Laser Technology

Scrivener Publishing

100 Cummings Center, Suite 541J

Beverly, MA 01915-6106

Adhesion and Adhesives: Fundamental and Applied Aspects

The topics to be covered include, but not limited to, basic and theoretical aspects

of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface

and interfacial analysis and characterization; unraveling of events at interfaces;

characterization of interphases; adhesion of thin films and coatings; adhesion

aspects in reinforced composites; formation, characterization and durability of

adhesive joints; surface preparation methods; polymer surface modification;

biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of

diamond-like films; adhesion promoters; contact angle, wettability and adhesion;

superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series

will include, but not limited to, green adhesives; novel and high- performance

adhesives; and medical adhesive applications.

Series Editor: Dr. K.L. Mittal

P.O. Box 1280, Hopewell Junction, NY 12533, USA

Email: usharmittal@gmail.com

Publishers at Scrivener

Martin Scrivener (martin@scrivenerpublishing.com)

Phillip Carmical (pcarmical@scrivenerpublishing.com)

Laser Technology

Applications in Adhesion

and Related Areas

Edited by

K.L.Mittal and Wei-Sheng Lei

This edition first published 2018 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA

and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA

© 2018 Scrivener Publishing LLC

For more information about Scrivener publications please visit www.scrivenerpublishing.com.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or

transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or other-

wise, except as permitted by law. Advice on how to obtain permission to reuse material from this title

is available at http://www.wiley.com/go/permissions.

Wiley Global Headquarters

111 River Street, Hoboken, NJ 07030, USA

For details of our global editorial offices, customer services, and more information about Wiley prod-

ucts visit us at www.wiley.com.

Limit of Liability/Disclaimer of Warranty

While the publisher and authors have used their best efforts in preparing this work, they make no rep-

resentations or warranties with respect to the accuracy or completeness of the contents of this work and

specifically disclaim all warranties, including without limitation any implied warranties of merchant-

ability or fitness for a particular purpose. No warranty may be created or extended by sales representa-

tives, written sales materials, or promotional statements for this work. The fact that an organization,

website, or product is referred to in this work as a citation and/or potential source of further informa-

tion does not mean that the publisher and authors endorse the information or services the organiza-

tion, website, or product may provide or recommendations it may make. This work is sold with the

understanding that the publisher is not engaged in rendering professional services. The advice and

strategies contained herein may not be suitable for your situation. You should consult with a specialist

where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other

commercial damages, including but not limited to special, incidental, consequential, or other damages.

Further, readers should be aware that websites listed in this work may have changed or disappeared

between when this work was written and when it is read.

Library of Congress Cataloging-in-Publication Data

ISBN 978-1-119-18493-5

Cover image: Supplied by Wei-Sheng Lei

Cover design by Russell Richardson

Set in size of 11pt and Minion Pro by Exeter Premedia Services Private Ltd., Chennai, India

Printed in the USA

10 9 8 7 6 5 4 3 2 1

v

Contents

Preface xiii

Part 1: Laser Surface Modification and Adhesion Enhancement 1

1 Topographical Modification of Polymers and Metals by Laser Ablation to Create Superhydrophobic Surfaces 3

Frank L. Palmieri and Christopher J. Wohl1.1 Introduction 31.2 Wetting Theory 61.3 Laser Ablation Background 12

1.3.1 Ablation Mechanics 121.3.2 Ablation in Metals 131.3.3 Ablation in Polymers 16

1.4 Preparation of Superhydrophobic Surfaces by Laser Ablation 181.4.1 Hydrophobic Organic Substrates 181.4.2 Hydrophilic Organic Substrates 261.4.3 Hydrophilic Substrates with Hydrophobic Coatings 321.4.4 Hydrophilic Inorganic Substrates 43

1.4.4.1 Metallic substrates 441.4.4.2 Silicon substrates 511.4.4.3 Ceramic Substrates 55

1.5 Summary 56 References 57

2 Nonablative Laser Surface Modification 69

Andy Hooper2.1 Introduction 692.2 Part 1 – Nonablative Laser Skin Photorejuvenation 70

2.2.1 Introduction 702.2.2 Nonablative Laser-Based Skin Treatments 72

vi Contents

2.2.3 Review of Nonablative Laser-Based Skin Treatments Based on Laser Type 732.2.3.1 Lasers Emitting at 532 nm 732.2.3.2 Lasers Emitting at 511, 578, 585, and

600 nm Wavelengths 752.2.3.3 Lasers Emitting at 780 nm 762.2.3.4 Lasers Emitting at 980 nm 762.2.3.5 Lasers Emitting at 1064 nm 762.2.3.6 Lasers Emitting at 1320 nm 772.2.3.7 Lasers Emitting at 1450 nm 782.2.3.8 Lasers Emitting at 1540 nm 782.2.3.9 Lasers Emitting at 2940 nm 80

