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«BIOLOGICALLY INSPIRED HAIRY SURFACES FOR LIQUID REPELLENCY By SHU-HAU HSU A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF ...»

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BIOLOGICALLY INSPIRED HAIRY SURFACES FOR LIQUID REPELLENCY

By

SHU-HAU HSU

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

© 2010 Shu-Hau Hsu To my lovely Mom and Dad

ACKNOWLEDGMENTS

I would like to first and foremost thank Dr. Wolfgang Sigmund, who is not only just an adviser but an incredibly energetic mentor and scientist. His vision, compassion, support, understanding and guidance helped me through the entire work. I would also like to thank my committee members, Drs. Moudgil, Baney, El-Shall, and C-Y Wu for their constructive comments. Also, I would like to thank Dr. Tonia Hsieh at Temple University for the extensive discussions on biological system. I would like to recognize the help of the staff of MAIC (Materials Analytical Instrument Center) and PERC (Particle Engineering Research Center) regarding the characterization of the surface properties.

There are also a lot of students and friends who without their help I would not have finished this work. I would like to acknowledge all the past and current members in the Sigmund‘s group for assisting me in many ways during my work. I particularly thank former group member, Yi-Chung Wang, for the preparation of the fluorocarbon coated samples. Special thanks would need to go to the people who had lived in Arbor (Howard, Ian, Kenneth, Sophia, Pei-Ching and Ray) for sharing the life in Gainesville.

Last but not least, I am extremely grateful to my parents for their love and unselfish support throughout my study in United States. Without them, this dissertation would have never been accomplished.

TABLE OF CONTENTS

page ACKNOWLEDGMENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

Abstract

CHAPTER 1 INTRODUCTION

Water Repellent Interfaces: A Green Technology

Superhydrophobic Surfaces in Nature

Research Objectives

2 PRINCIPLES OF LIQUID-REPELLENT SURFACES

Issues of Wetting on Solid Surfaces

Surface Tension

Wetting on Ideal Smooth Surfaces

Wetting on Roughened Surfaces I: Wenzel Model

Wetting on Roughened Surfaces II: Cassie-Baxter Model

Transition Between Cassie-Baxter and Wenzel State

Contact Angle Hysteresis and Sliding Angle

Water Repellent Surfaces from Nature

Plant Kingdom

Animal Kingdom

3 FABRICATION OF ARTIFICIAL HAIRY SURFACES

Experimental Work of Making Hairy Surfaces

Self-Assembly of Hair-Like Structure

Materials Selection

Experimental procedure

Characterization

Making Hairy Surfaces via Moulding Techniques

Casting with lithographed moulds

Casting with natural leaves

Casting with commercial porous membrane

Characterization of surface morphology

Characterization of thermal properties

Characterization of mechanical properties

Experimental Results

Making Hairy Surface via Self-Alignment of Carbon Nanotubes

Colloidal carbon nanotubes

Results and discussion

Making Hairy Surfaces via Moulding Technique

Casting with lithographed moulds

Casting with natural leaves

Making Hairy Surfaces via Moulding with Commercial Membranes................. 53 Membrane casting on elastomer

