Numerical investigation of air mediated droplet bouncing on flat surfaces

A liquid droplet can bounce off a flat substrate independent of surface wettability if the impact occurs at low velocities, i.e., We of less than seven. In this case, the droplet spreads on a sub-micrometer air layer and rebounds subsequently without any direct contact with the surface. We have numerically investigated the process of air layer formation beneath the droplet. The numerical simulations are validated using experimental results available in the literature based on morphology of the droplet interface and thickness of the air layer. Numerical results revealed that the formation of a high pressure zone at the center of impact deforms the droplet to a kink shape at the moment of impact. The deformation leads to displacement of high pressure zone from center to kink edge of the droplet interface. Further investigation of pressure and velocity of air beneath the droplet divulged that high pressure region at the kink edge suppresses air flow at the inner region while accelerating flow at the outer region. In addition, it is demonstrated that fluid flow at the kink edge where droplet interface has the minimum distance from the substrate resembles Couette flow. It is demonstrated that the deformation of droplet along with displacement of high pressure region from the center to kink edge are responsible for stabilizing the air layer beneath the droplet and consequently spreading and receding of droplet over a thin air cushion.