2.2.4 Combined Techniques 812.2.5 Conclusions for Part 1 – Nonablative Laser Skin

Photorejuvenation 812.3 Part 2 –Formation of Micro-/Nano-Structures and LIPSS

in Materials by Nonablative Laser Processing 822.3.1 Introduction 822.3.2 Review of Micro-/Nano-Structures and LIPSS 83

2.3.2.1 Micro-/Nano-Structures and LIPSS Formation in Metals 83

2.3.2.2 Micro-/Mano-Structures and LIPSS Formation in Semiconductors 85

2.3.2.3 Micro-/Nano-Structures and LIPSS Formation in Dielectrics 86

2.3.2.4 Micro-/Nano-Structures and LIPSS Formation in Polymers 86

2.3.2.5 Micro-/Nano-Structures and LIPSS Formation in Multiple Materials 87

2.3.3 Part 2 –Conclusion for Formation of Micro-/Nano-Structures and LIPSS in Materials by Nonablative Laser Processing 87

2.4 Part 3 – Nonablative Laser Surface Modification to Alter the Surface Properties of Materials 872.4.1 Introduction 882.4.2 Examples of Nonablative Laser Surface Modification

to Alter the Surface Properties of Materials 882.4.3 Conclusions for Part 3 – Nonablative Laser Surface

Modification to Alter Surface Properties 922.5 Summary 93 References 94

Contents vii

3 Wettability Characteristics of Laser Surface Engineered Polymers 99

D.G. Waugh and J. Lawrence3.1 Introduction 993.2 Lasers for Surface Engineering 101

3.2.1 Infrared Lasers for Surface Engineering 1013.2.2 Ultraviolet Lasers for Surface Engineering 1023.2.3 Ultrafast Pulsed Lasers for Surface Engineering 104

3.3 Laser Surface-Engineered Topography 1053.4 Laser Surface-Engineered Wettability 1103.5 Summary 116 References 117

4 Laser Surface Modification for Adhesion Enhancement 123

Wei-Sheng Lei and Kash Mittal4.1 Introduction 124

4.1.1 Mechanisms or Theories of Adhesion 1244.1.2 Methods of Surface Modification for Adhesion

Enhancement 1264.2 Basic Mechanisms of Laser Surface Modification 127

4.2.1 Absorption of Laser Radiation in a Material 1284.2.1.1 Linear Absorption 1294.2.1.2 Nonlinear Absorption 129

4.2.2 Photo-Chemical Process 1304.2.3 Photo-Thermal Process 132

4.2.3.1 Conventional Heat Flow Model 1324.2.3.2 Two-Temperature Model 1354.2.3.3 Ablation Rate and Ablation Spot Size 137

4.3 Laser Induced Surface Modification of Metal Substrates to Enhance Adhesion 1384.3.1 Laser Induced Surface Cleaning and Activation

for Adhesion Improvement 1384.3.2 The Dominant Role of Mechanical Interlocking

for Adhesion Improvement 1414.3.3 Laser Surface Patterning 1424.3.4 Laser Surface Topography Modification to

Enhance Adhesion of Hard Coatings on Metals 1454.3.5 Laser Surface Modification to Enhance

Metal-to-Metal Adhesive Bonding 1504.3.6 Laser Surface Modification of Metal Substrates

to Enhance Adhesion of Polymeric Materials 155

viii Contents

4.4 Laser Induced Surface Modification of Polymers and Composites to Enhance Their Adhesion 1584.4.1 Adhesion Improvement due to Laser Treatment 1594.4.2 Changes in Surface Morphology of Laser

Treated Surfaces 1634.4.3 Chemical Modification of Laser Treated Surfaces 164

4.5 Summary 167 References 168

5 Laser Surface Modification in Dentistry: Effect on the Adhesion of Restorative Materials 175

Regina Guenka Palma-Dibb, Juliana Jendiroba Faraoni, Walter Raucci-Neto and Alessandro Dibb5.1 Introduction 1755.2 Dental Structures 1805.3 Adhesion of Restorative Materials 1855.4 Laser Light Interaction with the Dental Substrate 1905.5 Dental Structure Ablation and Influence on Bond

Strength of Restorative Materials 1935.6 Summary 2005.7 Prospects 200 References 200

Part 2: Other Applications 209

6 Laser Polymer Welding 211

Rolf Klein6.1 Introduction to Laser Polymer Welding 2116.2 Theoretical Background 213

6.2.1 Reflection, Transmission and Absorption Behaviors 2136.2.2 Heat Generation and Dissipation 2266.2.3 Laser Welding Processes 239

6.3 Factors Affecting Polymer Laser Welding 2426.3.1 Types of Processes for TTLW 2426.3.2 Adapting Absorption to Welding Process 2506.3.3 Design of Joint Geometry 255

6.4 Practical Applications 2576.5 Testing and Quality Control 2616.6 Future Prospects 2636.7 Summary 263 Acknowledgements 263 References 266

