surface characterization, adhesion, and friction properties of hydrophobic leaf surfaces and ...

100 Pages·2005·2.82 MB·English

SURFACE CHARACTERIZATION, ADHESION, AND FRICTION PROPERTIES OF
HYDROPHOBIC LEAF SURFACES AND NANOPATTERNED POLYMERS FOR
SUPERHYDROPHOBIC SURFACES

A Thesis
Presented in Partial Fulfillment for
the Degree Master of Science in the
Graduate School of the Ohio State University

By
Zachary Travis Burton, B.A.
*****

The Ohio State University
2005

Master’s Examination Committee:
Prof. Bharat Bhushan, Advisor Approved by
Prof. Rebecca Dupaix

Advisor
Mechanical Engineering Graduate Program
ABSTRACT

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for
various industrial applications. Certain plant leaves are known to be hydrophobic in
nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax
film on the surface of the leaf. The purpose of this study is to fully characterize the leaf
surface and to separate out the effects of the microbumps and the wax on the
hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and
without wax, are studied. Using an optical profiler and an atomic/friction force
microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and
without wax, were made to fully characterize the leaf surface. Using a model that
predicts contact angle as a function of roughness, the roughness factor for the
hydrophobic leaves has been calculated, which is used to calculate the contact angle for a
flat leaf surface. It is shown that both the microbumps and the wax play an equally
important role in the hydrophobic nature as well as adhesion and friction of the leaf. This
study will be useful in developing super-hydrophobic surfaces.
The next logical step in realizing hydrophobic surfaces that can be produced in
the lab is to learn from the hydrophobic leaves and design surfaces based off of that
learning. This study looks at polymer surfaces that have a nanopattern present on the
ii
surface that mimics the surface found on hydrophobic leaves, such as lotus. This study
explores the effect of nanopatterning on hydrophobicity, adhesion and friction for two
different hydrophilic polymers, poly(methyl methacrylate) (PMMA) and polyurethane
acrylate (MINS) with two types of patterned asperities, low aspect ratio and high aspect
ratio. In addition to the polymers, a hydrophobic coating was deposited on the surface of
the patterned PMMA to study the effect of roughness on the contact angle, along with
adhesion and friction. Relative contribution due to change in contact angle and real area
of contact are explored. Scale dependence on adhesion and friction was also studied using
AFM tips of various radii. Since applications of these surfaces will require operation in
varying environments, the effect of relative humidity is investigated.
Tribological issues in micro/nanoelectromechanical systems (MEMS/NEMS) and
BioMEMS/NEMS are one of the concerns for the reliability of these devices. Silicon
based components for MEMS/NEMS devices have been the standard but new materials,
such as polymers, are gaining use due to their advantages over silicon in the area of
BioMEMS. Adhesion and friction properties of two polymers of interest, poly(methyl
methacrylate) (PMMA) and polydimethylsiloxane (PDMS), along with a single-crystal
silicon substrate, Si (100), are investigated by using a novel microtriboapparatus capable
of adopting BioMEMS components used in microfluidic devices. The adhesion and
friction properties of PMMA and PDMS are dependent upon the surrounding
environment and to study these issues, three different liquids are applied to the interface
of the contacts in two different forms, a drop and a film. Other environmental effects are
also investigated in this study by looking at relative humidity, temperature, rest time and
different gaseous environments such as nitrogen gas and reduced pressure.
iii
Dedicated to my family
iv
ACKNOWLEDMENTS

