Maintenance of air layer and drag reduction on superhydrophobic surface

Hydrophobic surface for drag reduction on marine vehicles and structures has been proposed for many years. However, the drag reduction effect has been found to be unstable under high flow speed/pressure conditions because of the destruction of air-water interface. In this paper, an air layer was maintained experimentally by continuous air injection on hydrophobic surfaces. Good hydrophobicity was found to benefit the spread of air bubbles and the formation of a wide and flat air layer on solid surfaces. Based on this recognition, a method combining air injection and surface hydrophobicity adjustment was proposed to maintain the air layer. Through flow field analyses, it has been found that the roughness of irregular micro-structures on superhydrophobic surfaces became dominant and the frictional drag was increased in the Wenzel state. However through air injection, the Cassie state was able to be recovered and the slip could also be re-formed at the solid surface. Because of the slip effect, vortices and shears in the turbulent boundary layer were restrained, and a drag reduction up to 20% was obtained in the study. This method was promising to maintain a robust air-water interface on superhydrophobic surfaces and sustain the drag reduction effect in engineering applications.