Mechanism Study About The Wettability And Flow Drag Reduction On Superhydrophobic Surface

The wettability is one of the key features of solid surface, usually the contact angle of droplets on the surface is used to measure wettability, when the contact angle is greater than 150°, this surface is called super-hydrophobic surface. In report, the chemical energy and roughness of surface is the key factor to decide the contact angle. 120°is the limit if only depend on the chemical process for smooth surface. Therefore, the microstructure on the surface is the key factor to get superhydrophobic surface. With the development of microprocessing technology, man-made superhydrophobic surface is possible. Because of its broad application, the wettability and application of superhydrophobic surface become a hot research.In this paper, the method of micro-etching and chemical processing on surface is used to manufacture the super-hydrophobic surface with a variety of micro-nano-structure. The wettability is studied in some special conditions like under-water pressure and condenation. In the experiment of under-water pressure, the size of gap of the micro-structure determine its ability of bearing pressure under water, through the survey of critical contact angle. This experiment proves that the ability of nano-structured super-hydrophobic surface bearing pressure under water is better than those of two tier structured surface and micro-structured surface. In the experiment of condensation, the third contact state of condensing was discovered which is different from the Wenzel and Cassie state. A new formulation to predict the apparent contact angle of the third state is proved. Through the observation of condensing droplets, the contact line is discovered which is the symbol of the third state and the key reason of that the droplet can't roll off quickly.For the application of super-hydrophobic surface about drag reduction, this paper adopts macro-model (k-εmodel) and micro-model (LES model) to simulate the flow on super-hydrophobic surface. By the macro-model, the influence to the effect of drag reduction with different Reynolds numbers, different diameters, different gas-liquid contact area and different micro-structure are researched. It is found that in turbulent flow, there is a critical Reynolds number. When the Reynolds number of flow is larger than the critical Reynolds number, the flow shows drag reduction and vice versa. When the Reynolds number is same, the effect of drag reduction is enhanced with the decrease of diameter and the increase of gas-liquid contact area. The effect of drag reduction of streamwise micro-structure is greater than spanwise micro-structure. By the micro-Model, the transition location and coherent structure of turbulence are investigated in ordered to find the mechanism of turbulent drag reduction. The simulation shows that the super-hydrophobic surface can't cause significant change of turbulent transition location and the second vortex in the turbulent coherent structure is the key factor to cause the drag reduction.