Contents ix

7 Laser Based Adhesion Testing Technique to Measure Thin Film-Substrate Interface Toughness 269

Soma Sekhar V. Kandula7.1 Introduction 2707.2 Modification of Laser Spallation Technique to Measure

Thin Film-Substrate Interface Fracture Toughness 2757.2.1 Sample Preparation 2777.2.2 Experimental Procedure and Analysis 278

7.3 Parametric Studies 2837.3.1 Effect of Test Film Thickness 2847.3.2 Effect of Amplitude of the Stress Pulse 2857.3.3 Effect of Shape of the Stress Pulse 2867.3.4 Effect of Thin Film Properties 2867.3.5 Effect of Thin Film Inertial Layer 2887.3.6 Effect of Amplitude and Gradient of Residual

Stresses on the Thin Film Delamination 2897.4 Validation of Dynamic Delamination Protocol 2907.5 Summary 294 References 294

8 Laser Induced Thin Film Debonding for Micro-Device Fabrication Applications 299

Wei-Sheng Lei and Zhishui Yu8.1 Introduction 2998.2 The Mechanism of Laser Induced Debonding (LID) 3018.3 Thin Film Patterning by Laser Induced Forward Transfer 306

8.3.1 Background 3068.3.2 Thin Film Transfer Mechanisms in a LIFT Process 308

8.4 GaN Film Lift-Off for High-Brightness LEDs and High Power Electronics 3098.4.1 Background 3098.4.2 The Laser Lift-Off Process 311

8.5 Dielectric Passivation Layer Opening for Interconnect Formation in Crystalline Silicon Solar Cells 3138.5.1 Background 3138.5.2 Laser Process for Making Local Contact Openings 314

8.6 Laser Induced Wafer Debonding for Advanced Packaging Applications 3168.6.1 Background 3168.6.2 The Laser Induced Wafer Debonding Process 318

8.7 Summary 319 References 320

x Contents

9 Laser Surface Cleaning: Removal of Hard Thin Ceramic Coatings 325

S. Marimuthu, A.M. Kamara, M F Rajemi, D. Whitehead, P. Mativenga and L. Li9.1 Introduction 3269.2 Chemical Etching of Hard Thin Coatings 3289.3 Typical Experimental Set-up for Excimer Laser Removal

of Thin Coatings 3289.4 Experimental Results on Excimer Laser Removal of Thin

Coatings 3299.4.1 Laser Removal of Titanium Nitride from Tungsten

Carbide 3299.4.1.1 Removal of Titanium Nitride from

Tungsten Carbide Cutting Insert 3299.4.1.2 Removal of Titanium Nitride from

Tungsten Carbide Micro-Tool 3329.4.2 Laser Removal of Titanium Aluminium Nitride

from Tungsten Carbide 3389.4.3 Laser Removal of CrTiAlN Coatings from High

Speed Steel 3459.5 Online Monitoring of Laser Coating Removal Process 354

9.5.1 Online Monitoring Using Probe Beam Reflection (PBR) System 355

9.5.2 Online Monitoring Using Laser Plume Emission Spectroscopy 357

9.6 Discussion of Excimer Laser Coating Removal Mechanisms 3589.7 Finite Element Modelling of Excimer Laser Removal

of Thin Coatings 3629.8 Performance Evaluation of Laser Decoated Mechanical Tool 366

9.8.1 Evaluation of Wear Performance 3669.8.2 Surface Roughness of Machined Parts 3679.8.3 Environmental Footprints in Cutting 3689.8.4 Energy Consumption and Footprints for

Laser Decoating 3709.8.5 Comparison of the Energy Footprints for the

Different Steps 3719.9 Summary 372 Acknowledgments 373 References 373

Contents xi

10 Laser Removal of Particles from Surfaces 379

Changho Seo, Hyesung Shin and Dongsik Kim10.1 Introduction 38010.2 Dry Laser Cleaning (DLC) 38210.3 Steam Laser Cleaning (SLC) 38610.4 Laser Shock Cleaning (LSC) 39510.5 Novel Laser Cleaning Techniques 400

10.5.1 Matrix Laser Cleaning (MLC) 40010.5.2 Wet Laser Cleaning (WLC) 40110.5.3 Wet Laser Shock Cleaning (WLSC) 40210.5.4 Combination of DLC and LSC 40210.5.5 Combination of LSC and SLC 40210.5.6 Laser-Induced Thermocapillary Cleaning 40310.5.7 Droplet Opto-Hydrodynamic Cleaning (DOC) 403

10.6 Summary 404 Acknowledgements 407 References 408

Index 417

xiii

Preface

The acronym Laser is derived from Light Amplification by Stimulated Emission of Radiation. The first theoretical description of stimulated emis-sion of radiation was given by Einstein in 1917. However, it took four decades for the technical realization of a laser source when in 1960 T.H. Maiman developed a solid state ruby laser emitting red laser radiation. Since the advent of the ruby laser, there has been an exponential progress in designing many different lasers with unique and specific characteristics, as lasers have found myriad applications in a host of industries for a legion of purposes. In fact, lasers are ubiquitously used and here an eclectric catalog of examples where lasers have been used efficiently and effectively should suffice to underscore the widespread utility of lasers: mechanical engineering operations (e.g., micromachining), adhesion promotion, plas-tics welding, surface modification, dentistry, surgery, microelectronics, patterning, MEMS (microelectromechanical systems) and NEMS (nano-electromechanical systems).

