Abstract

This talk will review the drag-reduction capabilities of riblets and superhydrophobic surfaces, with particular emphasis on the physical mechanisms at play. Riblets are a special type of roughness made up of small surface grooves aligned in the direction of the flow. Textured superhydrophobic surfaces, when immersed in water, can entrap pockets of air, so that a water stream can effectively slip over them. In both cases, the reduction of friction is proportional to the texture size, for vanishingly small sizes. At larger sizes, different phenomena appear, degrading the performance and limiting the range of sizes of technological interest. We will discuss both the drag-reducing and the drag-degrading mechanisms, and propose coatings with directional porosity as an alternative to riblets.