Fabrication of Nanowire Based Dewetted Surfaces: Improvement of Underwater Stability and Development of Superhydrophobicity Regeneration Property

Abstract

Surfaces should have a high roughness and low surface energy for dewetting water and repelling various liquids. Therefore studies for fabricating optimized micro/nano structures and controlling surface chemistry are essential for developing dewetted surfaces such as superhydrophobic surfaces and slippery liquid infused porous surfaces (SLIPS). Also stability problems of both superhydrophobic surfaces and SLIPS should be solved for practical applications of dewetted surfaces in diverse fields. In the present thesis, therefore, diverse facile methods for the fabrication of various superhydrophobic surfaces and SLIPS are introduced and new approaches for enhancing underwater stability of superhydrophobic surface were proposed. Furthermore, a regenerative property of underwater superhydrophobicity using photoelectrochemical hydrogen generation system is firstly developed. Finally, a drag reducing property, a representative application of dewetted surfaces, of superhydrophobic surfaces and SLIPS is analyzed. In chapter 1, research motivation and theoretical backgrounds about superhydrophobic surfaces, SLIPS and drag reduction are introduced. In chapter 2, superhydrophobic surfaces with quasi-aligned W18O49 nanowire (NW) arrays are fabricated using a simple thermal evaporation and surface chemistry modification method. The fabricated superhydrophobic W18O49 NWs surface shows reliable stability in submerged underwater conditions, exhibiting silvery surfaces by total reflection of water layer and air pockets. The stability of superhydrophobicity in underwater conditions decreases exponentially as hydrostatic pressure applied to the substrates increases. In addition, variations in stability are investigated according to changes of the surface energy of W18O49 NW arrays. As surface energy decreases, the underwater stability of superhydrophobic surface increases sharply. Based on these results, the models explaining tendencies of underwater superhydrophobic stability resulting from hydrostatic pressure and surface energy are designed. In chapter 3, to overcome limited lifetime of underwater superhydrophobicity, a novel method for regenerating a continuous air interlayer on superhydrophobic ZnO nanorod/Si micropost hierarchical structures (HRs) via the combination of two biomimetic properties of natural leaf: superhydrophobicity from the lotus leaf effect and solar water splitting from photosynthesis is introduced. The designed n/p junction in the ZnO/Si HRs allows for highly stable gas interlayer in water and regeneration of the underwater superhydrophobicity due to the unique ability of the surface to capture and retain a stable gas layer. Furthermore, a model to determine the optimum structural factors of hierarchical ZnO/Si surfaces that aid the formation of an air interlayer to completely regenerate the superhydrophobicity is designed. This model satisfactorily predicts the regeneration of underwater superhydrophobicity under various experimental conditions. In chapter 4, the underwater stability of superhydrophobic surfaces is enhanced by optimizing surface structures using SiC/Si interlocked structures. These structures have an unequaled stability of underwater superhydrophobicity and enhanced drag reduction capability, with a lifetime of plastron over 18 days and maximum drag reduction ratio of 56%. Furthermore, through photoelectrochemical water splitting on a hierarchical SiC/Si nanostructure surface, the limited lifetime problem of air pockets is overcome by refilling the gas layer, which also provides continuous drag reduction effects.