Droplet dynamics over a super hydrophobic surface

Super hydrophobic surfaces are surfaces having a set of tiny protrusions (from micro to nano scale) capable of producing large “apparent” contact angles approaching 180°. Liquid droplet cannot wet these surfaces. Droplets can glide on the surface with a negligible friction. Some phenomenological models were proposed earlier to predict conditions for a droplet to be maintained at such a state, without success. In this work, all interactions forces between droplet and wall are revisited. This includes forces exerted by the wall, either in the normal direction (surface tension and hydrostatic) or the tangential direction (frictional shear). This also includes drag force of surrounding air as the droplet moves. For all these forces simplified phenomenological models were proposed, based on either order of magnitude analysis or detailed 3D simulation to investigate fluid behavior in such complicated flow fields. Obtained simple forms were successful in reproducing, at least qualitatively, observed behavior. Adjusting parameters can easily be defined and fitted on experimental data later on to improve model precision.