Development of Scalable and Hierarchical Superhydrophobic Surfaces for Drag Reduction
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This dissertation discusses the technology to reduce friction drag of water flow using superhydrophobic surfaces. Water drops show very diverse and interesting behavior when interacting with solid surfaces of different wettabilities. Depending on the texture of the underlying surface and the solid surface energy, the water drop may display states ranging from super-wetting (superhydrophilic) to near-perfect non-wetting (superhydrophobic). Superhydrophobicity, in particular, refers to a state that occurs on a textured surface, on which the liquid may not penetrate into the surface asperities. Consequently, the drop partially sits on air that is entrapped between numerous asperities of the textured surface. The entrapped air forms a barrier between the solid surface and the liquid drop, thus forming a solid-liquid-air composite interface. Among various properties of a superhydrophobic surface, one particularly interesting and of potential impact is the reduction in skin friction drag. The air layer entrapped in the noted composite interface functions as a lubrication layer for bulk water, and it has been known that, at least theoretically, it is possible to decrease the viscous dissipation in the boundary layer for water flowing past a superhydrophobic surface. The objective of this current research is to comprehend the role of surface texture on the static and dynamic interactions of the liquid with such superhydrophobic surfaces, and to incorporate the obtained understanding to develop superhydrophobic surfaces that display large reduction in drag. The first part of this dissertation introduces the basics of wetting and non-wetting phenomena on solid surfaces, both smooth and textured. Emphasis will be laid on understanding the correlation between the texture topography of a surface and its wettability to liquids, and how superhydrophobic surfaces form composite interfaces and display high repellency against water. The second part of the dissertation delves into the dynamics of water on the textured surfaces, with the focus on the correlation between the texture topography and the slip length, a measure of drag reduction. This research discusses how it is virtually impossible for a superhydrophobic surface with a single-scale texture to achieve a large slip length without compromising the robustness against external perturbations. The current study then presents the key hypothesis – a hierarchically-textured superhydrophobic surface can achieve both large slip length and high robustness simultaneously. The current study validates the said hypothesis through empirical study of the reduction in drag in laminar and turbulent regime, using rheometry and particle image velocimetry respectively. Final part of the dissertation gives an outlook on the future work that is necessary for further improvement of this field of wetting science.