Membrane casting on thermoplastic polymers

Issues of the current membrane casting

General Discussion

The Artificial Hairy Surfaces

The Scope of the Casting Process

Durability of the Cast Structure

Summary

4 WETTABILITY OF HAIRY SURFACES

Contact Angle Measurement

Concerns of Contact Angle Measurement

Fitting Model of Contact Angle Measurement

Experimental Work of Wettability Evaluation

Static Contact Angle Measurement

Contact Angle Hysteresis Measurement

Video Assessment of the Surface Hydrophobicity

Surface Tension Determination

Contact Area and Theoretical Contact Angle Interpretation

Wetting Property of Hairy Plants

Observations on Hairy Leaves

Droplets on Hairy Plants

Wetting Property of Artificial Hairy Surfaces

Contact Angles of Cast PDMS Surface

Contact Angles of Cast Thermoplastic Surface

Polypropylene substrate

LDPE substrate

PVDF substrate

Perfectly Hydrophobic Response

Self-Cleaning Ability

General Discussion

Summary

5 TOWARDS SUPEROLEOPHOBIC SURFACES

Experimental Work of Plasma Treatment

Introduction

Surface Treatment with Water Plasma

Surface Treatment with Fluorocarbon Plasma Deposition

Characterization of Plasma Treated Surface

Surface morphology

Surface chemistry

Contact angle measurement





Experimental Results

Surface with Water Plasma Treatment

Surface with Fluorocarbon Coating

General Discussion

Summary

6 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

Conclusions

Suggestions for Future Work

The Role of Elasticity on Water Repellency

Quantitative Study of Self-Cleaning Effect

LIST OF REFERENCES

BIOGRAPHICAL SKETCH

–  –  –

3-1 List of materials for casting hairy surfaces in this study

3-2 Glass transition temperature (Tg) and melting point (Tm) of the thermoplastics used in this work..

3-3 Mechanical properties of the thermoplastics used in mould casting................... 89 4-1 Measured surface tensions of the liquids used in contact angle measurement

4-2 Contact angles of all the cast thermoplastic surfaces and the theoretical contact angles calculated from Cassie-Baxter theory.

5-1 Liquid drops on plasma treated 0.6m-peeled PP samples.

5-2 Contact angles of low surface tension liquids on cast PP surfaces before and after CF coatings.

–  –  –

1-1 SEM images of lotus leaf surface.

SEM image of lady‘s mantle leaf, stem of Dicliptera Suberecta and stem of 1-2 tomato.

2-1 Schematic diagram of liquid molecules at the surface and the surface tension.. 36 2-2 Wetting behavior of a liquid droplet on solid surface and their mathematical models.

Contact angle hysteresis is the angle difference between adv and rec.............. 38 2-3 2-4 Relationship between sliding angles and contact angle hysteresis

2-5 Few examples of microscopic mophology of water-repellent leaf surfaces........ 39 SEM micrographs of a lady‘s mantle leaf

2-6 SEM images of the hair layer of the water strider‘s body.

2-7 2-8 Hairs protrude from the leg of fish spider, and Plastron of fish spider, visible as a silver envelope around the body and legs.

3-1 Schematic procedure of self-alignment of functionalized carbon nanotubes with polyelectrolytes.

3-2 The procedure of high-aspect-ratio surface structure by photolithography and e-beam lithography.

3-3 Creating negative PDMS elastomer mould by using leaf Dicliptera Suberecta as a positive mould

3-4 Two commercial membranes, AAO and PC, used for casting

3-5 The procedure of casting PDMS elastomer with polycarbonate and alumina membranes

3-6 The procedure of casting thermoplastics substrate with polycarbonate membrane. The membrane is removed by either direct peeling or dissolving.... 72 3-7 Tg is determined by the midterm point B from curve obtained by DSC analysis.

3-8 Standard dog-bone shaped samples for ultimate tensile test.

3-9 A typical tensile stress-strain curve and the represented properties for polymeric materials.

3-10 Zeta-potential of functionalized and non-functionalized carbon nanotubes showing the shift of the IEP (isoelectrical point).

3-11 SEM picture of Si substrate after immersed into colloidal CNTs suspension..... 74 3-12 The surface structure of cast PDMS elastomer from moulds developed by photolithography and by electron beam lithography

SEM pictures showed the hairs structures of Dicliptera Suberecta and Lady‘s 3-13 Mantle.

3-14 The surface morphology of cast PDMS elastomer by using Dicliptera Suberecta as master moulds.

Lady‘s Mantle was dehydrated after curing process making it very difficult to 3-15 be removed from the PDMS elastomer.

A negative PDMS mould after peeling off the leaf of Lady‘s Mantle

3-16 3-17 The interface of PDMS substrate cast by PC membrane

3-18 PDMS substrate cast with different pore size PC membrane.

3-19 The microstructure of PP substrate cast with AAO membrane (=0.2m)......... 79 3-20 PP substrate cast with different pore size PC membrane. The membrane was dissolved after casting.

3-21 Top-viewed post structure of cast PP substrate after dissolving the membrane.

3-22 SEM images of PP substrate cast with different pore size PC membranes........ 82 3-23 SEM images of LDPE substrate cast with different pore size PC membranes... 83 3-24 SEM images of PVDF substrate cast with different pore size PC membranes... 84 3-25 SEM images of PS and PMMA substrates cast with PC membrane.................. 85 3-26 DSC analysis curve of PP substrate

3-27 DSC analysis curve of LDPE substrate

3-28 DSC analysis curve of PVDF substrate

3-29 DSC analysis curve of PS substrate

3-30 DSC analysis curve of PMMA substrate

3-32 Comparsion between the surface microstructure on PP substrate after peeling off the PC membrane and the microstructure of water strider‘s abdomen.

