Probability of Nucleation in a Metastable Zone: Cooling Crystallization and Polythermal Method

In polythermal experiments, typically the observed dependence of the metastable zone width (MZW - often characterized as the undercooling δT) on the experimental path is attributed to the variation of the nucleation rate of solute crystals along this path. In this work, we analyze the experimental data on the MZW for several solutes available in the literature and show that the path-dependence of MZW observed during this cooling crystallization may not always exist when one considers the supersaturation of nucleation. This result is in line with our previous observations on the MZW of various other solutes exhibited during evaporative crystallization. In this context, the assumptions behind the traditional theoretical explanations of MZW are examined, and the possible issues in obtaining nucleation kinetics from polythermal MZW experiments are discussed. A consistent method for determining the nucleation rate parameters, which leverages the complementary nature of both the induction time (IT) and the MZW experiments, is proposed. This work aims to bring the often-overlooked fundamental variables of crystal nucleation into the forefront of the analysis of polythermal experiments.