  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.
