Browsing by Author "Choi, Wonjae"
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Item Development of Scalable and Hierarchical Superhydrophobic Surfaces for Drag Reduction(2017-12) Pillutla, Venkata; Choi, WonjaeThis 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.Item Influence of Textural Statistics on Drag Reduction by Scalable, Randomly Rough Superhydrophobic Surfaces in Turbulent Flow(American Institute of Physics Inc., 2019-04-22) Rajappan, A.; Golovin, K.; Tobelmann, B.; Pillutla, Venkata; Abhijeet; Choi, Wonjae; Tuteja, A.; Mckinley, G. H.; Pillutla, Venkata; Abhijeet; Choi, WonjaeWe investigate the influence of statistical measures of surface roughness on the turbulent drag reduction (DR) performance of four scalable, randomly rough superhydrophobic (SH) textures. Each surface was fabricated using readily scalable surface texturing processes to generate a random, self-affine height profile on the base substrate. The frictional drag on all four SH surfaces was measured when fully submerged in shear-driven turbulent flow inside a bespoke Taylor-Couette apparatus at Reynolds numbers in the range 1 × 10⁴ ≲ Re ≲ 1 × 10⁵. An "effective" slip length quantifying the overall drag-reducing ability for each surface was extracted from the resulting Prandtl-von Kármán friction plots. Reductions in the frictional drag of up to 26% were observed, with one of the hierarchically textured surfaces exceeding a wall shear stress of 26 Pa (corresponding to a Reynolds number Re ≈ 7 × 10⁴) before the onset of flow-induced plastron collapse. The surface morphology of each texture was characterized using noncontact optical profilometry, and the influence of various statistical measures of roughness on the effective slip length was explored. The lateral autocorrelation length was identified as the key textural parameter determining the drag-reducing ability for randomly rough SH textures, playing the role analogous to the spatial periodicity of regularly patterned SH surfaces. A large autocorrelation length, a small surface roughness, and the presence of hierarchical roughness features were observed to be the three important design requirements for scalable SH textures for optimal DR in turbulent flows. © 2019 Author(s).Item A Stacked Polymer Film for Robust Superhydrophobic FabricsYoo, Youngmin; You, Jae Bem; Choi, Wonjae; Im, Sung GapA robust superhydrophobic fabric was achieved by depositing a stacked polymer film composed of a poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (p(V4D4)) layer and a poly(1H,1H,2H,2H-perfluorodecylacrylate) (p(PFDA)) layer. The polymer film was deposited by initiated chemical vapor deposition (iCVD), a solventless process that allows conformal coating of the stacked polymer film on various micro-structured substrates. The two polymeric layers most likely formed a covalent bonding at their interface, and thus the stacked polymer film was characterized by both strong hydrophobicity and enhanced mechanical robustness originated from highly cross-linked p(V4D4) and p(PFDA). The surface topography of superhydrophobic coating was systematically tunable by controlling the operating parameters of iCVD process and a hierarchical structure was obtained by a simple one-step iCVD process. The film was also highly transparent in the wavelength range from 380 nm to 780 nm. Fabrics coated with this stacked polymer film displayed chemical robustness even after exposure to different chemicals including acetone, toluene, H2SO4, and KOH. The fabric also maintained its water repellency even after 20 000 cycles of the abrasion test and after 75 cycles of the laundry test.