Dynamics of collapse of air films in drop impact.

Liquid drops hitting solid surfaces deform substantially under the influence of the ambient air that needs to be squeezed out before the liquid actually touches the solid. Nanometer- and microsecond-resolved dual wavelength interferometry reveals a complex evolution of the interface between the drop and the gas layer underneath. For intermediate impact speeds (We∼1…10) the layer thickness can develop one or two local minima-reproduced in numerical calculations-that eventually lead to the nucleation of solid-liquid contact at a We-dependent radial position, from a film thickness >200  nm. Solid-liquid contact spreads at a speed involving capillarity, liquid viscosity and inertia.