Enhancement of air stability of superhydrophobic surface using air replenishment methods

Abstract

Reduction of frictional drag by using superhydrophobic (SHPo) surfaces has gained significant attention due to its potential applications. The presence of an air plastron, which refers to a pocket of trapped air between micro/nano structures of a submerged SHPo surface, plays a crucial role in achieving valid drag reduction effect. However, several factors can deplete the air plastron, leading to deterioration in drag reduction performance. In this study, two feasible solutions are proposed to address the depletion problem of air plastron. First, a surface air injection method is proposed, which can be used as a controllable strategy under partial replenishment conditions where the replenishment rate of air is smaller than the depletion rate. An air injection layer is added to a ridged multi-layered SHPo (ML-SHPo) surface to supply air through the surface. Dynamic behaviors of the air plastron on the ML-SHPo surface are directly visualized using synchrotron X-ray imaging technique. The temporal evolution of the depletion process on the ridged ML-SHPo surface is monitored under laminar flow conditions to gain insights into the underlying basic physics of enhanced air stability induced by surface air injection. Second, a solar-powered method utilizing photothermal effect of a carbon-base materials is proposed to replenish air plastrons on superhydrophobic surfaces, and its practical applicability is experimentally checked. To enhance the photothermal effect, the surface is coated with carbon nanotubes (CNTs), which possess nanochannels that facilitate the generation of nano- and microbubbles. The formation of these bubbles on the surface is directly observed using X-ray and optical imaging techniques under no-flow conditions. To understand the mechanism of bubbles generation, the photothermal effect on the CNT-coated surface is quantitatively analyzed. Solar irradiation gives rise to high surface temperature of the surface. This high temperature might decrease the air solubility of water surrounding the surface due to the temperature gradient.