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Floating on Air
A drop impacting on a solid surface deforms before the liquid makes contact with the surface. We directly measure the time evolution of the air layer profile under the droplet using high-speed color interferometry, obtaining the air layer thickness before and during the wetting process and the volume of the entrained droplet. This volume shows a maximum as function of the impact velocity. We physically explain this maximum as a balance between capillary and inertial effects. The experiments are complemented by numerical simulations, based on potential flow for the impacting droplet and a lubrication theory for the gas layer in between the droplet and the surface, and by scaling laws which we derived analytically. The work is then extended in various directions : For the drop impact on a hot surface heated above the liquid’s boiling point, the droplet either immediately boils when it contacts the surface (‘‘contact boiling’’), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (‘‘gentle film boiling’’), or both forms the Leidenfrost layer and ejects tiny droplets upward (‘‘spraying film boiling’’). We also look at the maximum spreading of impacting droplets