In 1756, Leidenfrost observed that a droplet of water deposited on a hot surface does not instantly vaporise, but remains a droplet for a considerable duration of time and slides freely across the solid surface. This is known as the Leidenfrost effect. The effect can easily be seen in the kitchen by sprinkling a few drops of water on a hot frying pan.
The water droplet remains a droplet because a layer of water at the solid-water interface vaporises and forms a cushion of air. This insulates the rest of the drop and prevents it evaporating.
The two videos below, from Vakarelski et al, show (respectively) the break-up of a leidenfrost layer and a race between falling spheres.
- Terminal velocity and drag reduction measurements on superhydrophobic spheres
G. McHale, N.J. Shirtcliffe, C.R. Evans and M.I. Newton, Appl. Phys. Lett. 94 (6) (2009) 064104
- Drag reduction by Leidenfrost vapor layers
I.U. Vakarelski, J.O. Marston, D.Y.C. Chan and S.T. Thoroddsen, Physical Review Letters 106 (21) (2011) 214501