Abstract
A full understanding of growth rate dispersion (GRD), whereby seemingly identical crystals grow at different rates under identical conditions, has not yet been achieved despite thirty years of research into the phenomenon. The current study investigated the effect of a crystal's 'growth rate history' on the current crystal growth of sucrose, potash alum and KDP crystals. All three crystals, which differ in terms of crystal structures and thermodynamic properties, had similar results showing that the growth history of a crystal had a significant effect on the future crystal growth rate of that crystal. In particular, if a crystal had a period of high growth in a high supersaturation environment, the subsequent growth of the crystal in a lower supersaturation had a lower rate than a crystal that had been kept in the lower supersaturation environment. These results can be explained by the effect of high growth rates on the growing surface of a crystal. It was observed that crystals grown in high supersaturation solutions had a rougher surface (on a macroscopic rather than molecular scale) than those grown in low supersaturation solutions. This phenomenon only occurs if growth occurs above a critical level of the supersaturation, that we term the roughening transition. The roughening transition supersaturation for a crystalline species depends on the surface energy of the crystal, with species having a relatively large surface energy having a low roughening transition threshold. This situation could be quite important during either nucleation or growth periods. The initial nuclei form at a higher supersaturation than later nuclei, and this may relate to differences in initial growth rates for apparently identical nuclei. In addition, if local levels of supersaturation vary, there will be an effect on the inherent crystal growth rate dispersion, in addition to random fluctuations in growth rate.
