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«UNIVERSITY OF CALIFORNIA Santa Barbara A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive A Dissertation submitted in partial satisfaction of ...»

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UNIVERSITY OF CALIFORNIA

Santa Barbara

A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive

A Dissertation submitted in partial satisfaction of the

requirements for the degree Doctor of Philosophy

in Materials

by

Michael Thomas Northen

Committee in charge:

Professor Kimberly Turner, Chair

Professor Noel MacDonald

Professor Anthony Evans

Professor Jacob Israelachvili

March 2006

The dissertation of Michael Thomas Northen is approved.

_____________________________________________________

Anthony Evans _____________________________________________________

Jacob Israelachvili _____________________________________________________

Noel MacDonald _____________________________________________________

Kimberly Turner, Committee Chair March 2006 A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive Copyright © 2006 by Michael Thomas Northen iii

ACKNOWLEDGEMENTS

This manuscript is dedicated to my loving parents Tom and Cheri, for with their gentle guidance and unfaltering support I was able to make a journey longer than I ever imagined. I’d also like to thank my brother Trent for being amazing enough that it has always been okay to follow in his footsteps.

For making this body of work possible, and enticing me to do a PhD in the first place, I would like to thank my advisor and friend Professor Kimberly Turner. Also there have been many friends and colleagues that without their help I would not have been able to traverse this path, or at least wouldn’t have had so much fun in the process. A few key people I would like to thank are Alicia Soliz, for all her love and support; Tellef Tellefson and Laurent Pelletier, for just being darn good friends;

Michael Requa, Marco Aimi and Emily Parker, for always being there as friends and colleagues; Masa Rao and Brian Thibeault, for their extensive processing advice;

and Jim and Robin Cooper for being friends and helping me along at critical points in this journey. I’d also like to recognize all the staff and faculty at UCSB, in particular the Materials and Mechanical Engineering Departments, for making this institution such a dynamic and supportive environment, where creativity and innovation can flourish.

iv

VITA OF MICHAEL THOMAS NORTHEN

March 2006 EDUCATION Associates of Arts/Science in English/Engineering, Santa Rosa Junior College, 1999 Bachelor of Science in Mechanical and Environmental Engineering, University of California, Santa Barbara, June 2001 (summa cum laude with distinction) Master of Science in Materials, University of California, Santa Barbara, June 2002 Doctor of Philosophy in Materials, University of California, Santa Barbara, March 2006 (expected)

PROFESSIONAL EMPLOYMENT

1999-2001 Tutor for the Disabled Students Office 2001-2002: Teaching Assistant, Departments of Materials and Mechanical Engineering, University of California, Santa Barbara Summer 2002: Summer Mentorship Internships in Nanosystems, Science, Engineering and Technology (INSET), University of California, Santa Barbara.

2003-2006: Supervising Mentor for Internships in Nanosystems, Science, Engineering and Technology (INSET), University of California, Santa Barbara.

2001-2006: Research Assistant, Department of Materials, University of California, Santa Barbara.

PROFESSIONAL SERVICE

2005: Reviewer for the journal Nanotechnology, an Institute of Physics Publication.

2006: Reviewer for the journal Nanotechnology, an Institute of Physics Publication.

PUBLICATIONS

Northen, M. T., Greiner, C., Arzt, E., Turner, K.L. (2006). "A bio-inspired reversible adhesive." Science (Submitted).

–  –  –

Northen, M. T., Turner, K.L. (2005). "Batch fabrication and characterization of nanostructures for enhanced adhesion." Current Applied Physics (In Press).

Northen, M. T., Turner, K.L. (2005). “Meso-scale adhesion testing of integrated micro- and nano-scale structures.” Sensors and Actuators A (In Press).

Northen, M. T., Turner, K.L. (2004). Single High Aspect Ratio Pillar Supports. ECS, The 206th Meeting of the Electrochemical Society, Waikiki, Hawaii, The Electrochemical Society.

Northen, M. T., Turner, K.L. (2004). Single High Aspect Ratio Pillar Support Structures: Multi-scale Chip Integrated Conformal Structures. IMECE04, 2004 ASME International Mechanical Engineering Congress and Exposition, Anaheim, California, USA.

