Oscillating crystal growth rates

Abstract A number of papers in the literature have reported that under certain conditions the growth rate of a crystal may not assume a unique value at a given supersaturation. These papers implied either that identical crystals growing in a supersaturated solution under identical conditions did not all grow at the same growth rate or that a given growing crystal was alternating between two values of growth rate. In this work the existence of such a phenomenon was clearly demonstrated by experiments with sodium chloride crystal whiskers in which the growth was monitored semi‐continuously. It was observed that instead of being a continuous process, crystal growth was intermittent in nature. Growth was characterized by two alternating periods, a growth period during which rapid growth occurred and a period of small or non‐existent growth. This work also showed that this type of intermittent growth could account for another phenomenon previously reported in an investigation with potassium alum crystals; that in certain supersaturation regions the growth rates appeared to be insensitive to changes in supersaturation. It is also pointed out that the existence of periodic rates will complicate the expressions for the kinetics of growth used in the analysis of crystallizers and that the insensitivity of growth rates to supersaturation may affect the stability of continuous crystallizers.