I wish to thank my advisor, Prof. Bharat Bhushan, for giving me the opportunity
to work in his lab for my graduate research. His support, knowledge and encouragement
made this research possible and he has given me the tools to take on any challenge I face
in the future. I would also like to extend my sincere note of appreciation to the other
member of my Master’s graduate committee, Prof. Rebecca Dupaix.
I want to thank all of my colleagues at Nanotribology Laboratory for Information
Storage and MEMS/NEMS (NLIM). Everyone provided a positive atmosphere that made
it a pleasure to work everyday. I want to thank Dr. Nikhil Tambe for getting me started
with the AFM and helping me with any technical questions and editorial comments.
Other members of NLIM that have provided help and support in their own way are
Carmen LaTorre, Dr. Zhenhua Tao, Dr. Guohua Wei, Dr. Dharma Tokachichu, Dr.
Huiwen Liu, Eric Cai, Walt Hanson, Tony Alfano, Andy Wright, and Tom Hayes. Last
but certainly not least, I would like to thank Caterina Runyon-Spears for her help in
preparing paper work for publications during the course of my research.
But most of all I would like to thank my family, who without their support and
encouragement throughout my life, I would not be where I am today. The unconditional
love and support they have shown me is more important than anything else that has ever
been given to me. Thank you.
v
VITA

June 30, 1981…………………………... Born – Ypsilanti, MI
2003…………………………………….. B.A. Physics, Kalamazoo College
2003 – Present………………………….. Graduate Research Associate, The Ohio State
University

PUBLICATIONS
1. B. Bhushan and Z. Burton, “Adhesion and friction properties of polymers found
in microfluidic devices.” Nanotechnology, 16, 467, (2005).

FIELDS OF STUDY
Major field: Mechanical Engineering

vi
TABLE OF CONTENTS
Abstract...............................................................................................................................ii
Dedication..........................................................................................................................iv
Acknowledgments...............................................................................................................v
Vita.....................................................................................................................................vi
List of Tables......................................................................................................................x
List of Figures....................................................................................................................xi
Chapters:

1 Introduction......................................................................................................................1
2 Surface characterization, adhesion and friction properties of hydrophobic leaf surfaces
and nanopatterned polymers...............................................................................................3
2.1 Introduction...............................................................................................................3
2.2 Experimental details..................................................................................................9
2.2.1 Instrumentation..................................................................................................9
2.2.2 Samples............................................................................................................11
2.2.3 Roughness factor..............................................................................................15
2.2.4 Test matrix for nanopatterned polymers..........................................................17
2.3 Results and discussion............................................................................................18
2.3.1 Hydrophobic leaf surfaces...............................................................................18
2.3.1.1 Contact angle measurements.....................................................................19
vii
2.3.1.2 Surface characterization using an optical profiler....................................20
2.3.1.3 Leaf characterization using an AFM.........................................................25
2.3.1.3.1 Surface characterization.....................................................................25
2.3.1.3.2 Adhesive force and friction of hydrophobic leaves...........................32
2.3.1.3.3 Dynamic shrinking effects of the leaf................................................33
2.3.2.1 Contact angle measurements.....................................................................36
2.3.2.2 Adhesion studies and scale dependence...................................................37
2.3.2.3 Effect of relative humidity on adhesive force and coefficient of friction.41
2.4 Summary.................................................................................................................45
3 Adhesion and friction properties of polymers in microfluidic devices..........................49
3.1 Introduction.............................................................................................................49
3.2 Experimental...........................................................................................................55
3.2.1 Instrumentation................................................................................................55
3.2.2 Samples............................................................................................................58
3.2.3 Test matrix.......................................................................................................62
3.2.4 Contact angle measurements............................................................................64
3.3 Results and Discussion...........................................................................................64
3.3.1 Adhesive force and coefficient of friction in dry and liquid environments.....64
3.3.2 Effect of environment on adhesive force and coefficient of friction...............72
3.3.2.1 Effect of relative humidity........................................................................72
3.3.2.2 Effect of temperature................................................................................73
3.3.2.3 Effect of rest time......................................................................................75
3.3.2.4 Effect of various gaseous environments...................................................76
viii
4 Summary........................................................................................................................80
List of References.............................................................................................................82
ix
LIST OF TABLES

Table Page
2.1 Bump statistics for lotus and colocasia.......................................................................24

3.1 Selected physical properties for various samples.......................................................61

x