  Phase change under reduced environmental pressures is key to understanding
liquid discharge and propulsion processes for aerospace applications. A
representative case is the sessile water droplets exposed to high vacuum, which
experience complex phase change and transport phenomena that behave so
differently than that under the atmosphere. Here, we demonstrate a previously
unexplored aspect of the mechanism governing icing droplet self-launching from
superhydrophobic surfaces when exposed to low pressures (~100 Pa). In contrast
to the previously reported recalescence-induced local overpressure underneath
the droplet that propels icing droplet self-jumping, we show that the
progressive recalescence over the free surface plays a significant role in
droplet icing and jumping. The joint contribution of the top-down vaporization
momentum and bottom-up local overpressure momentum leads to
vaporization-compression-detaching dynamics of the freezing droplets. We
delineate the jumping velocity of the icing droplet by analyzing droplet
vaporization mediated by freezing and substrate structuring, and reveal jumping
direction coupled with the spatially probabilistic ice nucleation. Our study
provides new insights into phase change of supercooled droplets at extreme
conditions seen in aerospace and vacuum industries.
