Effects of pressure gradient on stability and drag reduction of superhydrophobic surfaces

Air layers in microstructures of submerged superhydrophobic (SHPo) surfaces enable liquids to slip over these surfaces, thereby inducing drag reduction. However, the instability of entrapped air hinders the utilization of such surfaces in practical applications. Pressurization, air diffusion, and condensation have been regarded as the main mechanisms of the instability. Here, the pressure gradient is proposed as another mechanism that has to be considered for application in marine vehicles and channel surfaces. SHPo surfaces with differently sized ridges are fabricated, and partitioned structures are adopted for the ridges to regulate their length. The pressure gradient condition results in the deformation of air–water meniscus in the transparent ridge structures until air in these structures has been depleted. The critical pressure gradient for air depletion is measured according to the ridge size. Ridges with small widths and lengths sustains air entrapment even at high pressure gradient conditions, and they enhance the drag reduction efficiency by suppressing the deformation of air–water meniscus. Theoretical models are also derived to demonstrate the stability condition and the performance of drag reduction based on the structural sizes of ridges.