Ripening of faceted ice crystals

Abstract

Ripening of an ensemble of progressively faceting ice crystals in aqueous fructose solution is studied by characterising the change in average crystal size, number density, and size distribution with time, as well as tracking individual crystals, so as to follow the evolution of crystal anisotropy with time. The results are compared to theoretical models developed for an ensemble of spherical crystals, which consider the ripening process to be limited by either: (i) diffusion processes, or (ii) reaction kinetics at the crystal–fluid interface. It is found that the results obtained conform neither with true diffusion, nor reaction-controlled, ripening. It is suggested that the system appears to be approaching “near” diffusion control at long times, but that initially when the crystals are sufficiently small, there may be a period where growth onto the prism plane is limited by reaction kinetics. The growths of the prism and basal planes of ice crystals in the ensemble are found to display marked anisotropy, presumably as a result of the relative roughening transitions of these planes. However, since the prism plane appears to facet while the crystals are still comparatively small, it is suggested that the crystals are able to grow towards their Wulff shape, which is thought to be a hexagonal column at −19°C.