Many laser parameters such as wavelength emitted, pulse duration, power, pulse repetition rate dictate the function and performance of a laser source. A panoply of laser sources is available for different tasks, and there is tremendous activity in ameliorating the existing laser sources as well as in devising more versatile and more efficient laser systems.

Considering the voluminous literature available dealing with laser sci-ence and technology, one will need a multi-volume compendium to cover all facets of lasers. However, in this book we have focused on the applica-tions of laser technology in adhesion and allied areas. Lasers play a signifi-cant role in the domain of adhesion. For example, polymers are used for a chorus of industrial applications as polymers have a number of desirable bulk traits, but these materials suffer from lack of adhesion due to their low reactivity and low surface free energy. A number of different techniques (e.g., corona, plasma, flame, UV/ozone, wet chemical) are commonly har-nessed to modify polymer surfaces and render them adhesionable. But laser surface treatment offers some distinctive features and advantages.

xiv Preface

These days one of the mantras is: green and laser technology offers a green (environmentally-benign) alternative without noxious emissions.

In the adhesion-related arena, two examples where lasers have played a very useful role can be cited as follows. One is directly from Nature’s treasure-trove and it is said that Nature does not waste time in trifling things and Nature is a great teacher. Here we are referring to the behavior and significant trait (self-cleaning) of the Lotus Leaf. Since the recognition/popularization of the Lotus Leaf Effect in 1997, there has been an explo-sive growth of interest in replicating the surface chemistry and topography of the Lotus Leaf using an array of techniques and in this venture laser technology has found much application. Another example is the removal of particles from surfaces. In the field of microelectronics, with the ever-shrinking feature size there is patent need to remove a few nanometer size particles and lasers have been found to be capable of removing such small particles. Apropos, the antonymous field of debonding has also benefited from the lasers, as lasers have been utilized to debond materials (e.g., thin films and coatings) from variegated substrates.

Now coming to this book which contains 10 chapters written by inter-nationally renowned subject matter experts is divided into two parts: Part 1: Laser Surface Modification and Adhesion Enhancement, and Part 2: Other Applications. The topics covered include: Topographical modifica-tion of polymers and metals by laser ablation to create superhydrophobic surfaces; nonablative laser surface modification; laser surface engineering of materials to modulate their wetting behavior; laser surface modification to enhance adhesion; laser surface modification in dentistry; laser polymer welding; laser based adhesion testing technique to measure thin film-sub-strate interface toughness; laser induced thin film debonding for micro-device fabrication applications; laser surface removal of hard thin ceramic coatings; and laser removal of particles from surfaces.

This unique book should be of great interest, value and usefulness to those in materials science, chemistry, physics and engineering. The book is profusely illustrated and copiously referenced. The information consoli-dated in this book should be of much value and relevance to R&D per-sonnel engaged in adhesion and adhesive bonding, surface modification (both physical and chemical) for a host of applications, polymer welding, cleaning (removal of hard thin coatings and nanometer size particles from surfaces), dentistry, device fabrication, micro and nanostructures forma-tion, and unravelling thin film/substrate adhesion behavior.

Now comes the important and fun part of writing a Preface as it pro-vides the opportunity to thank those who were instrumental in material-izing this book. First and foremost, we are beholden to the authors for their

Preface xv

interest, enthusiasm, unwavering cooperation and contributions which were a desideratum to bring out this book. Our appreciation is extended to Martin Scrivener (publisher) for his sustained commitment and steadfast support for this book project, and for giving this book a body form.

K.L. MittalHopewell Jct., NY, USA

E-mail: ushaRmittal@gmail.com

Wei-Sheng LeiApplied Materials Inc.,

Sunnyvale, CA, USAE-mail: Wei-Sheng_Lei@amat.com

Part 1

LASER SURFACE MODIFICATION

AND ADHESION ENHANCEMENT

3

K.L.Mittal and Wei-Sheng Lei (eds.) Laser Technology: Applications in Adhesion and Related Areas,

(3–68) © 2018 Scrivener Publishing LLC

1

Topographical Modification of Polymers and Metals by Laser Ablation to Create

Superhydrophobic Surfaces

Frank L. Palmieri* and Christopher J. Wohl

NASA Langley Research Center, Hampton, VA, USA

AbstractThe applications for superhydrophobic surfaces are nearly limitless: self-cleaning

coatings, corrosion resistance, ice mitigation, non-stick cookware, and anti-fog

surfaces to name a few. The last few decades of research have shown repeatedly

that synergy of surface chemistry and topography must be harnessed to attain

superhydrophobicity. Over the same time frame, laser technology has advanced

such that fast and ultrafast lasers with sufficient power for surface ablation are

now available to both researchers and high volume manufactures to modify the

topography and chemistry of materials. In this chapter, laser processing methods

to prepare hydrophobic and superhydrophobic surfaces are reviewed. Brief back-

grounds in wetting theory and laser ablation are provided to prepare the reader.

