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©2011 Liehui Ge All Rights Reserved ©2011 LIEHUI GE ALL RIGHTS RESERVED SYNTHETIC GECKO ADHESIVES AND ADHESION IN GECKOS A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Liehui Ge May, 2011 ABSTRACT Geckos’ feet consist of an array of millions of keratin hairs that are hierarchically split at their ends into hundreds of contact elements called “spatula(e)”. Spatulae make intimate contacts with surface and the attractive van der Waals (vdW) interactions are strong enough to support up to 100 times the animals’ bodyweight. Tremendous efforts have been made to mimic this adhesion with polymeric materials and carbon nanotubes (CNT). However, most of these fall short of the performance of geckos. “Contact splitting principle”, based on Johnson–Kendall–Roberts (JKR) theory, predicts that a vertically aligned carbon nanotube array (VA-CNT) will be at least 50 times stronger than gecko feet. Although 160 times higher adhesion was recorded in atomic force microscopy (AFM) measurements, macroscopic VA-CNT patches often show low or even no adhesion to substrates. The behavior of VA-CNT hairs near the contact interface has been explored using a combination of mechanical, electrical contact resistance, and scanning electron microscopic (SEM) measurements. Instead of making the expected end contacts, carbon nanotubes make significant side-wall contacts that increase with preload. Adhesion of side-wall contact CNTs is determined by the balance of adhesion in the contact region and the bending stiffness of the CNTs, thus a compliant VA-CNT array is required to make adhesive patches. iii Macroscopic patches of compliant VA-CNT array have been fabricated. Patches of uniform array have adhesive strength similar to that of geckos (10 N/cm2) on a variety of substrates and can be removed easily by peeling. When the array is patterned to mimic the hierarchical structures of gecko foot-hairs, strength increases up to four times. VA- CNT-based gecko adhesives are self-cleaning, non-viscoelasticity and give good strength in vacuum. These properties are desired in robotics, microelectronics, thermal management and outer space operations. Current theory still cannot completely explain adhesion of gecko feet. A series of experiments have been carried out to measure adhesion at different temperatures using a single protocol with two species of gecko that had been previously studied (G. gecko and P. dubia). Strong evidence of an effect of temperature was found but the trend was counterintuitive given the thermal biology of geckos and it violated the prediction by van der Waals interactions. Consequently, other factors (e.g., humidity) that could explain the variation in the observed clinging performance were examined. Evidence was found, unexpectedly, that humidity is likely an important determinant of clinging force in geckos. Both van der Waals and capillary forces fail to explain the shear adhesion data at the whole animal scale. Resolution of this paradox will require examination of the physical and chemical interaction at the interface and particular way in which water interacts with substrate and setae at the nanometer scale. iv ACKNOWLEDGEMENTS I owe my deepest gratitude to my advisor, Dr. Ali Dhinojwala, whose encouragement, patience, support and guidance have enabled me to develop a deep understanding of the subject of adhesion of gecko feet. He has also been a source of inspiration and creativity during the course of my study. I would like to thank my committee members, Dr. Gary R. Hamed, Dr. Li Jia, Dr. Darrell H. Reneker and Dr. Peter H. Niewiarowski for their valuable comments and suggestions. I would like to show my gratitude to those who have made contribution to my work. Dr. Lijie Ci, Dr Anubha Goyal, Dr. Sunil Pal from Dr. Pulickel Ajayan’s research group at Rice University and Rensselaer Polytechnic Institute have provided high quality carbon nanotube samples and offered help in developing synthetic gecko adhesives. Dr. Darrell H. Reneker generously donated his tube furnace when I needed it most. Dr. Peter H. Niewiarowski and his students Stephanie Lopez, Alyssa Stark have performed a lot of experiments and taken good care of geckos. Dr. Todd Blackledge allowed me to use his equipment. REU interns, Emily Hagen and Rachel Shi, have carried out experiments. I am also indebted to other members of the group for supporting me. I would also like to thank National Science Foundation for financial support and The University of Akron for tuition and stipend scholarship . Lastly, I offer my regards and blessings to all of those who supported me in any respect during the completion of the project. v Lastly, I offer my regards and blessings to my family for supporting me in any respect during the completion of this dissertation. My wife, Zhiwei Rong and my daughter, Angela Ge have always been good companions. My father, Gangzheng Ge and my stepmother, Xifen Huang have always provided encouragement and support. Last but not least, I want to thank my late mother Hao Shen whom I have missed most for all the sacrifices she made for where I am today. vi TABLE OF CONTENTS Page LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES .............................................................................................................x CHAPTER I. INTRODUCTION ...................................................................................................1 II. BACKGROUND .....................................................................................................9 2.1 Adhesion in Geckos ................................................................................ 10 2.2 Modeling of Gecko Adhesion ................................................................. 35 2.3 Design Guidelines for Synthetic Gecko Adhesives ................................ 62 2.4 Fabrication and Application of Gecko-Inspired Adhesives .................... 64 III. EXPERIMENTAL .................................................................................................80 3.1 Fabrication of CNT-based Synthetic Gecko Adhesives .......................... 80 3.2 Characterizations of Adhesion Properties ............................................... 83 3.3 Synthetic Gecko Tape ............................................................................. 90 3.4 Live Gecko Experiment .......................................................................... 95 IV. RESULTS AND DISCUSSION ..........................................................................101 4.1 Adhesion of VA-CNTs.......................................................................... 101 vii 4.2 Synthetic Gecko Tape ........................................................................... 124 4.3 Mechanism of Adhesion in Geckos ...................................................... 135 V. CONCLUSIONS..................................................................................................145 5.1 Adhesion of VA-CNTs.......................................................................... 145 5.2 Mechanism of Adhesion in Geckos ...................................................... 154 BIBLIOGRAPHY ............................................................................................................157 viii LIST OF TABLES Table Page 2.1 Mechanisms of adhesion excluded by experiments. ............................................. 21 2.2 Adhesion force of spatulae in dry and wet environment ...................................... 26 2.3 Pull-off force and scaling efficiency of contact shapes ........................................ 47 2.4 Adhesion of live Tokay gecko on rough surface .................................................. 53 2.5 Comparison of polymer-based synthetic gecko adhesives ................................... 72 2.6 Comparison of CNT-based synthetic gecko adhesives ......................................... 76 3.1 Sex, size, and toe pad area of geckos .................................................................... 97 4.1 Average maximal adjusted adhesive force by species and temperature. ............ 137 ix LIST OF FIGURES Figure Page 2.1 The hierarchical structures on adhesive toe pads of geckos ................................. 11 2.2 TEM images of sections through a single Tokay gecko seta.. .............................. 13 2.3 Adhesion force of a single seta ............................................................................. 16 2.4 Shear force from sliding setae............................................................................... 18 2.5 Adhesion of a single spatula. ................................................................................ 19 2.6 Evidence for van der Waals forces. ...................................................................... 24 2.7 Effect of humidity on adhesion ............................................................................. 28 2.8 Effect of temperature on clinging ability .............................................................. 31 2.9 Self-cleaning of gecko feet ................................................................................... 34 2.10 Scaling of contact element size and density with body mass ............................... 38 2.11 The interaction between two surfaces. .................................................................. 42 2.12 Peel zone model of spatula ..................................................................................
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