Reproduced in part with permission from Badge, I., Sethi, S., Dhinojwala, A. Langmuir 2011, 27, 14726–14731 Copyright [2011] American Chemical Society TUNING WETTABILITY AND ADHESION OF STRUCTURED SURFACES A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Ila Badge May, 2014 TUNING WETTABILITY AND ADHESION OF STRUCTURED SURFACES Ila Badge Dissertation Approved: Accepted: _______________________________ _______________________________ Advisor Department Chair Dr. Ali Dhinojwala Dr. Coleen Pugh _______________________________ _______________________________ Committee Member Dean of the College Dr. Mesfin Tsige Dr. Stephen Z.D. Cheng _______________________________ _______________________________ Committee Member Dean of the Graduate School Dr. Li Jia Dr. George R. Newkome _______________________________ _______________________________ Committee Member Date Dr. Darrell Reneker __________________________ Committee Member Dr. Robert Weiss ii ABSTRACT Structured surfaces with feature size ranging from a few micrometers down to nanometers are of great interest in the applications such as design of anti-wetting surfaces, tissue engineering, microfluidics, filtration, microelectronic devices, anti- reflective coatings and reversible adhesives. A specific surface property demands particular roughness geometry along with suitable surface chemistry. Plasma Enhanced Chemical Vapor Deposition (PECVD) is a technique that offers control over surface chemistry without significantly affecting the roughness and thus, provides a flexibility to alter surface chemistry selectively for a given structured surface. In this study, we have used PECVD to fine tune wetting and adhesion properties. The research presented focuses on material design aspects as well as the fundamental understanding of wetting and adhesion phenomena of structured surfaces. In order to study the effect of surface roughness and surface chemistry on the surface wettability independently, we developed a model surface by combination of colloidal lithography and PECVD. A systematically controlled hierarchical roughness using spherical colloidal particles and surface chemistry allowed for quantitative prediction of contact angles corresponding to metastable and stable wetting states. A well-defined roughness and chemical composition of the surface enabled establishing a correlation between theory predictions and experimental measurements. iii We developed an extremely robust superhydrophobic surface based on Carbon- Nanotubes (CNT) mats. The surface of CNTs forming a nano-porous mesh was modified using PECVD to deposit a layer of hydrophobic coating (PCNT). The PCNT surface thus formed is superhydrophobic with almost zero contact angle hysteresis. We demonstrated that the PCNT surface is not wetted under steam condensation even after prolonged exposure and also continues to retain its superhydrophobicity after multiple frosting- defrosting cycles. The anti-wetting behavior of PCNT surface is consistent with our model predictions, derived based on thermodynamic theory of wetting. The surface of gecko feet is a very unique natural structured surface. The hierarchical surface structure of a Gecko toe pad is responsible for its reversible adhesive properties and superhydrophobicity. van der Waals interactions is known to be the key mechanism behind Gecko adhesion. However, we found that the wettability, thus the surface chemistry plays a significant role in Gecko adhesion mechanism, especially in the case of underwater adhesion. We used PECVD process to deposit a layer of coating with known chemistry on the surface of sheds of gecko toes to study the effect that wettability of the toe surface has on its adhesion. In summary, we demonstrated that PECVD can be effectively used as means of surface chemistry control for tunable structure-property relationship of three types of structured surfaces; each having unique surface features. iv DEDICATION I would like to dedicate this dissertation to my parents, Ravindra and Vasanti Badge, for their unconditional love, support and words of encouragement throughout my Ph.D. v ACKNOWLEDGEMENTS I would like to take this opportunity to thank several individuals who have contributed to my success in the graduate school and helped make the Ph.D. experience one of the most valuable ones in my life. First and foremost, I would like to thank my advisor, Dr. Ali Dhinojwala, for his valuable guidance and encouragement at every step of graduate school. None of the work presented here would have been possible without his support. Under his guidance, I have learnt to be more disciplined and organized, to thoroughly and critically analyze the problems and learn from failures. Having him as my mentor has made me not only a better scientist but also a better person in the past few years. I would like to acknowledge and express my gratitude towards my committee members, Dr. Mesfin Tsige, Dr. Li Jia, Dr. Darrell Reneker and Dr. Robert Weiss, for agreeing to be on my dissertation committee and their valuable advice and feedback from time to time. I would like to thank Dr. Li Jia and his former graduate student Dr. Sarang Bhawalkar for collaborating in the project of study of wettability using patterns formed with colloidal lithography. I would also like to thank Dr. Peter Niewiarowski and Alyssa Stark for collaboration in the work of gecko adhesion and also for numerous valuable discussions. I would like to thank Dr. Bojie Wang for training me to use SEM and TEM techniques, which I heavily relied upon in my research. I would also like to thank Ed vi Laughlin for his excellent machine work and Jack Gillespie for his help with glass related work. I would like to specially mention my gratitude towards both of them for their valuable ideas and suggestions in part designs. I would like to thank Goodyear Tire and Rubber Company for the funding and giving me an opportunity to work as an industrial intern. I would like to thank all my group members, past and present, for their help in my research. The communications on a daily basis and exchange of ideas in small chats have always helped me solve problems related to the research. I would like to specifically mention my gratitude to Dr. Sunny Sethi for teaching me carbon nanotube synthesis technique and mentoring me in the initial years of my Ph.D. I would like to thank Dr. Frederic Siffer for training me to use the PECVD set up. I would specially like to thank Dr. Vasav Sahni, Dr. Michael Heiber, Mena Klittich and Alyssa Stark for proof reading manuscripts and abstracts and their valuable feedback for the same. I would like to thank Mena Klittich for her help in proof reading my dissertation as well. Last but not the least I would like to thank my friends and family, for their unconditional love and support, for sharing the moments of happiness and for helping me get through tough times. I would like to thank my parents and my younger brother, Mayur Badge for believing in me and their constant love and support. I would specially like to thank friends in Akron who have been a family away from home. I thank Kushal Bahl, Saurabh Batra, Sarang Bhawalkar, Diya Bandyopadhyay and Poonam Songar for all their support and being just wonderful friends. vii TABLE OF CONTENTS Page LIST OF TABLES ............................................................................................................ xii LIST OF FIGURES ......................................................................................................... xiv CHAPTERS I. INTRODUCTION ..................................................................................................1 II. BACKGOUND ........................................................................................................5 2.1 Wetting ............................................................................................................5 2.1.1 Wetting by water ....................................................................................6 2.2 Contact Angle ..................................................................................................7 2.2.1 Background ............................................................................................7 2.2.2 Young’s Equation .................................................................................8 2.2.3 Static and dynamic contact angles .........................................................9 2.2.4 Surface energy versus surface roughness ............................................12 2.2.5 Wenzel model ......................................................................................15 2.2.6 Cassie-Baxter model ............................................................................17 2.3 Designing surface with desired wettability ..................................................19 2.3.1 Superhydrophobic surfaces ..................................................................20 viii 2.3.2 Superwetting surfaces .........................................................................33 2.3.3 Oleophobic and omniphobic surfaces ..................................................34 2.4 Thermodynamic stability of wetting states ....................................................38 2.5 Loss of superhydrophobicity .........................................................................41 2.6 Adhesion of structured surfaces.....................................................................45