Northen, M. T., Turner, K.L. (2005). Multi-scale compliant structures for use as a chip-scale dry adhesive. Transducers '05, The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea.

INVITED TALKS

Jawaharlal Nehru Center for Advanced Scientific Research, December 17, 2004, Bangalore, India Max Planck Institute, December 7, 2005, Stuttgart, Germany TALKS ECS, The 206th Meeting of the Electrochemical Society, Joint Meeting, October 3Waikiki, Hawaii USA IMECE04, 2004 ASME International Mechanical Engineering Congress and Exposition, November 13-20, 2004, Anaheim, California USA ISRS 04, International Student Research Symposium, December 20-22, 2004, IIT Madras, Chennai, India AMN-2, Advanced Materials and Nanotechnology, 6-11 February 2005, Queenstown, New Zealand

–  –  –





Hilton Head 2006: A Solid State Sensor, Actuator and Microsystems Workshop, Hilton Head Island, South Carolina, June 4 - 8, 2006 AWARDS Student of the year, Mechanical and Environmental Engineering Department, University of California, Santa Barbara, 2001

FIELDS OF STUDY

Major Field: Biomimetic Micro/Nanotechnology Studies in micro/nanofabrication techniques for use in bio-inspired systems with Professor Kimberly Turner.

–  –  –

The gecko adhesive has been of scientific interest for over two millennia, ever since Aristotle observed a gecko running up and down a tree. Since then, advances in optical and electron microscopy have provided increased information on the structure of the pad of the gecko’s foot, for it is the structure that leads to the adhesion and not the chemistry. Each toe contains many ridges, or scansors, displaying arrays of ~100 μm long, ~5 μm wide setae, each branching into hundreds of smaller fibers called spatulae, ~200 nm across and 5 nm thick. The combination of the setal flexibility and the nanoscale compliance of the spatulae creates a large amount of intimate surface contact, enhancing van der Waals interactions, and promoting adhesion.

In this work, an adhesive—inspired by the gecko—was micro/nanofabricated.

There were two main thrusts in this work. The first was to show the importance of the hierarchical structure on the performance of the gecko adhesive, and the future need to for multiple levels of compliance in synthetic dry adhesives. The second thrust was to create a surface with controllable adhesion. While the adhesion

–  –  –

adhesion on and off that provides the technological driving force for mimicking the gecko system.

The microscale setae of the gecko have been replicated by microfabricating flexible silicon dioxide freestanding structures ~100 μm long and ~10 μm wide.

These structures were coated with aligned vertical polymeric nanorods ~4 μm tall and ~200 nm in diameter, analogous to the gecko spatulae. Testing of the synthetic structures shows that the multi-scale system provides a 5-fold increase in adhesion over nanorods alone, demonstrating the need for a hierarchical structure. To create a switchable adhesive, the silicon dioxide microstructures were replaced with nickel micro-paddles. When the nickel structures were placed in a magnetic field, a conformation change was induced, rotating the paddles away from an adherent surface, and adhesion was reduced by a factor of 40. This is the first demonstration of a surface displaying reversible and controllable adhesion.