The preparation of superhydrophobic surfaces by laser ablation is divided into four

sections based on substrate materials and hydrophobic coatings: 1) hydrophobic

organic substrates, 2) hydrophilic organic substrates, 3) hydrophilic substrates

with hydrophobic coatings, and 4) hydrophilic inorganic substrates.

Keywords: Surface free energy, contact angle, reentrant, hierarchical structures,

wettability, laser ablation, superhydrophobicity

1.1 Introduction

Superhydrophobic surfaces have been the subject of thousands of research articles and patents since 1977 when scientists began studying and

*Corresponding author: frank.l.palmieri@nasa.gov; franklpalmieri@yahoo.com

4 Laser Technology

re-creating the properties of the lotus leaf [1–3]. Research on superhydro-phobicity has exploded in the last few decades as scientists search for self-cleaning, anti-fouling, corrosion-resistant coatings to protect everything from industrial infrastructure to sunglasses [4, 5]. A keyword search for “superhydrophobic” on The Web of Science™ shows a dramatic increase in the number of new publications starting around 2002 (Figure 1.1), possibly spurred by a 1996 paper on superhydrophobic fractal surfaces and a 1997 review of hydrophobic plants [6, 7].

Superhydrophobicity requires an advancing water contact angle (ACA) > 150° and a sliding angle (SA) < 10° (the angle with respect to gravity required for a drop to move on a surface). A low contact angle hyster-esis (CAH, the difference between the ACA and receding contact angle (RCA)) is implicit with a low SA and is often used to classify surfaces as superhydrophobic. Smooth materials with low surface free energy have been prepared with contact angles (CAs) up to approximately 120° and low CAH, but cannot meet the superhydrophobicity requirements without additional topography [8–12]. For example, polished silicon wafers cov-ered with densely packed trifluoromethyl groups achieved a water contact angle (WCA) of 119° and had a CAH of 6° [9]. Smooth surfaces covered with a liquid-like monolayer of covalently bound poly(dimethylsiloxane) resulted in an impressive CAH of 1° in some cases, but the ACA was less than 107° in all cases [8]. Porous solids filled with fluorinated fluid, given the name slippery liquid infused porous surfaces (SLIPS), resulted in effec-tively smooth surfaces with moderate ACA and CAH characteristics [12]. The scale and morphology of roughness for a SLIPS surface do not impact either the WCA or CAH, but have a profound effect on the retention of the

Figure 1.1 The number of publications using the word “superhydrophobic” each year

since 1998.

0

200

400

600

800

1000

1200

1400

1998 2000 2002 2004 2006 2008 2010 2012 2014 2016

Nu

mb

er

of

pu

bli

cati

on

s re

late

d t

o

"Su

pe

rhy

dro

ph

ob

ic"

Year

Topographical Modification of Polymers and Metals 5

infused liquid under high shear conditions [10]. Although these smooth, low energy surfaces demonstrated hydrophobicity and even low CAH in some cases, none of them achieved a WCA greater than 150° because the surfaces lacked topography.

The dependence of wettability on both the scale and form of surface topography has been thoroughly studied by observation of natural and synthetic surfaces [13]. Both random and regular topographies on the nanometer to micrometer scale impact the macroscopic wetting proper-ties of solids. Naturally occurring superhydrophobic surfaces may contain structures with a combination of length scales (i.e. hierarchical struc-tures). Synthetic surfaces, mimicking topographies found in nature, can be prepared using a litany of fabrication techniques which may be ran-dom (e.g. phase separation and abrasion) or precise (e.g. lithography, self-assembly and micromachining). Precise fabrication methods are widely used to prepare regular arrays on the nano- and micro- scales. A combina-tion of regular, microscale structures and random, nanoscale structures is often used to prepare hierarchical topographies. One versatile technique for the fabrication of hierarchical nano/microstructures is micromachining by laser ablation.

Although conceived much earlier in science fiction, lasers were first developed in the 1960’s and were accompanied almost immediately by research on the ablation of materials [14]. As laser technology developed, systems capable of high precision micromachining were developed for applications from chemical analytics to microfabrication to medicine. Modern systems provide a fast, efficient, low environmental impact means of generating microstructured surfaces.

