Growth Kinetics of Ice Single Crystal from Vapour Phase and Variation of its Growth Form

Abstract

First we briefly review the experimental studies on growth forms and mechanisms of ice crystals from the vapour, i.e. snow crystals. Then we discuss why the basic habit of ice alternate three times with decreasing temperature: from plates (A) to columns (B) at -4°C, to plates (C) at -10°C, and to columns (D) at -20°C∼-35°C. The surface of ice just below 0°C is covered with a quasi liquid layer, whose thickness or coverage 9 decreases with falling temperature, and therefore the growth mechanism of a crystal face changes also as follows: I) Vapour-Quasi Liquid-Solid-Mechanism (∂<1), II) Adhesive Growth on a surface strongly adsorbed by H2O molecules themselves (0.02<∂<l), and III) Two Dimensional Nucleation Growth on a surface with low eigen adsorption (∂<0.02). The type of surface structure and consequently the growth mechanism depend on the surface orientation and temperature. Therefore the complicated habit change is caused mainly by the combination of surface growth mechanisms of the {0001}- and {1010}-face. Furthermore diffusion field surrounding a crystal contributes to the third habit conversion from plates (C) to columns (D) in dependence on supersaturation, and to the formation of the marked habits with extreme axis ratio (c/a»1, «1). Finally we discuss why the transition from polyhedral to skeletal to dendritic morphology occurs with increasing supersaturation. This morphological instability of polyhedral crystal is due to the inhomogeneity in supersaturation over its surface. In order to pursue the problem, we must self-consistently solve three dimensional diffusion field surrounding a polyhedral crystal and growth kinetics at the surface with supersaturation inhomogeneity.