–  –  –

I. Introduction

II. Background

A. Biological Adhesion

1. Overview

2. Adhesion Mechanics

3. Adhesion forces

B. MEMS Fabrication Techniques

1. Photolithography

2. Wet etching

3. Dry Etching

4. Electron Beam Deposition

5. Sputtering

C. Micro/Nanotribology

1. Nanoindentation

2. Wyko Optical Profilometer

III. Detailed Micro/Nanofabrication

A. Silicon Dioxide Platforms Supported by Single Crystal Silicon Pillars

B. Multi-Scale Integrated Structures

C. Nickel Multi-Scale Integrated Structures

IV. Single High Aspect Pillar Support Structures

A. Introduction

–  –  –

1. Fabrication

2. Equipment

C. C. Results and Discussion

1. Characterization

2. Future Work

D. Conclusions

E. Acknowledgments

F. Figures

V. A batch fabricated biomimetic dry adhesive

A. Introduction

B. Fabrication

C. Experimental

D. Results and Discussion

E. Summary

F. Figures

VI. Batch Fabrication and Characterization of Nanostructures for Enhanced Adhesion 84 A. Introduction

B. Experimental

1. Fabrication

2. Characterization

C. Results and Discussion

–  –  –

E. Acknowledgments and Correspondence

F. Figures

VII. Meso-Scale Adhesion Testing of Integrated Micro- and Nano-Scale Structures 97 A. Introduction

B. Fabrication

C. Experimental

D. Result and Discussion

E. Conclusion

F. Figures

VIII. A bio-inspired reversible adhesive

IX. Concluding Remarks and Future Work

X. References

Appendix

A.1 Micromechanical Nanoindenter Testing

–  –  –

Figure II-1 Dude, a male Tokay Gecko (Gekko Gecko) sticking to a wall surface (Image by Jeff Clark, www.jeffclarkphotography.com).

Figure II-2 Gecko hierarchical adhesive system. Electron micrographs of: A. the terminal spatular structure, scale bar 500 nm; B. the heavily branched spatular end of a setal stalk, scale bar 10 μm; C. setae extending from a lamellae surface, scale bar 20 μm. D. setal array showing the branched structures of the setae, scale bar 100 μm.

Figure II-3 Images of Dude, a Tokay Gecko, on a glass surfaces. Note the orientation of the toes relative to Dude’s leg and body orientation on the glass surface.

Figure II-4 Image of Dude the Gecko peeling his tarsus from the surface (Image by Jeff Clark)

Figure II-5 Illustration of the contact splitting phenomenon in nature. Plot shows that as the animal’s mass increases then so does the necessary density of termini in the adhesion system. Figure from reference (27)

Figure II-6 Diagram of the parallel plate sensing mechanism in the Hysitron™ nanoindenter system

Figure II-7 Interference microscope design for the Wyko NT1100 optical profiler. 27 Figure II-8 Fringing pattern see on a tilted surface (top). Illustration of the intensity versus height for interfering light on a tilted surface.

–  –  –

dioxide platform supported by a single crystal silicon pillar. Right image offers a magnified view of the pillar. Scale bars are 20 μm and 10 μm left and right respectively.

Figure III-2 SHARPS fabrication process flow schematic.

Figure III-3 Electron micrograph of the organic looking polymeric nanorods, ‘organorods,’ scale bar 10 μm

Figure III-4 Electron micrographs of SHARPS structures coated with organorods.

Scale bars are 20 μm and 5 μm, left and right respectively.

Figure III-5 (Top) Optical images of a water droplet on untreated (left) and treated (right) organorod surfaces. (Bottom) Corresponding electron micrographs of the structure, scale bars 2 μm.

Figure III-6 Electron micrographs of released organorod coated nickel paddle structures. Opposing paddles coated with organorods scale bar 20 μm (top).

Magnified view with dimensions of organorods, scale bar 2 μm (bottom-left).

Array of paddles, scale bar 100 μm (bottom-right)

Figure IV-1 Electron micrographs of various SHARPS structures (From Top and Left to Right) Square topped array, round top, slotted square, slotted round, branched finger, branched fine finger, radial meander, serpentine

Figure IV-2 SHARPS fabrication process flow.

Figure IV-3 Optical micrographs showing pillar geometry along the long axis.

(From left to right) Hexagonal, triangular, square, octagonal.

Figure IV-4 SHARPS structure with modified pillar geometry along the long axis. 57

–  –  –

the bottom left image. (Bottom left) SHARPS structure after sputtering a half micrometer of titanium. (Bottom right) same structure after sputtering ~5 μm of titanium.

Figure IV-6 Nanoindenter bending stiffness experimental data and theoretical plot originating at the center of the SHARPS platform. (Top) Diamond cross section (Bottom) Square cross section.

Figure IV-7 (Top) before and after images of a probe pushing laterally on the structure. (Bottom left) Superposition of images illustrating deformation distance: distance between blue and yellow lines. (Bottom right) Schematic representation of 3-dimensional deformation.

Figure IV-8 SHARPS structures after sputtering ~5 μm titanium

Figure IV-9 Comparison of adhesion vs. applied normal force between a polyamide bead and the substrate or a radial meander SHARPS structure.



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