Although it is not the subject of this chapter, a great deal of research has been devoted to laser ablation as a means of cleaning surfaces, creat-ing polar and reactive species, and creating topography to improve bond-ing with coatings and adhesives [15, 16]. Laser ablation can increase or decrease the surface free energy depending on the residual chemical spe-cies and resulting topography. For example, Lawrence and coworkers pub-lished a series of papers on laser ablation of stainless steel and aluminum to modify wettability. They showed that laser ablation with a defocused beam at high powers resulted in smoother surfaces with lower WCAs [17–19].

In this chapter, the basic theory for macroscopic wetting behavior is presented with an emphasis on the understanding and modeling of hydro-phobic and superhydrophobic phenomena. The physics of laser ablation is described with specific surface modification examples of both inorganic and organic substrates. Finally, a literature review is presented for (super)hydrophobic surfaces prepared by laser ablation of inorganic substrates

6 Laser Technology

with and without hydrophobic coatings. Special attention is given to sur-faces that exhibit (super)hydrophobicity without chemical modification after ablation (i.e. laser ablation results directly in (super)hydrophobicity).

1.2 Wetting Theory

This section provides the reader with a theoretical background of wetting phenomena. The development of macroscopic wetting theory with some attention to microscopic and stochastic models for prediction of wetting behavior will be emphasized. Many topics that are referenced in later sec-tions of this chapter (i.e. wetting states, pinning theory, etc.) are described here in greater detail.

The basic theory that describes the interaction at a solid/liquid/vapor interface is given by Young’s equation from 1805,

cos SV SL

LV

(1.1)

which relates the CA, , to the interfacial free energies of the solid-liq-uid (

SL), solid-vapor (

SV), and liquid-vapor (

LV) interfaces [20]. The CA

is a macroscopic, thermodynamic quantity because it is independent of intermolecular forces which are acting over much shorter distances than dimensions of the wetted interface. No information about microscopic shape of the contact profile can be derived from the macroscopic CA [21]. The apparent CA is observable by a wide variety of techniques[22] and is the basis of several models to calculate surface free energies[23–25]. Solids with a high

SV generally exhibit low CAs (< 90°), whereas low

SV surfaces

exhibit high CAs (> 90°). Control over the solid surface free energy and, in turn, over the wetting properties is the goal of thousands of materials researchers.

Wenzel observed and published the first significant advance in under-standing the impact of topography on CA [26]. His 1936 paper describes how surface roughness enhances the hydrophobicity of hydrophobic sur-faces and the hydrophilicity of hydrophilic surfaces. His modification to Young’s equation is,

cos cosApp

SV SL

LV

rr 0

(1.2)

Topographical Modification of Polymers and Metals 7

where App is the apparent CA observed on a rough surface, r is a roughness factor defined as the ratio of real surface area to flat surface area and 0 is the intrinsic CA on an ideal surface, which replaces Young’s CA. An ideal surface is smooth, homogeneous, rigid, insoluble and non-reactive with the contacting liquid. Because r is always greater than one, adding rough-ness only increases the numerator of equation (Wenzel) which drives the apparent CA away from 90° [26].

The Wenzel model successfully predicts the apparent CA of rough, homogeneous surfaces. In 1944, Cassie and Baxter [27] proposed a model to predict the apparent CA on rough, heterogeneous surfaces composed of two different materials.

cos cos cosApp f f1 1

0

2 2

0 (1.3)

In equation (1.3), f1 and f

2 are the complementary fractions of the real

surface areas with intrinsic CAs given by 1

0 and 2

0, respectively [27]. For porous surfaces where f

2 is the area fraction of air entrapped under a drop-

let, 2

0 is 180°, and equation (1.3) simplifies to equation (1.4).

cos cosApp f f0 (1.4)

This simplified form of the Cassie-Baxter equation is commonly used for describing superhydrophobic surfaces where f

1 = f and is referred to as

the Cassie-Baxter coefficient. When Wenzel’s roughness factor, r, is applied to the Cassie-Baxter model, a combined, Wenzel/Cassie-Baxter model can be written.

cos cosApp rf f0 1 (1.5)

Unlike the Wenzel model, the Cassie-Baxter model predicts the pos-sibility of an apparent CA greater than 90° even with an intrinsic CA < 90° which means intrinsically hydrophilic substrates can be topographically modified to be (super)hydrophobic without further chemical modification if the topography results in trapped air. Equating (1.2) with (1.5), we obtain a relationship between f, r, and the critical intrinsic CA (

c) predicted for

the transition between the Wenzel and Cassie-Baxter wetting states.

cos c

f

r f

1 (1.6)

8 Laser Technology

Because f < 1 < r, the transition from the Wenzel state to a Cassie-Baxter state requires that

c > 90°, but (super)hydrophobicity has been demon-

strated on substrates with 0 < 90° which requires a Cassie-Baxter wetting state [28–30]. In fact, the waxy coating found on the lotus leaf and the surfaces of several other superhydrophobic plants have a 0 ~ 75° [28]. The failure of equation (1.6) to predict superhydrophobicity on these surfaces has been attributed to the reentrant surface structures, characterized by sidewall angles > 90°, i.e. the surface normal vector intersects the surface interface more than once as in Figure 1.2.

