TITLE:
Does the Homogeneous Ice Nucleation Initiate in the Bulk Volume or at the Surface of Super-Cooled Water Droplets? A Review
AUTHORS:
Gianni Santachiara, Franco Belosi
KEYWORDS:
Supercooled Droplets, Homogeneous Nucleation, Nucleation Rate
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.4 No.4,
October
9,
2014
ABSTRACT: The formation of ice in clouds can occur
through primary processes, either homogeneously or heterogeneously triggered by
aerosol particles called ice nuclei, as well as through secondary processes.
The homogeneous ice nucleation process involves only pure water or solution
droplets. Homogeneous freezing is crucial for the microphysics in the formation
of high-altitude cirrus and polar stratospheric clouds, and also in the
glaciation of thunderclouds, at temperatures below about 235 K. Nucleation
rates in supercooled water have been measured using different experimental
techniques: expansion cloud chambers, water-in-oil emulsions, levitation
methods, free falling droplets, supersonic nozzles, field measurements, and
molecular dynamics simulations. An important question concerns the possibility
that the nucleation process in supercooled water can occur not only in the
interior volume of the droplet, but even at or close to its surface. Even if
there is no conclusive evidence, the majority of experimental and theoretical
results suggest that the contribution of surface nucleation increases with
decreasing radius of the supercooled droplets, and the surface (or sub-surface)
nucleation rate is prevalent for droplets with radius lower than about 5 μm.
If homogeneous freezing initiates at the droplet surface, the freezing rate
should depend on the droplet size, and even a slight contamination by molecules
within the surface layer could hamper the rate of the nucleation process.