Bouncing off the Walls: The Influence of Gas-Kinetic and van der Waals Effects in Drop Impact

A model is developed for liquid drop impact on a solid surface that captures the thin film gas flow beneath the drop, even when the film's thickness is below the mean free path in the gas so that gas kinetic effects (GKE) are important. Simulation results agree with experiments, with the impact speed threshold between bouncing and wetting reproduced to within 5%, while a model without GKE overpredicts this value by at least 50%. To isolate GKE, the pressure dependence of the threshold is mapped and provides experimentally verifiable predictions. There are two principal modes of contact leading to wetting and both are associated with a van der Waals driven instability of the film.