Mechanisms of Contact Nucleation: Insight from Laboratory Experiments

The presence of ice in mixed phase clouds occurs at temperatures too warm for homogeneous freezing, requiring a foreign catalyst to initiate heterogeneous freezing through one of 3 modes, deposition, immersion, or contact. Mechanisms of ice contact nucleation in the atmosphere are reviewed in light of recent experiments. Contact mode has the capability to catalyze ice formation at temperatures where other modes cannot. Previous work has demonstrated the difference in freezing behaviors between contact and immersion mode, but the aerosol-droplet interactions were poorly quantified. Recent work has been able to compare the two modes with more constrained studies, allowing a closer examination of the physical mechanisms.

The apparent enhancement of contact freezing temperatures over immersion does not yet have a commonly accepted theory. The prevailing theories are: 1)Subcritical ice embryos form as aerosols approach droplets which cause freezing upon immersion (Cooper), 2)As adsorbed water molecules reorient to be part of the bulk liquid, their transition results in a transient higher free energy zone which lowers the nucleation barrier (Fukuta), 3)Line tension and the orientation of crystal plane growth with the contact line facilitates nucleation (Djikaev & Ruckenstein).

We will also discuss the possibility that contact mode is really just modified immersion mode with some property unaccounted for, but the physical mechanisms are the same. We will use our data and others to compare these three extensions to classical nucleation theory, and discuss further experiments and data needed to support the theories.