A Comprehensive Model of Snow Crystal Faceting

Abstract

Crystal faceting can emerge via two broad physical mechanisms: anisotropic attachment kinetics on growing crystals and anisotropic surface energies on near-equilibrium crystals. For the case of the ice/vapor system, anisotropic attachment kinetics is the dominant faceting mechanism, while the possible occurrence of equilibrium faceting has been debated for many decades. In this investigation we examine ice/vapor faceting at low supersaturations over the temperature range -15C<T<0C, where evidence of a roughening transition has been previously reported. Our findings indicate that a comprehensive attachment kinetics model can explain all the experimental data to date, while assuming an essentially isotropic surface energy (which is supported by other considerations). Specifically, our kinetic model naturally explains the observed disappearance of prism faceting on slowly growing ice crystals in vacuum at T>-2C, thus suggesting that snow crystal faceting is caused by anisotropic attachment kinetics even at extremely slow growth rates.