GECKO AND BIO-INSPIRED HIERARCHICAL FIBRILLAR ADHESIVE STRUCTURES EXPLORED ...

166 Pages·2012·4.69 MB·English

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

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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
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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.

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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
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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.

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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
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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 ........................................................ 24
2.4 Rational Design Based on Geometric Replications of Gecko Adhesive System .... 26
2.5 Rational Design beyond Geometric Replications of Gecko Adhesive System ....... 33
2.6 Self-Cleaning in Artificial Fibrillar Adhesives. ...................................................... 38
2.7 Summery and Outlook ............................................................................................. 39
III. DYNAMIC SELF-CLEANING IN GECKO SETAE ............................................... 41
3.1 Introduction ............................................................................................................. 41
3.2 Materials and Methods ............................................................................................ 43
3.3 Results ..................................................................................................................... 46
3.3.1 Self-cleaning of toe pads in unrestrained geckos ............................................. 47
3.3.2 Observations of dirt particles on live gecko toe pads ...................................... 49
3.4 Discussion ................................................................................................................ 51
3.4.1 Gecko detachment mechanisms with and without digital hyperextension ...... 51
3.4.2 Dynamic self-cleaning mechanism .................................................................. 56
3.4.3 Effect of DH on dynamic self-cleaning ........................................................... 64
3.4.4 Revisiting the easy detachment mechanism of gecko feet. .............................. 67
3.5 Conclusions ............................................................................................................. 71
IV. MECHANICAL ANALYSES OF THE HIERARCHICALLY STRUCTURED
CARBON NANOTUBE ARRAYS FOR DRY ADHESIVE APPLICATIONS ............. 72
4.1 Introduction ............................................................................................................. 72
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4.2 Outline of the Multiscale Modeling and Simulation Approaches ........................... 74
4.3 Fully Atomistic Molecular Dynamics ..................................................................... 76
4.3.1 Introduction ...................................................................................................... 76
4.3.2 Methodology .................................................................................................... 78
4.3.3 Results .............................................................................................................. 80
4.3.4 Discussion ........................................................................................................ 89
4.3.5 Conclusions .................................................................................................... 103
4.4 Coarse Grained Molecular Dynamics .................................................................... 103
4.4.1 Model description .......................................................................................... 103
4.4.2 Results and discussion ................................................................................... 109
4.4.3 Conclusions .................................................................................................... 116
4.5 Finite Element Analysis......................................................................................... 117
4.5.1 Model description .......................................................................................... 118
4.5.2 Results and discussion ................................................................................... 122
4.5.3 Conclusions .................................................................................................... 138
V. CONCLUSIONS AND FUTURE WORK ............................................................... 139
5.1 Conclusions ........................................................................................................... 139
5.2 Recommended Future Works ................................................................................ 141
REFERENCES ............................................................................................................... 143

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LIST OF TABLES
Table Page
4-1 Parameters calculated for the adhesion enhancement of various materials………...102

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