  When a volatile drop impacts on a superheated solid, air drainage and vapor
generation conspire to create an intermediate gas layer that delays or even
prevents contact between the liquid and the solid. In this article, we use
high-speed synchronized reflection interference and total internal reflection
imaging to measure the short-time dynamics of the intermediate gas film and to
probe the transition between levitation and contact. We observe that the
substrate temperature strongly affects the vertical position of the liquid-gas
interface and that the dynamic Leidenfrost transition is influenced by both air
and vapor drainage (\emph{i.e}, gas drainage), and evaporation, the later
giving rise to hitherto unreported vertical oscillations of the gas film that
can trigger liquid-solid contact. We first derive scaling relations for the
height of the gas film trapped under the drop's centerline, called the dimple
height, and the minimum film thickness at short times. The former is set by a
competition between gas drainage and liquid inertia, similarly as for
isothermal impacts, while the later strongly depends on the vapor production.
The gas pressure, at the location where the minimum thickness is reached, is
determined by liquid inertia and vapor production and ultimately balanced by
the increasing interfacial curvature, determining the minimum thickness. We
show that, in the low impact velocity limit, the transient stability of the
draining gas film remarkably makes dynamic levitation less demanding than
static levitation. We characterise the vertical gas film oscillations by
measuring their frequency and monitoring their occurrence in the parameter
space spanned by surface temperature and impact velocity. Finally, we model the
occurrence of these oscillations and account for their frequency through an
hydrodynamic mechanism.
