Effects of surface air injection on the air stability of superhydrophobic surface under partial replenishment of plastron

Frictional drag reduction using superhydrophobic (SHPo) surface has attracted great attention due to its potential for practical application. Air plastron (i.e., pockets) trapped between structures in a submerged SHPo surface serves a crucial role in the drag reduction effect. However, the air plastron on SHPo surface can easily be depleted by various factors, resulting in the deterioration of the drag reduction performance. This study proposed a surface air injection method to resolve the depletion problem and explored its effect on the enhancement of the air stability of SHPo surface as a controllable strategy under partial replenishment conditions in which replenishment rate is less 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. The dynamic behavior of the air plastron on the surface is directly visualized by an X-ray imaging technique. The temporal evolution of depletion length on the ridged ML-SHPo surface is monitored under laminar flows to understand the underlying basic physics of the enhanced air stability caused by surface air injection. The depletion rate of air plastrons on SHPo surfaces depended on the nondimensional air injection rate ( Cq). The dynamic behavior in the air depletion regime is closely associated with flow condition and Cq. The stability of air plastron is enhanced by 12%-87% as the Cq value increases. In addition, a simple scaling relationship between the depletion rate and Cq is proposed to predict the depletion rate of air plastrons (longevity) on ridged SHPo surfaces.