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

TL;DR

This study experimentally explores how impacting ethanol drops behave on heated sapphire surfaces under various pressures, revealing that contact and transition boiling are primarily governed by contact line dynamics and are weakly affected by impact velocity.

Contribution

It demonstrates the influence of ambient pressure on boiling regimes and establishes a link between static and dynamic Leidenfrost temperatures, highlighting the role of contact line dynamics.

Findings

The boundary between contact and transition boiling weakly depends on impact velocity.

The boundary temperature correlates with the static Leidenfrost temperature across pressures.

The dynamic Leidenfrost temperature increases with impact velocity and varies with ambient pressure.

Abstract

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