Enhanced air stability of ridged superhydrophobic surface with nanostructure

Air plastron in a superhydrophobic (SHPo) surface works as a lubricant that induces drag reduction on the surface. By air plastron, air is trapped between structures of the SHPo surface. However, air plastron is easily depleted by static water pressure or external flow conditions. Various nanostructures have been introduced to enhance the air stability of SHPo surfaces. In this study, the effects of such nanostructure on the air stability were experimentally investigated under high water pressure and flow conditions. Polyvinyl chloride solution was employed to form the nanostructure on the ridged SHPo surface. The critical pressure for the depletion of air plastron is 70% higher on SHPo surfaces introduced with the nanostructure than on surfaces without the nanostructure. Pressure drops (ΔP) in rectangular channels with the SHPo surface on the bottom side were measured to quantify the air stability under a flow condition. ΔP gradually decreases as the air plastron disappears on both SHPo surfaces. The hierarchical ridged surfaces with the nanostructure showed better air stability under static and flow conditions compared with the simple ridged surfaces without the nanostructure. The present results are helpful to understand the effects of the nanostructure on the air stability and its drag reduction mechanism.