  Volatile drops deposited on a hot solid can levitate on a cushion of their
own vapor, without contacting the surface. We propose to understand the onset
of this so-called Leidenfrost effect through an analogy to non-equilibrium
systems exhibiting a directed percolation phase transition. When performing
impacts on superheated solids, we observe a regime of spatiotemporal
intermittency in which localized wet patches coexist with dry regions on the
substrate. We report a critical surface temperature, which marks the upper
bound of a large range of temperatures in which levitation and contact coexist.
In this range, with decreasing temperature, the equilibrium wet fraction
increases continuously from zero to one. Also, the statistical properties of
the spatio-temporally intermittent regime are in agreement with that of the
directed percolation universality class. This analogy allows us to redefine the
Leidenfrost temperature and shed light on the physical mechanisms governing the
transition to the Leidenfrost state.
