Impact and bouncing of Leidenfrost Jets

The Leidenfrost effect consists of a liquid droplet surfing over its own evaporating vapor layer. It is now widely studied, essentially because of its analogy with a drop resting on a perfectly super-hydrophobic substrate. This effect has also been evidenced in systems different from the original Leidenfrost droplets: for example, drops over granular materials, sublimating solids over a heated solid substrate, or a droplet over a heated liquid. Surprisingly, the case of liquid jets has been studied solely for macroscopic jets impinging on heated plates at normal incidence to characterize the heat transfer and subsequent cooling of the plate at the contact boiling transition. In this study, we show for the first time that the Leidenfrost effect exists for millimetric impinging jets. The influence of several parameters, jet radius and velocity, incident angle and substrate temperature are experimentally investigated. We mainly focus on the minimal thickness of the vapor cushion and on its typical area over which the jet bounces on its own vapor layer. We also show that a directed percolation-like transition appears just before the jet enters contact boiling on the heated substrate.