Vapor layer evolution during drop impact on a heated surface

When a liquid drop impacts on a sufficiently hot surface above the boiling point, a vapor layer is formed between the drop and the surface, preventing direct contact between them and as a result levitating the drop, known as the Leidenfrost effect. Understanding the evolution of the vapor layer is largely unexplored despite its importance in estimating heat transfer in cooling systems of thermal or nuclear power plants. The side-profile visualization of the vapor layer, as absolutely required for investigating its evolution, has been however unavailable by conventional optical microscopy. In this study, by employing ultrafast X-ray phase contrast imaging, we directly visualize the profiles of the vapor layers during liquid drop impact on a hot surface and elucidate the evolution of the vapor layers during spreading and retraction of the drop as functions of impact height and surface temperature. We reveal that the evolution is governed by the propagation of capillary waves generated in retraction and the wavelength of capillary waves $\mathrm{\lambda }$ is inversely proportional to the impact height h with a relation $\propto {\frac{\sigma}{\rho h}} \propto {\mathrm{We}}^{-1}$ where We is weber number. Capillary waves that converge at the center of the vapor layers are linked to the bouncing behavior of the drop.