The entrapment of air between reentrant structures occurs due to con-tact line pining at an outside corner of the surface structure where addi-tional advancement of the contact line would reduce the microscopic CA below 0. Wang and Chen established a set of criteria for air entrapment based on an energy balance. Air entrapment is predicted when the depth of a pore is greater than the depth of intrusion by a liquid [31]. This is depicted in Figure 1.3, where the liquid front may make contact with a neighboring structure and entrap air before contacting the bottom of the pore depending on the microscopic geometry and 0 [32].

Figure 1.2 The local angle of a surface topographical structure with respect to the average

surface tangent is shown here as Top

. A non-reentrant feature (a) has Top

< 90° and only

one intersection with the surface normal vector (arrows protruding from surface). Three

possible reentrant structures are shown (b-d) which have Top

> 90° and intersect with the

surface normal vector in at least two places.

Top

(a) (b) (c) (d)

Figure 1.3 A representation of a liquid pinning on a reentrant surface with a 0 of 70°

leading to entrapped air. (a) shows a liquid front advancing on a reentrant surface. In (b),

the liquid front maintains a CA of 70° as it advances around the reentrant surface feature.

Pinning occurs at the point on the circular surface feature where further wetting forces the

CA to deviate from 70°. In (c) the liquid front makes contact with the top of a neighboring

structure before the liquid is intruded to the bottom of the pore, causing gas entrapment.

Fluid

Trapped air

Air

Solid Substrate

70

(a)

70

(b)

70

(c)

Topographical Modification of Polymers and Metals 9

For the entrapment of air, the Cassie-Baxter state requires a liquid bridge to form between surface asperities. For stability, the three-phase contact line must remain pinned on the asperities, and the liquid bridge cannot make contact with any solid surface within the pore. As the span of the liq-uid bridge increases or the intrinsic CA of the surface decreases, the liquid bridge penetrates farther into the pore due to gravitational forces acting on the liquid. If this penetration depth is equal to the topography height, the entire surface will be wetted, and superhydrophobicity will be lost as the pore assumes a Wenzel state. The microscopic pinning of the contact line on surface topography can be used to explain the wetting behavior in the Wenzel state for the so-called rose petal effect where droplets simultane-ously exhibit superhydrophobic CAs and very high CAH [33–35].

The design of topographies to entrap air on wetted surfaces has been the focus of many researchers [7, 28–30, 36]. Cao et al. stated that a reentrant surface with a topography angle (

Top) > (180 - 0) would prevent a liquid

from penetrating into a pore [30]. Wang and Chen went further to provide a model to predict intrusion depth (h

i) into a pore of width d

v in terms of

an intrusion angle and the 0 of the substrate [31].

hd

i

v 1

2

0

0

cos

sin (1.7)

For air entrapment, surface topography must accommodate both Top

> and depth of pore > h

i. This model was applied to 1) square arrays

of cylindrical pillars, 2) square arrays of square pillars, and 3) hexagonal arrays of square pillars all with

Top = 90°. The intrusion depth was small-

est for geometry 2) and greatest for geometry 3). Wang and Chen’s model accurately predicted air entrapment for experimental results obtained by others on structures with

Top~ 90° [31, 37, 38]. Finally, the model was

applied to a reentrant geometry, allowing for > 90°, which predicted entrapped air for 0 < 90°.

Tuteja et al. [28] proposed a more generalized model to predict the stability and hydrophobicity of the Cassie-Baxter state on reentrant sur-faces based on two dimensionless geometric parameters, H* and D*. The dimensionless height (H*) is the ratio of the maximum pore depth (h

2), the

vertical distance between the contact line and pore bottom, to the sagging depth (h

1), the depth of liquid penetration past the contact line. When the

liquid-air interface is farther from the bottom of the pore, the value of H* is greater and the Cassie-Baxter state is more stable. The dimensionless dis-tance between asperities, D*, is the inverse of f. When the distance between

10 Laser Technology

asperities is greater, more of the surface is covered with entrapped air, the value of D* is greater and the surface is more hydrophobic

The forms of H* and D* are geometry dependent, and are given for a surface covered in cylindrical fibers and for micro-hoodoo structures (a specific form of reentrant structure). For cylinders:

Hh

h

Rl

D

cap*

cos2

1

0

2

2 1 (1.8)

Df

R D

R*

1 (1.9)

Here, R is the radius of the fiber or hoodoo head, l gcap LV / , is the density, g is gravitational acceleration, and D is half the characteristic spacing between the fibers or hoodoo heads. For hoodoo structures:

Hh

h

R H l

D

cap*

cos2

1

0

2

2 1 (1.10)

DW D

D* (1.11)

where W is the width of the micro-hoodoo head and H is the height of the support column. These dimensionless parameters predicted the stability of tall, reentrant structures with large spacing (nanonails and micro-hoo-doos) to have the greatest hydrophobicity and stability [28, 29].

