How air deforms the free surface just before disk impact on a liquid bath

When a flat disk impacts onto a liquid bath, a layer of air is trapped between the disk and the free surface, a phenomenon known as air cushioning. The air layer is pushed out radially at increasing speeds, causing the water surface to be lifted up towards the approaching disk. This qualitative observation is traditionally ascribed to Bernoulli suction occurring in the low-pressure region created by the large air velocities in the gap. Here, by means of a novel high-speed imaging technique that uses the free surface as a mirror, we quantitatively measure the time evolution of the free surface profile. We show that, whereas the manner in which the surface below the center of the disk is pushed down is consistent with potential theory, this is not the case for the elevation of the free surface below the disk's edge. Instead, the surface lifting appears to be initiated by a Kelvin-Helmholtz instability occuring under the edge of the approaching disk.