From levitation to breakup: Leidenfrost droplet impacts on liquid films

Recently, researchers have explored the dynamic Leidenfrost process, where a droplet impacting a dry substrate heated above a critical temperature levitates due to vapor layer formation at the interfacial gap. In applications such as spray cooling, droplets may encounter surfaces that have already been wetted. Therefore, it is imperative to examine dynamic Leidenfrost for droplet impacts on liquid-covered substrates. This talk explores several key differences in the Leidenfrost process when a droplet impinges on a heated liquid versus a heated solid surface. In experiments, a room-temperature ethanol droplet was impacted on a nonadecane film heated above the droplet’s saturation temperature. The impact outcomes, plotted in a film temperature-Weber number map, were classified into three regimes: (i)No Leidenfrost, (ii)Regular Leidenfrost, and (iii)Leidenfrost with breakup. These results show that the critical film temperature required for transition from No Leidenfrost to Regular Leidenfrost increases with the Weber number, following a power-law relationship. Furthermore, an increase in liquid film thickness promotes this transition. At higher Weber numbers, the Leidenfrost impacts exhibit a subsequent breakup, i.e., the outcome transitions to Leidenfrost with breakup. This transition is insensitive to film temperature but is inhibited by increased film thickness. We will discuss the competing mechanisms and scaling laws involved in the transitional boundaries.