Drag reduction with lubricant rings over a patterned liquid-infused cylinder

Liquid-infused surfaces (LISs), where a lubricant is infused within surface structures, have recently been developed for drag reduction. In the present study, by patterning alternating superhydrophobic strips and superhydrophilic strips over a rotating cylinder to produce a so-called patterned liquid-infused cylinder (p-LIC), the resulting wettability step effect leads to the formation of stable lubricant rings over the cylindrical surface and to consequent drag reduction. By increasing the height and viscosity ratio of the lubricant rings, or the width and depth of the wettability steps, the drag reduction rate can be enhanced, to a maximum value of 23.4% with the present p-LIC. The profiles of the mean azimuthal velocity at the superhydrophobic strip over the p-LIC indicate that the flow slips at the liquid–liquid interface, with the slip velocity being positively related to the drag reduction rate. These results provide a new approach to enhance the drag-reducing ability of LISs via the wettability step effect, and they can also serve as a guidance for the optimal design of drag-reducing LISs based on this approach.