Drop impact onto a heated surface in a depressurized environment

Abstract: We investigated the impact of a droplet on a heated surface in a depressurized environment, with a particular focus on the unique outcome observed under these conditions: magic carpet breakup. This phenomenon, first reported by Hatakenaka et al. [Int. J. Heat Mass Transf., 145, 118729(2019)], describes an explosive, widespread rebound of the drop. A newly-developed thin-film Fe-Ni thermocouple array with $20~\mathrm{nm}$ thick layers unveiled surface temperature during the magic carpet breakup. This high-speed surface temperature measurement was synchronized with total internal reflection (TIR) imaging. The bubble growth and the subsequent pressure release eventually led to an explosive rebound of the drop. The bubble grew almost linearly with a slight acceleration, significantly different from the asymptotic growth observed for the bubble on a superheated substrate in a liquid pool. The growth rate remained low even when the surface was superheated to delta T = 60 K, but it increased sharply afterward. The surface temperature decreased sharply as the measuring junction became wet but did not recover immediately after the ring-shaped contact region passed. Remarkably, the study captured liquid microdroplets forming at the receding contact line of a growing bubble via a side-view camera and TIR. The surface temperature remained relatively low due to the evaporation of microdroplets. The threshold for microdroplet formation is related to the bubble growth rate.