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«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»

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UNIVERSITY OF CALIFORNIA

Santa Barbara

Design and Characterization of Fibrillar

Adhesives

A Dissertation submitted in partial satisfaction

of the requirements for the degree of

Doctor of Philosophy

in

Mechanical Engineering

by

John W. Tamelier

Committee in Charge:

Professor Kimberly Turner, Chair

Professor Jacob Israelachvili

Professor Matthew Begley

Professor Glenn Beltz

June 2013

The Dissertation of

John W. Tamelier is approved:

Professor Jacob Israelachvili Professor Matthew Begley Professor Glenn Beltz Professor Kimberly Turner, Committee Chairperson June 2013 Design and Characterization of Fibrillar Adhesives Copyright c 2013 by John W. Tamelier iii Acknowledgements To fully acknowledge everyone who has helped me up to this point would require more pages than I am allowed. I will therefore say a few words about each person directly related to this body of work. Without my graduate advisor, Dr. Kimberly Turner, the research I carried out would not have been possible. She has allowed me to direct my professional development while offering advice and support when needed. Collaboration with Dr. Jacob Israelachvili has greatly expanded my knowledge outside of the main areas of my research and meetings with him offered many interesting investigations to pursue. The advice and suggestions received from Dr. Matthew Begley and Dr. Glen Beltz have greatly strengthened this body of work.

Working with the various people during my time at UCSB has greatly increased my happiness throughout this experience. Turner group members, both past and present, have provided a wonderful work environment. Their advice when problems arose saved many hours of work and their diverse personalities made working in the lab or office an enjoyable experience. Sathya Chary deserves special attention since we have shared this project over the past years. Collaborating with the researchers in the Israelachvili lab, Jing Yu and Saurabh Das, has allowed an environment for discussion with many experiments increasing the body of knowledge in this area. Dave Bothman, the Turner group lab manager, could iv always be relied upon to solve problems both inside and outside of the laboratory as well as offer useful suggestions for improvement. The UCSB Nanofabrication Facility staff deserve recognition for their maintenance of the cleanroom and processing advice. The staff in the Mechanical Engineering Department and Institute for Collaborative Biotechnologies have always helped to ensure that my research could progress.

My friends at UCSB and elsewhere have given me many lasting memories and much happiness. I am grateful to them for all the laughter, adventure, and life experiences they have provided. My family has always provided me with an overwhelming amount of support and encouragement. It was their sacrifices throughout my life that have given me the opportunity to pursue postgraduate education. Finally, the love, encouragement, and unbreakable faith of my wife, Giulia, has greatly helped me to complete this work.

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from the following: J. Tamelier, S. Chary, K.L. Turner, J. Yu, S. Das, and J.N. Israelachvili. Millimeter Size Patch Behavior of Gecko-Inspired Reversible Adhesive.

In: Proceedings of 2011 IEEE Sensors, October 28–31, 2011. ( c 2011 IEEE) Some of the content and figures in Chapter 4 are reprinted with permission from the following: J. Tamelier, S. Chary, and K.L. Turner. Vertical Anisotropic Micro-fibers for a Gecko-inspired Adhesive. Langmuir. 28(23):8746–8752. 2012.

( c 2012 American Chemical Society) Some of the content and figures in Chapter 5 are reprinted with permission from the following: S. Chary, J. Tamelier, and K.L. Turner. Articulation of angled semicircular microfibers for a gecko-inspired anisotropic adhesive. In: Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems, January 20–24, 2013. ( c 2013 IEEE) Some of the content and figures in Chapter 5 are reprinted with permission from the following: S. Chary, J. Tamelier, and K.L. Turner. A microfabricated gecko-inspired controllable and reusable dry adhesive. Smart Materials and Struc

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Some of the content and figures in Chapter 6 are reprinted with permission from the following: J. Tamelier, S. Chary, K.L. Turner. Optimal articulation strategy for a gecko-inspired controllable adhesive. In: Proceedings of Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), 2013 17th International, June 16–20, 2013. ( c 2013 IEEE)

