  The macroscopic dynamics of a droplet impacting a solid is crucially
determined by the intricate air dynamics occurring at the vanishingly small
length scale between droplet and substrate prior to direct contact. Here we
investigate the inverse problem, namely the role of air for the impact of a
horizontal flat disk onto a liquid surface, and find an equally significant
effect. Using an in-house experimental technique, we measure the free surface
deflections just before impact, with a precision of a few micrometers. Whereas
stagnation pressure pushes down the surface in the center, we observe a lift-up
under the edge of the disk, which sets in at a later stage, and which we show
to be consistent with a Kelvin-Helmholtz instability of the water-air
interface.
