Boiling regimes of impacting drops on a heated substrate under reduced pressure

摘要

We experimentally investigate the boiling behavior of impacting ethanol dropson a heated smooth sapphire substrate at pressures ranging from P = 0.13 bar toatmospheric pressure. We employ Frustrated Total Internal Reflection (FTIR)imaging to study the wetting dynamics of the contact between the drop and thesubstrate. The spreading drop can be in full contact (contact boiling), it canpartially touch (transition boiling) or the drop can be fully levitated(Leidenfrost boiling). We show that the temperature of the boundary betweencontact and transition boiling shows at most a weak dependency on the impactvelocity, but a significant decrease with decreasing ambient gas pressure. Astriking correspondence is found between the temperature of this boundary andthe static Leidenfrost temperature for all pressures. We therefore concludethat both phenomena share the same mechanism, and are dominated by the dynamicstaken place at the contact line. On the other hand, the boundary betweentransition boiling and Leidenfrost boiling, i.e. the dynamic Leidenfrosttemperature, increases for increasing impact velocity for all ambient gaspressures. Moreover, the dynamic Leidenfrost temperature coincides forpressures between P = 0.13 and P = 0.54 bar, whereas for atmospheric pressurethe dynamic Leidenfrost temperature is slightly elevated. This indicates thatthe dynamic Leidenfrost temperature is at most weakly dependent on the enhancedevaporation by the lower saturation temperature of the liquid.