Tip splitting in dendritic growth of ice crystals

Ice crystals growing vertically upwards in quiescent ultrapure water have been investigated at small subcoolings in the range of 0.035 <ΔT<1.0 K. At subcooling less than 0.35 K, dendritic tips split and the tip radius ${\mathit{R}}_2$ drops because two tips replace one when viewed from the basal plane. The growth velocity and the tip radius of the edge plane of the dendrite, however, are found to remain independent of time and unaffected by the morphological instabilities in the basal plane. In contrast, when the subcooling is greater than 0.35 K, the tips seem to grow with an invariant shape. The shape of drops of water that are in equilibrium with solid ice is observed to be close to a circular disk. From the shape of these drops, the anisotropy of the surface energy in the edge plane, which is an ellipse, is estimated to be about 30%, while the basal plane is found to be indistinguishable from a circle, despite the hexagonal structure of ice. We speculate here that tip splitting in ice dendrites may be a morphological instability caused by the coupling between lack of anisotropy in the basal plane and natural convection, which becomes significant at small ΔT.