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The climbing ability of the Tokay gecko has attracted much attention from scientists trying to reproduce the adhesive found on the animal’s feet. Many of the impressive properties of the adhesive have been attributed to the micrometer- and nanometer-sized fibers and how they are moved, or articulated, during adhesive placement and separation. Using microfabrication techniques, fibrillar structures were created and tested using a home-built testing apparatus specifically designed to characterize tribological properties over millimeter-sized areas. The results confirm that fiber geometry and fiber articulation are both important for geckoinspired adhesives.





Tilted and vertical microfabricated polydimethylsiloxane (PDMS) rectangular flaps were created to determine if non-circular structures, similar in shape to the terminal features of the gecko, could be used to create a controllable adhesive.

The control of the adhesive was achieved by varying shear length and direction to contact different parts of the flaps. A high adhesion force was achieved when contacting the side face of the flaps, lower adhesion force when contacting the top of the flaps, and almost zero adhesion force when contacting the edge between

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behavior, creating both a gripping direction and a releasing direction.

The effect of fiber shape was then explored by using a semicircular cross section. The PDMS fibrillar adhesive was the first single material vertical adhesive without fiber tip modification to exhibit adhesion and friction anisotropy. Using a simple microfabrication process, differences in contact area caused anisotropic behavior when displaced towards and against the flat face of the fiber. The vertical fibers even showed higher anisotropy values than some tilted structures.

As a continuation of the testing on vertical semicircular fibers, tilt was again added to the design. The adhesion forces and anisotropy of the PDMS angled fibers were higher than the previous designs, illustrating the importance of both fiber shape and tilt for anisotropy. The shear forces were also high and the shear pressure reached 88% of the value obtained when testing the two front feet of the gecko. The adhesive was then tested over 10,000 cycles and retained 118% of its initial shear force and 77% of its initial adhesion force. The performance of this adhesive is comparable to other high lifetime gecko-inspired adhesives.

As an alternative to the geometric design of the fibers, the influence of fiber articulation, that is how the fibers are moved, was explored using a common vertical testing procedure and was compared to an angled testing procedure. Both tests were shown to result in similar maximal shear and adhesion forces. The

–  –  –

have been shown to be important for robot stability when climbing. By choosing the correct approach angle, shear length, and retraction angle, a single testing procedure was shown to achieve a high shear force, a high adhesion force, and a high µ′ value, resulting in an optimal articulation.

–  –  –

5.1 SEM of Angled Semicircular Fibers................. 97

5.2 Angled Exposure Schematic for Creating Angled Semicircular Fibers 99

5.3 Shear Adhesion Forces of Angled Semicircular Fibers....... 103

5.4 Optical Images and Schematic Diagrams of Angled Semicircular Fiber Contact................................ 107

5.5 Shear Forces of Angled Semicircular Fibers............. 111 xv

5.6 Shear and Adhesion Force Endurance Test on Angled Semicircular Fibers.................................... 116

5.7 SEM Image of Angled Semicircular Fibers after 10,000 Test Cycles 118

6.1 Angled Load-Drag-Pull Test Schematic............... 128

6.2 SEM Image of Vertical Semicircular Fibers............. 130

6.3 Experimental and Simulation Preload Forces for Angled Approach Tests..................................... 132

6.4 Shear Adhesion Forces of Unpatterned PDMS for Vertical Testing 138

6.5 Shear Forces of Unpatterned PDMS for Vertical Testing...... 140

6.6 Shear Adhesion Forces of Vertical Semicircular Fibers for Vertical Testing................................... 142

6.7 Shear Forces of Vertical Semicircular Fibers for Vertical Testing. 144

6.8 Shear Adhesion Forces of Vertical Semicircular Fibers for Angled Testing Without Additional Shear Displacement............. 146

