Transition of slip mode at gas–liquid interface formed by wettability steps on an superhydrophobic surface

The gas–liquid interface (GLI) over a submerged superhydrophobic surface (SHS), at which the flow slips, is key to the reduction of frictional drag. In the study reported here, by patterning a hydrophilic test plate with a specifically designed superhydrophobic region, a wettability step is successfully used to induce millimeter-scale anisotropic slip at the GLI. With an increase in the height of the GLI, or an expansion of its size, the local slip length increases to become infinite in the center region of the GLI, i.e., the slip boundary condition gradually transforms from a finite slip mode to a hybrid mode. The effective slip length also increases with increasing height and size of the GLI. This considerably greater slip length at the GLI formed by wettability steps over an SHS can be exploited to design SHSs with enhanced slip to improve drag reduction in ocean engineering.