Transient dynamics in drop impact on a superheated surface

When a drop impinges on a superheated surface, a Leidenfrost vapor layer forms between the drop and the surface. Transient dynamics of the layer can play a critical role for cooling in power plants but is not fully understood. Here we successfully visualize transient dynamics of the layer using ultrafast x-ray imaging. We reveal that a vapor disk with a homogeneous thickness, developed during drop impact, grows in thickness following the Fourier law. At a certain thickness (12 +/- 2 mu m in this study) of the vapor disk, ripples generate at its periphery due to capillary waves, resulting in significant enhancement of drop vaporization.