Effect of surface micro/nano-structure on gas-water interface stability and flow drag reduction

The pressure-flow measurement method and flow visualization are used to study the drag reduction effect of 6 types of super-hydrophobic surfaces with different micro/nano-structure dimensions and the influence of the surface microstructure shape on the gas-water interface stability. The experimental results show that the various superhydrophobic surfaces have a certain drag reduction under laminar flow and turbulent flow conditions. At the same solid area fraction, the smaller the microstructure spacing is, the better the drag reduction effect is. The maximum drag reduction rate, which is (38.6±4.5)%, is achieved on the micro/nano hierarchical structure surface with the smallest structural spacing. The visualization experiment also found that the drag reduction rate is related to the microstructure level, microstructure size, channel flow pattern and microstructure morphology, which all have certain influence on the gas-water interface stability. The hierarchical micro/nano-structure can significantly improve the drag reduction, because the addition of the nano-secondary structure reduces the solid area fraction of the surface and improves the stability of the gas-water interface. In addition, a microchannel surface with doubly reentrant structures (umbrella structure), even though made of wettable material, can capture and sustain the air-water interface, thereby achieving superhydrophobic performance.