Wetting properties of surfaces and drag reduction

In this Chapter, the flow of simple incompressible Newtonian liquids across surfaces having mixed wettability arising from surface texture is considered. This commences with ideas relating to droplet and molecular mobility on smooth surfaces and progresses to include gas and vapour interfaces introduced by superhydrophobic/oleophobic surfaces and the Leidenfrost effect. Ideas developed are also applicable to self-healing slippery liquid impregnated/infused (porous) surfaces (SLIPS or LIS). Subsequently, there is a brief review of the literature focused on drag reduction by superhydrophobic surfaces and related ideas involving bubble and air layer drag reduction. The Chapter develops concepts of slip at surfaces, friction for internal flow through objects and drag for external flow across surfaces. It includes aspects of laminar and turbulent flow, and small and large scale objects with a focus on apparent slip arising from plastrons and surface-retained lubricating fluids on the surfaces of pipes, cylinders and spheres as model systems.