  Freezing of water droplets placed on the bare and superhydrophobic surfaces
of polymer wedges are studied both experimentally and computationally.
Two-dimensional numerical calculations of the transient temperature field in a
chilled polymer wedge show that the direction of heat flux from the droplet
through the thermal contact region with the wedge differs significantly from
the normal to the wedge surface. This is the physical cause of the recently
observed asymmetric cooling of the droplet. A novel approximate computational
model is proposed that takes into account the variable area of the water
freezing front in the droplet. This model gives a quantitative estimate of the
faster freezing of the droplet on the bare surface. The obtained numerical
results agree with the data of laboratory experiments. The velocity of the
crystallization front and the droplet deformation including the so-called
freezing tip formation are monitored in the experiment. The direction of the
freezing cone axis appears to be noticeably different for the cases of bare and
superhydrophobic wedge surfaces. This deviation is explained by the fact that
the direction of the freezing cone axis is controlled by the local direction of
the heat flux. For a hydrophobic wedge surface, the deviation of the freezing
tip from the vertical is smaller, because the reduced thermal contact area
reduces the influence of the heat flux direction at the wedge surface.