6.9 Shear Forces of Vertical Semicircular Fibers for Angled Testing Without Additional Shear Displacement................. 148

6.10 µ′ Values of Vertical Semicircular Fibers for Angled Testing Without Additional Shear Displacement.................... 150

6.11 Shear Adhesion Forces of Vertical Semicircular Fibers for Angled Testing with +40 µm of Shear Displacement............... 152

6.12 Shear Forces of Vertical Semicircular Fibers for Angled Testing with +40 µm of Shear Displacement................... 154

6.13 µ′ Values of Vertical Semicircular Fibers for Angled Testing with +40 µm of Shear Displacement...................... 156

6.14 Figure of Merit Values of Vertical Semicircular Fibers for 2.5◦ Angled Approach............................... 157

6.15 Figure of Merit Values of Vertical Semicircular Fibers for 90◦ Vertical Approach............................... 159

7.1 Angled Flaps with Hierarchical Nanofibers............. 168

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Introduction Much interest in the Tokay gecko is related to the animal’s impressive climbing ability. The gecko is able to climb at speeds of up to 1 m/s [8], an impressive feat for an animal only 20–50 centimeters long. The gecko is also able to support its 50-gram weight with its 230 mm2 pad area [45] on vertical and inverted surfaces.

The natural surfaces the gecko encounters such as rock and wood can have varying degrees of roughness, yet the gecko is able to stick to them all. The surfaces are not always clean and can contain a variety of different sized particles which would foul most adhesives. Despite these difficulties, the gecko is still able to run and walk without losing its adhesive ability.

When adhesives are typically discussed they may refer to drying adhesives such as rubber cement where a solvent evaporates and the adhesive hardens. This type of adhesive is meant to join two materials permanently and works in a different manner than the gecko adhesive. A separate type of adhesive commonly used Chapter 1. Introduction are pressure sensitive adhesives which can be found in adhesive tapes. Pressure sensitive adhesives are made of a complaint material which must be applied with a high enough preload to allow the adhesive to flow and and conform to the other surface. Once the two surfaces are close enough together, van der Waals forces become relevant for higher strength. Pressure sensitive adhesive typically use materials with a stiffness less than 100 kPa, in order to flow and create the large areas of contact [24]. This low elastic modulus causes pressure sensitive adhesives to foul easily, attach to unwanted surfaces, and attach to itself. The gecko adhesive functions in a completely different manner while still taking advantage of the intermolecular van der Waals forces [12].

Unlike conventional adhesives, the adhesive fibers found on the bottom of the gecko’s feet are composed a β-keratin, with a Young’s modulus of ≈ 1.5 GPa [84]. The hierarchical structure with nanometer-scale, micrometer-scale, and millimeter-scale features achieve intimate contact necessary for van der Waals forces by conformation of the fibers over the different length scales. The levels of hierarchy result in an effective elastic modulus of ≈100 kPa and allow the fibers to act as a more compliant material resulting in good contact with the opposing surface. The high modulus also causes the fibers to not be sticky by default, to remove undesired fouling particles from the fibers, and to not stick to other fibers.

The adhesive fibers are asymmetric in shape and rely on articulation (movement) Chapter 1. Introduction to attach or detach. Movement with the fiber tilt causes high shear and adhesion forces while movement against the tilt causes low shear and adhesion forces. In this way, the gecko can control the fibers’ adhesive properties.

1.1 Motivation Climbing robots are a natural fit for the adhesives being developed. With gecko-inspired adhesives integrated on a legged robot, the robot would no longer be restricted to flat or slightly angled terrain. Robots could have the freedom to climb vertical or traverse inverted surfaces in order to explore previously inaccessible areas. When the robot has reached its desired location, sensing applications or information gathering using other platforms could be implemented. The development of climbing robots have been undertaken by various research groups with climbing on vertical surfaces requiring the use bio-inspired adhesives [39, 79].



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