Micro-Structured Adhesives for Climbing Applications - Biomimetics

161 Pages·2009·59.8 MB·English

by  Aaron Parness

MICRO-STRUCTURED ADHESIVES FOR CLIMBING
APPLICATIONS
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF MECHANICAL
ENGINEERING
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Aaron Parness
December 2009
(cid:13)c Copyright by Aaron Parness 2010
All Rights Reserved
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I certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(Mark Cutkosky) Principal Adviser
I certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(Tom Kenny)
I certify that I have read this dissertation and that, in my opinion, it
is fully adequate in scope and quality as a dissertation for the degree
of Doctor of Philosophy.
(Beth Pruitt)
Approved for the University Committee on Graduate Studies.
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Abstract
Researchers seeking to expand the capabilities of mobile robots have begun looking to
biological systems for inspiration. One particularly agile creature, the gecko lizard, is
remarkably adept at maneuvering across both flat and vertical surfaces. Some species
ofgeckoareevencapableofclimbingacrossinvertedsurfaces. Thestudyofthegecko’s
adhesivesystem hasinformed thedesign ofseveral synthetic adhesives in recent years.
However, these adhesives generally fail to match the gecko adhesive’s performance in
one fashion or another. Many previous synthetic gecko adhesives are not reusable or
they lack a method of control. Others do not have the ability to conform to surfaces
with any roughness, or to distribute forces across the many thousands of fibers evenly.
This work focuses on the design of a gecko-like adhesive system that achieves the level
of performance necessary for implementation on a small-scale climbing robot.
A gecko’s adhesive structure has a strong directional preference, allowing the an-
imal a method to control the stickiness of its feet. When loaded from the tip of
the toe towards the palm, the material exhibits high adhesion in both the shear and
normal direction. However, when this load is released, or when the toe is loaded in
a different direction, no adhesion is present. The early part of this thesis focuses
on the creation of a synthetic micro-structure that also displays a directional adhe-
sive dependence. First, a presentation of a new lithographic process used to create
asymmetric wedge-shaped cavities in a photo-sensitive epoxy at the scale of tens of
microns is made. Elastomeric materials were cast into these molds to produce the
synthetic micro structures. Analysis and laboratory testing of this material show its
strong directional dependence. The material’s performance for robotic climbing ap-
plications was promising at small sample sizes when tested on smooth surfaces like
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glass. However, the material proved inadequate for climbing because its performance
could not be scaled to areas greater than about 1 cm2, nor could it adhere to rough
surfaces.
The gecko uses a multi-tiered hierarchy to insure that its millions of sub-micron
sized spatulae make intimate contact with a surface regardless of its roughness. The
gecko’s system conforms across multiple length scales, distributing climbing forces
and allowing rapid locomotion with seemingly minimal control effort. In the second
main thrust of this work, hierarchical suspensions for fibrillar adhesives are analyzed,
and three iterations of a synthetic hierarchical adhesive design are detailed. The use
of these hierarchical suspensions with modified wedge micro-structures was successful
enough to allow a mobile robot platform to climb multiple vertical surfaces ranging
in roughness from glass to drywall. Data from large patches, well over 100 cm2, are
also included. Discussion of these results and their implications for future climbing
applications conclude the thesis.
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Acknowledgements
This work was performed in part at the Stanford Nanofabrication Facility of NNIN,
supported by the National Science Foundation under GrantECS-9731293. The work
was supported in part by DARPA-Zman, the NSF Center of Integrated Nanomechan-
ical Systems (COINS), the CIS New Users Grant Program, and NSF NIRT (UCSB).
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Contents
Abstract iv
Acknowledgements vi
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Requirements for climbing adhesives 7
2.1 Controllable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Reusable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Conformation to surfaces . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Equal load distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Background 17
3.1 Gecko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.1 Mechanism of gecko adhesion . . . . . . . . . . . . . . . . . . 17
3.1.2 Gecko foot anatomy and physiology . . . . . . . . . . . . . . . 19
3.1.3 Mechanics of the gecko adhesive system . . . . . . . . . . . . . 20
3.2 Theoretical literature on gecko adhesion . . . . . . . . . . . . . . . . 22
3.3 Gecko-like synthetic adhesives . . . . . . . . . . . . . . . . . . . . . . 27
3.3.1 Sub-micron fibers . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.2 Hierarchical fibrillar adhesives . . . . . . . . . . . . . . . . . . 31
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3.3.3 Compliant micro-scale fibers for climbing . . . . . . . . . . . . 35
3.3.4 Film-based adhesives . . . . . . . . . . . . . . . . . . . . . . . 42
3.3.5 Fibrillar adhesives for wet environments . . . . . . . . . . . . 43
3.4 Alternative adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4 Microstructured adhesive fabrication 49
4.1 MicroWedge fabrication . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.2 Secondary Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.3 Improved wedge structures . . . . . . . . . . . . . . . . . . . . . . . . 61
5 Wedge results 67
5.1 Experimental testbed and methods . . . . . . . . . . . . . . . . . . . 67
5.2 Force-time data for microstructured adhesive . . . . . . . . . . . . . . 69
5.3 Limit curve data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.4 Lifetime tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.5 Scaling data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.6 Variations in wedge design . . . . . . . . . . . . . . . . . . . . . . . . 79
5.6.1 Shrinking the wedge design . . . . . . . . . . . . . . . . . . . 81
5.6.2 Increasing real area of contact . . . . . . . . . . . . . . . . . . 81
5.6.3 Chisel shaped wedge structures . . . . . . . . . . . . . . . . . 84
6 Hierarchical synthetic adhesive structures 87
6.1 Design and testing of a two-stage hierarchical adhesive . . . . . . . . 88
6.2 Improving the hierarchical adhesive’s real area of contact . . . . . . . 99
6.3 Hierarchy II and III scaling data . . . . . . . . . . . . . . . . . . . . . 107
6.4 Hierarchy II and III roughness data . . . . . . . . . . . . . . . . . . . 110
6.5 Application to a climbing robot . . . . . . . . . . . . . . . . . . . . . 114
7 Conclusion 116
7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.2 Future possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
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A Microfabrication recipes 119
A.1 1st stage: Aluminum patterning . . . . . . . . . . . . . . . . . . . . . 120
A.2 2nd stage: 50 µm wedges SU-8 mold . . . . . . . . . . . . . . . . . . 121
A.3 2nd stage: 20µm wedges or squeegees SU-8 mold . . . . . . . . . . . . 123
A.4 2nd stage: 10 µm wedges SU-8 mold . . . . . . . . . . . . . . . . . . 124
A.5 2nd stage: 20µm chisel tips SU-8 mold . . . . . . . . . . . . . . . . . 125
A.6 Process for 20µm oblique wedge mold . . . . . . . . . . . . . . . . . . 127
B Matrix method for plane frame structures 129
Bibliography 131
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List of Tables
3.1 GSA Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1 Material inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
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