Three-dimensional aspects of sidebranch formation during the growth of snow crystals

The nature of sidebranching in the facetted regions of snow crystals is investigated. It is shown that during the growth of a snow crystal, an initial sidebranch-pair forms after step bunching occurs on the leading prism planes, followed by step bunching on the adjacent flat basal face, producing a macrostep between the main branch and sidebranch feature. A Michelson interferometer–reflecting microscope setup was used to in-situ observe the three-dimensional morphology of a growing snow-crystal branch in a diffusion chamber. The macrostep on the basal plane was found to occur in the cases of simple facet growth, repetitive facet growth, and rounded tip growth, with the rounded tips forming at higher supersaturations. Thus, snow-crystal sidebranching involves a separation of the supply route of diffusing molecules on basal plane from the tip of the main-branch. Moreover, the sidebranches would fully sprout when the supersaturation increased, at which time the facetted branch tip abruptly narrowed and became rounded. These results indicate important roles of both the surface diffusion of water molecules and step migration for dendritic morphological instability on snow crystals.