GECKO AND BIO-INSPIRED HIERARCHICAL FIBRILLAR ADHESIVE STRUCTURES EXPLORED BY MULTISCALE MODELING AND SIMULATION A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Shihao Hu August, 2012 GECKO AND BIO-INSPIRED HIERARCHICAL FIBRILLAR ADHESIVE STRUCTURES EXPLORED BY MULTISCALE MODELING AND SIMULATION Shihao Hu Dissertation Approved Accepted Advisor Department Chair Dr. Zhenhai Xia Dr. Celal Batur Co-advisor Dean of the College Dr. Xiaosheng Gao George K. Haritos Committee Member Dean of the Graduate School Dr. Gregory Morscher George R. Newkome Committee Member Date Dr. Juay Seng Tan Committee Member Dr. Hendrik Heinz ii ABSTRACT Gecko feet integrate many intriguing functions such as strong adhesion, easy detachment and self-cleaning. Mimicking this biological system leads to the development of a new class of advanced fibrillar adhesives useful in various applications. In spite of many significant progresses that have been achieved in demonstrating the enhanced adhesion strength from divided non-continuous surfaces at micro- and nano- scales, directional dependent adhesion from anisotropic structures, and some tolerance of third body interferences at the contact interfaces, the self-cleaning capability and durability of the artificial fibrillar adhesives are still substantially lagging behind the natural version. These insufficiencies impede the final commercialization of any gecko inspired products. Hence here, we have focused our attentions on these critical issues in both (i) the gecko adhesive systems and (ii) the synthetic counterparts. (i) We tested the self-cleaning of geckos during locomotion and provided the first evidence that geckos clean their feet through a unique dynamic self-cleaning mechanism via digital hyperextension. When walking naturally with hyperextension, geckos shed dirt from their toes twice as fast as they would if walking without hyperextension, returning their feet to nearly 80% of their original stickiness in only 4 steps. Our dynamic model predicts that when setae suddenly release from the attached substrate, they generate enough inertial force to dislodge dirt particles from the attached spatulae. The predicted cleaning force on dirt particles significantly increases when the dynamic iii effect is included. The extraordinary design of gecko toe pads perfectly combines dynamic self-cleaning with repeated attachment and detachment, making gecko feet sticky yet clean. This work thus provides a new mechanism to be considered for biomimetic design of highly reusable and reliable dry adhesives and devices. (ii) A multiscale modeling approach has been developed to study the force anisotropy, structural deformation and failure mechanisms of a two-level hierarchical CNT structures mimicking the gecko foot hairs. At the nanoscale, fully atomistic molecular dynamics simulation was performed to explore the origin of adhesion enhancement considering the existence of laterally distributed CNT segments. Tube-tube interactions and the collective effect of interfacial adhesion and friction forces were investigated at an upper level. A fraction of the vertically aligned CNT arrays with laterally distributed segments on top was simulated by coarse grained molecular dynamics. The characteristic interfacial adhesive behaviors obtained were further adopted as the cohesive laws incorporated in the finite element models at the device level and fitted with experimental results. The multiscale modeling approach provides a bridge to connect the atomic/molecular configurations and the micro-/nano- structures of the CNT array with its macro-level adhesive behaviors, and the predictions from the modeling and simulation help to understand the interfacial behaviors, processes and mechanics of the gecko inspired fibrillar structures for dry adhesive applications. iv ACKNOWLEDGEMENTS I would like to thank all those who helped me through my PhD program. Without them, I could not have completed this multidisciplinary project. First of all, having Dr. Zhenhai Xia as my advisor is a blessing. In this four years span, he nurtures me not only a unique way of approaching intriguing scientific problems and demanding technical challenges, but also gives me many great opportunities to expose myself into the real scientific and engineering communities through a variety of conferences and symposium, presenting my research, talking with my peers and learning from the authorities. Luckily as his first PhD student here in Akron U, I have got the chance to experience the growth of our group from the very beginning. Those early days that he and I build up our lab pieces by pieces are still vividly pictured in my mind. Now, four years later he is supervising a group of highly motivated students in the area of advanced composite materials, biomimicry, energy related projects and biomedical devices. Here, I wish to express my truthful gratitude and appreciation to him for his support, patience and understanding at this critical stage of my life, and am also looking forward to continuing our strong connections in the future. Life is full of surprises. At the third year of my PhD program I met Dr. Peter H. Niewiarowski, from a Tropical Vertebrate Biology Class (Outfield in Florida), who later on helped make the third capture possible. He is a very knowledgeable and experienced experimental biologist, who has a special sense of humor and super fun to be around. We v recognized the same interest in gecko self-cleaning and initiated collaboration right away. This unexpected experience broadens my horizon and fosters me to think more critically across disciplines under different and often contrasting contexts. I would like to thank him for giving me a chance to work in his lab and using his resources especially for the geckos. I really appreciate the time and efforts he had put in our joint project. Besides, I believe that some of the interesting American conventions as well as his perspectives in life science will have a much longer impact of my career and life. I am very grateful to my co-advisor Dr. Xiaosheng Gao, for his supervision, countless intellectually stimulating conversations, kindness, always ready to help. I am also deeply thankful to Dr. Gregory Morscher, Dr. Juay Seng Tan and Dr. Hendrik Heinz for serving as my committee members. It is not an easy job to thoroughly and carefully read a PhD thesis and give constant feedbacks for improvement. From my proposal to the final defense, they offered me great encouragement and many positive suggestions. I would want to thank all our group members and fellow students in Dr. Xia’s group, Thanyawalai Sujidkul, Lipeng Zhang, Craig Smith, Lili Li, Jianbing Niu, Quan Xu and Jie Wen, for being so supportive and friendly. Those are the experiences that I will never forget. Finally, I wish to give my parents the full credit. Their endless love, care, support and understanding, from the very start of my life, are what make me to become me today. I am very thankful to them for giving me the strength to grown as a person and offering me the opportunities to stretch my potentials. vi TABLE OF CONTENTS Page LIST OF TABLES .............................................................................................................. x LIST OF FIGURES ........................................................................................................... xi CHAPTER I. INTRODUCTION .......................................................................................................... 1 1.1 Background ................................................................................................................ 1 1.2 Problem Statement ..................................................................................................... 2 1.3 Objectives .................................................................................................................. 2 1.4 Hypothesis ................................................................................................................. 3 1.5 Dissertation Outline ................................................................................................... 4 II. LITERATURE REVIEW .............................................................................................. 6 2.1 Hierarchical Fibrillar Structure of Gecko Toe Pads .................................................. 6 2.2 Polymer Based Fibrillar Dry Adhesives .................................................................... 9 2.2.1 Cast-molding .................................................................................................... 10 2.2.2 Electron-beam lithography ............................................................................... 12 2.2.3 Soft and rigiflex lithography ............................................................................ 13 2.2.4 Tip shape variations ......................................................................................... 15 2.2.5 Anisotropic fibrillar structures ......................................................................... 17 2.2.6 Hierarchical fibrillar structures ........................................................................ 20 vii 2.3 Carbon Nanotube Based Fibrillar Dry Adhesives ................................................... 23 2.3.1 Single-level CNT arrays/forests ....................................................................... 23 2.3.2 Multi-level patterned CNT arrays/forests