3-33 Images of Immersed regular PP substrate and PP substrate with artificial hairy structure.

3-34 SEM images of cast PP and PVDF substrates before and after being rubbed by fingers.

4-1 Contact angle measurement on a superhydrophobic surface with different diameters of syringe

4-2 Images of the same water droplet on a superhydrophobic surface under different fitting modes of the static contact angle

4-3 The equipment set-up of goiniometer.

4-4 The interface of DropSnake program.

4-5 Selected sequential images during contact/compression/release test on a superhydrophobic surface.

4-6 Schematic picture of Wilhelmy plate method of measuring liquid surface tension.

4-7 The fraction of contact area is estimated by adjusting the threshold of topview SEM pictures.

Water droplets on the leaf of Lady‘s Mantle

4-8 Stereo microscopic Images of a droplet on the leaf of Lady‘s Mantle............... 120 4-9 Variety of hair density of the leaves of Lady‘s Mantle leaves.

4-10 4-11 Morphology of different cast PDMS surfaces and their contact angles............. 121 4-12 The surface morphology of the membrane-dissolved PP surfaces and their sessile drop images.

4-13 The surface morphology of the membrane-peeled PP surfaces and their sessile drop images.

4-14 The surface morphology of the membrane-peeled LDPE surfaces and their sessile drop images.

4-15 The surface morphology of the membrane-peeled PVDF surfaces and their sessile drop images.

4-16 The images of contact angle measurement acquiring from goniometer. (a) a steel ball and (b) a droplet on cast PP hairy surface.

4-17 Selected images during contact/compression/release test on a perfectly hydrophobic hairy PP substrate.

4-18 Selected images during contact/compression/release test on a non-perfectly superhydrophobic PP substrate

4-19 Selected images during motion test on 0.6m-peeled hairy PP substrate........ 129 4-20 Schematic diagram of self-cleaning effect (Lotus effect)

4-21 Water droplets on an uncast PP sheet contaminated by carbon powder......... 131 4-22 Water droplets on an cast hairy PP surface contaminated by dirt particles...... 132 4-23 Two configurations of models for water-repellent hair pile.

4-24 The interaction between water droplet and artificial hairy surface.

5-1 Schematic diagram of the water plasma system used in this work................... 145 5-2 Photo of the water plasma chamber used in this work.

5-3 Schematic diagram of the STS deep reactive ion etch system.

5-4 Schematic diagram of the typical process of deep reactive ion etching............ 146 5-5 The morphology of hairy surface before and after Ar plasma

5-6 The surface morphology of CF coated PP samples after casting process....... 148 5-7 The hairy structure before and after plasma deposition of CF layer in DRIE chamber.

5-8 Contact angles of methanol-water mixture on hairy surfaces coated with different plasma power.

5-9 XPS results of CF coated surfaces under different plasma power

5-10 Low contact angle of nonpolar liquid (dodecane) on CF coated hairy PP surfaces.

5-11 Low surface tension liquid (dodecane) on PVA and (b) pHEMA substrates..... 152 Schematic diagrams of interfacial force when a liquid with contact angle c on 5-12 the non-reentrant and reentrant structure.

–  –  –

AAO anodic aluminum oxide ASTM American Society for Testing and Materials CAD computer aided design CB Cassie-Baxter CCD charge-coupled device

–  –  –

DRIE deep reactive ion etching DSC differential scanning calorimetry HMDS hexamethyl-disilazane ICP induced coupled plasma IEP iso-electrical point LDPE low-density polyethylene MeOH methanol MWCNT multi-walled carbon nanotube PC polycarbonate PDAC poly(diallyldimethylammonium chloride) PDMS polydimethyl siloxane PMMA polymethyl methacrylate PP polycarbonate

–  –  –

RFGD radio frequency glow discharge SEM scanning electron microscope THF tetrahydrofuran XPS x-ray photoelectron spectroscopy ALV contact area of liquid/vapor interface ASL contact area of solid/liquid interface ASV contact area of solid/vapor interface fs area fraction of liquid/solid interface fv area fraction of liquid/vapor interface

–  –  –



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