ID 257

"Magic carpet" breakup of a drop impacting onto a heated surface under reduced pressure

 

Abstract:

Drop impact onto a hot substrate results in a multitude of different drop outcomes, depending on the temperature of the substrate and the impact parameters: drop deposition, intensive heterogeneous nucleation, thermal atomization, transition boiling and finally drop rebound for temperatures above the Leidenfrost condition. In the present experimental study this parameter space has been extended to conditions of reduced ambient pressure (1 - 100 kPa), which are relevant to space applications. For reduced ambient pressure, impact onto a superheated smooth substrate with small impact velocities (0.1-0.46 m/s) has been captured using a high-speed camera. Under these conditions two new modes of drop outcome, tiptoeing and magic carpet breakup, have been identified, occurring for conditions between the film boiling regime and the deposition regime. These new modes involve direct contact between the drop and the substrate, resulting in fast intensive evaporation, leading to strong drop deformation, backlash and further disintegration. A detailed regime map has been assembled, which indicates the type of drop outcome for different pressures and wall temperatures. Finally, a hypothesis on the mechanism of magic carpet breakup and an analytical model based on classical bubble growth theory correlated with drop kinematics is presented; its agreement with the experiment regarding the bubble burst time is very good.