Abstract:
The freezing of a sessile droplet unveils fascinating physics, characterized by the emergence of a frost halo on the underlying substrate, the progression of the liquid-ice interface,
and the formation of a cusplike morphology at the tip of the droplet. We investigate the
freezing of a volatile sessile droplet, focusing on the frost halo formation, which has not
been theoretically explored. The formation of the frost halo is associated with the inherent
evaporation process in the early freezing stages. We observe a negative evaporation flux
enveloping the droplet in the initial stages, which indicates that vapor produced during
freezing condenses on the substrate close to the contact line, forming a frost halo. The
condensate accumulation triggers reevaporation, resulting in a temporal shift of the frost
halo region away from the contact line. Eventually, it disappears due to the diffusive nature
of the water vapor far away from the droplet. We found that increasing the relative humidity
increases the lifetime of the frost halo due to a substantial reduction in evaporation that
prolonged the presence of net condensate on the substrate. Increasing liquid volatility
increases the evaporation flux and condensation occurs closer to the droplet, as a higher
amount of vapor is in the periphery of the droplet. We also found that decreasing the
thermal conductivity of the substrate increases the total freezing time. The slower freezing
process is accompanied by increased vaporized liquid, resulting in condensation with its
concentration reaching supersaturation.
