Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction

Abstract

Nature has developed materials, objects, and processes that function from the macroscale to the nanoscale. The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices, and processes which provide desirable properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature to provide properties of interest. There are a large number of objects including bacteria, plants, land and aquatic animals, and seashells with properties of commercial interest. Certain plant leaves, such as Lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning phenomenon is widely known as the "Lotus effect." These surfaces with high contact angle and low contact angle hysteresis with a self-cleaning effect also exhibit low adhesion and drag reduction for fluid flow. In this thesis, the theoretical mechanisms of the wetting of rough surfaces are presented followed by the characterization of natural leaf surfaces. The next logical step is to realize superhydrophobic surfaces based on understanding of the leaves. Next, a comprehensive review is presented on artificial superhydrophobic surfaces fabricated using various fabrication techniques and the influence of micro-, nano-and hierarchical structures on superhydrophobicity, self-cleaning, low adhesion, and drag reduction. An aquatic animal, such as a shark, is another model from nature for the reduction of drag in iii fluid flow. The artificial surfaces from the shark skin have been created, and the influence of structure on drag reduction efficiency is discussed. Furthermore, oleophobic surfaces can be used as a biomimetic coating that prevents contamination of the underwater parts of ships by biological and organic contaminants, including oil. The thesis discusses the wetting behavior of oil droplets on various superoleophobic surfaces. iv DEDICATION Dedicated to my parents, my wife, and my lovely daughters v ACKNOWLEDGMENTS The work presented in this dissertation would not have been possible without the support of many people. I would like to first thank my advisor, Professor Bharat Bhushan who was abundantly helpful and offered invaluable assistance, support and guidance. I will always remember the time spent with him. I would like also to extend my sincere appreciation for my doctoral committee members: Professor J. William Rich, Professor

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