Universal behavior of the initial stage of drop impact.

During the early stages of the impact of a drop on a solid surface, pressure builds up in the intervening thin lubricating air layer and deforms the drop. The extent of the characteristic deformation is determined by the competition between capillary, gravitational, and inertial forces that has been encapsulated in a simple analytic scaling law. For millimetric drops, variations of the observed deformation with impact velocity V exhibit a maximum defined by the Weber and Eötvös numbers: We=1+Eo. The deformation scales as V(1/2) at the low-velocity capillary regime and as V(-1/2) at the high-velocity inertia regime, in excellent agreement with a variety of experimental systems.