Bouncing-to-Merging Transition during Droplet Impact on Heated Subcooled Liquid Film

Droplet impact on mildly heated liquid films is important in applications such as cooling and printing. However, most previous studies on heated substrates have focused on superheated conditions, in which the substrate temperature exceeds the liquid saturation temperature, and evaporation strongly alters the impact dynamics. In this study, we experimentally investigate droplet impact on heated subcooled films, where the film temperature remains below saturation, with particular emphasis on how heating modifies the critical conditions for the transition from noncoalescence to coalescence outcomes. Regime maps obtained at different temperatures are used to characterize these transitions. We further derive a scaling relation for interfacial gas layer drainage that incorporates phase-change effects. The combined scaling analysis and experiments with different liquids reveal the key physical mechanisms governing the critical impact speed required for achieving complete coalescence on mildly heated films.