Magic carpet breakup of a drop impacting onto a heated surface in a depressurized environment

摘要

Drop impact onto a hot substrate leads to various hydrodynamic outcomes depending on the substrate temperature and the impact parameters: deposition, drop dancing, thermal atomization, and finally rebound at temperatures above the dynamic Leidenfrost condition. In the present experimental study this parameter space has been extended to conditions of a depressurized environment (1-101 kPa), which are relevant to space applications such as a reentry spaceship or a rocket engine. The impact onto a superheated smooth substrate with relatively small impact velocities (0.1-0.46 m/s) has been captured using a high-speed video camera system. Under these conditions two new modes of drop outcome have been identified; magic carpet breakup, in which the drop splashes immediately after the impact, leading to extensively large lateral deformation and upward drop movement, and tiptoeing, in which the drop bounces back with a time scale similar to capillary rebound (so-called rebound) accompanied by a significant fluctuation of the outer shape and secondary droplet generation triggered by successive attach-and-detach cycles. These new modes occur at a temperature range between the deposition regime and the capillary rebound regime. Detailed regime maps indicating the type of drop outcome for different pressures and substrate temperatures have been constructed. Moreover, a hypothesis on the mechanism of magic carpet breakup is presented and a theoretical model, in which a classical bubble growth model for pool boiling is extended to bubble growth in a drop and correlated with drop kinematics, is developed. The numerical solution is in good qualitative agreement with the experimental data for the bubble burst time and for the change in drop velocity. (C) 2019 The Authors. Published by Elsevier Ltd.