Habits of ice grown in supercooled water and aqueous solutions

Abstract

Abstract At small supercoolings the direction of growth of ice in pure water and aqueous solutions is parallel to the basal plane. As the supercooling is increased the growth direction of the dendrites become inclined at an angle to the basal plane, the angle increasing with supercooling. The most general habit of growth now has the form of two symmetrically situated hollow hexagonal pyramids joined at their apices. At still larger supercoolings further growth ensues from the initial dendrites comprising the pyramidal segments and complex three-dimensional structures are produced which are nevertheless single crystals. It is noted that the pyramidal growth habit occurs also in ice crystals formed from the vapour phase at high supersaturations and in Tyndall figures produced in the melting of ice. The non-rational growth of the dendrites is explained in terms of deposition predominantly on basal surfaces on one side of the dendrite tip and on prism faces on the other, the direction of growth being determined by the resultant of the two component growth velocities. It is suggested that the relatively rapid rapid increase of the c-axis velocity with supercooling is due to growth in the transitional regime in which the rate of surface nucleation is the controlling factor. This mechanism may explain also the effects of impurities on the growth in aqueous solutions.