Plastron replenishment on superhydrophobic surfaces using bubble injection

Abstract

While the air lubrication by bubble injection and superhydrophobic (SHPo) surfaces have been investigated vigorously for flow control, for example, underwater drag reduction, further advancement seems to be delayed. For the former, large air flow rate is required for the meaningful performance, and furthermore, the injected bubbles do not stay over the surface willingly. Depletion (diffusion) of the trapped air pockets on the SHPo surface is a critical issue for the latter. In the present water-tunnel experiments, we show that the above-mentioned challenges can be successfully overcome by combining the two methods; that is, the plastron on SHPo surfaces can be replenished in turbulent flows with a very small amount of air, even after the surface is fully wetted. To analyze the phenomena, the bubble–plastron interaction is visualized and quantified while introducing bubbles over the SHPo surfaces (with random roughness or longitudinal grooves) in the turbulent boundary layer flow of ReL=0.3−1.1×106. The plastron on SHPo surfaces with longitudinal grooves is retained in a film-like shape with a quite smaller amount of air than that with random roughness. By quantifying the light intensity from the surface, we suggest a scaling relation between the effective plastron thickness and surface light intensity, which would serve as a criterion for the successful plastron replenishment. Finally, the morphology of the plastron is classified into different regimes, depending on the Reynolds number, air flow rate, and surface roughness types.