Liaoning Province Shiyan High School, where he completed his high school educations.

Kejia attended Dalian University of Technology in September 2001, where he completed his bachelor degree in chemical engineering in 2005 and master degree in materials science in 2008. In December 2008, he joined Tulane University pursuing his

Ph.D. degree in chemical engineering. During his time as a doctoral student, he worked under the supervision of Prof. Noshir, S. Pesika. His research focus was on design and fabrication of smart biomimetic adhesives. He has authored in 10 peer-reviewed journals and one patent, and has presented his research several times in international conferences.

His honors include Distinguished Graduate Student Award (2012) and Outstanding

Research Award (2012) from the Department of Chemical and Biomolecular Engineering,

Tulane University; Student Award of Pressure Sensitive Tape Council (2013).