Preparation, Hydrophobic Stability And Application Research Of Biomimetic Superhydrophobic Surfaces

Biomimetic superhydrophobic surfaces are non-wetting and low adhesion, and therefore have widely applications in several fields, such as water-proof, self-clean, underwater drag reduction, micro-flow control and micro electro mechanical systems. In the present thesis, preparation, water-repellent stability and application in boiling-induced refrigeration were investigated, and the main conducted researches can be summarized as follows:(1) Based on wet etching and ultrasonic cavitation, an ultrasonic etch method was proposed for fabricating micro-and nanoscale hierarchical structures, and superhydrophobic surfaces with hierarchical roughness were successfully fabricated on several kinds of steels. Compared to regular etching, the chemical corrosion of ultrasonic etching was enhanced and modified by coupling with cavitation, which can effectively etch the grain boundaries and subgrain boundaries, and therefore special hierarchical structures can be fabricated on solid surfaces.(2) The cell model of superhydrophobic interface with arrayed pillars was established, by regarding the water-air part of the interface as a self-adaptive meniscus, the stability of Cassie interface was studied in the views of mechanics and thermodynamics. The results indicate that, the mechanical stability of Cassie interface is mainly depend on surface free energy of solid and contact line density, and influenced by the fractional area of solid and geometrical shape of pillar. During the transition from ground state to high energy state that driven by pressure, increment in potential energy results in a meniscus energy barrier, which determines the thermodynamic stability of Cassie state. Choosing raw material with low free energy, constructing hierarchical structures, minimizing microstructure scaling and giving preference to inverted trapezoidal pillars benefit the stability of Cassie interface under pressure.(3) Based on the total reflection of underwater superhydrophobic interface and vacuum technique, a new method was proposed to test the stability of superhydrophobic state, and wetting behavior of three types of superhydrophobic surfaces were investigated, and the wetting behaviours of pressure induced Cassie-Wenzel transition were discussed. The results show that, without entrapped air between superhydrophobic interface, the wetting transition of a typical surface covered by asperities has four stages:non-wetting stage, primary wetting stage, enhanced wetting stage, and complete wetting stage; the critical pressure in primary wetting stage agrees with the theoretical one, and the entrapped air can enhance the stability and wetting reversibility. During the transition, papillary microstructures adapts to external pressure by enhancing the capillary force via increasing density of three-phase contact line in the wetting process, while the columnar microstructures adapts to external pressure by increasing the curvature of meniscus that hangs between pillars, and such differences lead to the self-organized criticality of the wetting transition for the latter.(4) A theory model was estabilished to study the energy barriers during Cassie-Wenzel wetting transition driven by pressure, and the features and characteristics of the three kinds of energy barriers were discussed. The results indicate that the energy barrier during the transition involves three terms:meniscus energy barrier, capillary energy barrier and the air energy barrier. The meniscus energy barrier, which results from the dilation of meniscus and separates the Cassie state from transition state, is self-adaptive and reversible; the capillary energy barrier, which comes from resistance of capillary force around the contact line and separates the transition state from Wenzel state, is directional and dissipative; the air energy barrier, which stems from the compression of entrapped air between surface microstructures and exists only when air is entrapped, is dissipative, partly reversible and time-efficient, and makes up the main cause in inducing dewetting when the pressure is released.(5) The cavitation reaction and gas enrichment of superhydrophobic interface under vacuum atmosphere were experimentally studied. The results indicate that, due to the interfacial cavitation, the saturated vapor pressure of superhydrophobic interface is higher than that of water and therefore forms the interfacial bubble. The gas of the interfacial bubble comes from the enrichment of dissolved air, the capillary evaporation of meniscus that hang between pillars and the cavitation reaction of hydrophobic solid-water interface. Without the gas enrichment of interface, the cavitation and evaporation cannot form the macro bubble, and the cavitation reaction and the equivalent dispersion of the interface enhances as the temperature rises. And a humiditying method with zero energy consumption was proposed based on the aformentioned study.(6) The boiling behavior of hydrophobic and superhydrophobic interfaces under vacuum and its application in refrigeration were studied, the film boiling model of superhydrophobic interface was preliminary established, and a new refrigeration method was proposed. The results show that, with the increase in hydrophobicity of solid-water interface, the boiling pattern transits from nucleate boiling to film boiling; and such a boiling was the composite effects of evaporation and film boiling, which involves capillary evaporation, interface cavitation and gas enrichment. Vacuum-induced boiling can be used for refrigeration, and the refrigerating efficiency was mainly depended on the vacuum degree and superhydrophobic interface density, and theoretical energy efficiency ratio can be more than1.