Vertical impact of a water jet on a hot plate: From a growing drop to spray formation

In this paper, we experimentally investigate the impact of a submillimetric water jet on a horizontal surface heated well above the "static" Leidenfrost temperature of water. We observe the transition from a regime where a single drop grows at the impingement point to a regime of spray formation. The main control parameter appears to be the jet Weber number (We). The first regime persists until We less than or similar to 30 whereas the spray formation occurs for We greater than or similar to 40. Surprisingly, we found no influence of the hot plate's temperature on the reported phenomena. We particularly focus on the second regime, where the liquid jet spreads on the plate, forming a liquid sheet that eventually lifts off and breaks into droplets. We characterized this regime by the radius r(c) of the liquid sheet when it is still in contact with the plate and the angle of ejection theta of the droplets. We further examine the ejected droplets by characterizing their speed and sizes. Simple models are proposed to predict the dependencies and order of magnitudes of r(c) and theta through the establishment of scaling laws. We also aim to predict the critical Weber number at which the transition between the two regimes occurs. Our models show reasonable agreement with our experimental data. Finally, we compare the energy transferred from the jet to the droplets with results reported in the literature for impacts on unheated surfaces, finding a difference of nearly a factor of 2.