Continuum wetting models based on macroscopic quantities, although greatly advanced since the time of Thomas Young, cannot explain all wet-ting phenomena. Stochastic models may provide insight into the micro-scopic mechanisms of observed wetting behaviors. Mean field theory (MFT) is a simplified probability model where the average effect of an ensemble on a body is statistically determined. Monson and coworkers used MFT with a lattice gas model to predict the 3-D density distribution for liquid droplets on smooth and textured surfaces [39, 40]. The density distribution provided information about CA and penetration of liquids into pores. It also predicted hybrid wetting states where Cassie-Baxter (CB) and Wenzel states occur simultaneously in distinct regions along the solid-liquid interface. This model might explain the observed petal state of

Topographical Modification of Polymers and Metals 11

some droplets that exhibit high CAs indicative of a CB wetting state while simultaneously exhibiting high CAH, indicative of a Wenzel wetting state.

MFT can be used to model wetting behavior for nanoscale droplets or topographical features, which is challenging to observe experimentally. Malonoski et al. used lattice density functional theory (DFT) to study the wetting of nanodroplets on nanostructured surfaces and showed that line tension, which was originally proposed by Gibbs but was neglected in Young’s equation, must be considered on the nanoscale [41]. Malonoski proposed a simple expression for 0 based on Young’s equation.

cos 0 SV SL

LV CR (1.12)

The term is composed of the Gibbs line tension ( ), the strength of the interaction potential ( ), and the radius of the circle of contact between the drop and an ideal surface (R

C) which acts to increase the intrinsic

CA. As the drop size (and RC) increases to the macroscale, the line ten-

sion term becomes vanishingly small and Young’s equation dominates the expression. Other authors used a similar treatment to model the effect of line tension on 0 [42, 43]. Checco and Guenoun used noncontact atomic force microscopy to measure CAs of nanoscopic alkane droplets on sili-con wafers coated with octadecyltrichlorosilane, and found that the intrin-sic CA increased with increasing drop size[44]. The experimental data did not correlate well with model systems based on line tension effects. Rosso and Virga stated that measuring line tension from these experiments was exceptionally difficult and values ranging from 10–12 to 10–5 N were reported [45, 46].

Wang et al. [47] used a DFT analysis and concluded that the Cassie-Baxter wetting of micro-rough surfaces with microscopic droplets was modeled well using macroscopic equations. In contrast, Wenzel and transitional (i.e. between Cassie-Baxter and Wenzel states) wetting state characteristics predicted by macroscopic models deviated significantly on micro-rough surfaces.

Given the developments over the past two centuries, wetting phenom-ena remain as a highly active area of fundamental and applied research. It should be noted that, almost exclusively, the study of wetting phenomena has been confined to relatively steady-state systems, i.e., low speed dynam-ics, in which case inertia effects can be ignored. Recently, research on the wetting and liquid transfer between two surfaces was investigated at dif-ferent separation velocities where it was determined that the transfer ratio

12 Laser Technology

(the ratio of liquid transferred to the second surface relative to the amount remaining on the first) did not converge to 0.5 as a result of an asym-metric liquid bridge between the surfaces at high separation velocities [48]. The design of practical, superhydrophobic surfaces stands to benefit from better understanding of microscopic effects on macroscopic wetting behavior.

1.3 Laser Ablation Background

Laser ablation is the removal of material from a surface using laser radia-tion. Ablation can be performed on any material that absorbs the incident radiation making it a highly versatile technique for creating topography on polymers, ceramics and metals. Laser radiation can be focused to an ulti-mate resolution (R) given by the Rayleigh criterion where λ is wavelength, NA is the lens numerical aperture, and k is a system constant of order 1.

R kNA

(1.13)

A focused beam combined with robotic motion control can be used to machine regular arrays of 3D microstructures which are often covered with irregular nanostructures leading to a hierarchical topography [49]. Radiation can also be used to create random micro- and nano- scaled structures which are inherently formed during many ablation processes due to the variety of physical and chemical mechanisms that occur simul-taneously during substrate irradiation.

1.3.1 Ablation Mechanics

Photo-chemical, photo-physical, and photo-thermal processes can occur individually or in combination to cause ablation. Photo-chemical ablation (photoablation) is the disassociation of chemical bonds due to the absorp-tion of photons which typically requires a fluence of 800–1000 mJ/cm2 for organic materials. As much as an order of magnitude higher fluence is needed to cause photoablation in metals and ceramics [50]. Photons in the ultraviolet range (100–400 nm) are absorbed by most materials within one micrometer of the surface and have sufficient energy to disassociate cova-lent bonds. Additionally, lasers with short pulse duration (~10 picoseconds or less) can have sufficient peak power to enable multiphoton absorbtion and photoablation. A laser pulse of sufficient power can disassociate a large