Simulations reveal the shape of crystal nuclei to be consistent with classical theory

Though nucleation is an important natural phenomenon that is central to many processes from precipitation to corrosion, it is poorly understood at the atomic level because of the small size of the nucleus and the short timescale of its formation. Currently, little is known about the shapes of nuclei at the atomic level.

By performing molecular dynamics simulations of nucleation in a Lennard-Jones system, Cheng et al. obtained the average shape of a single crystal nucleus and found it to be smooth and close to spherical, but with fluctuations in its instantaneous configurations. Additionally, the authors identified a linear relationship between the equilibrium shape of the nucleus and its excess free energy.

“The very nice thing here is that, from this average shape with small deviations from a perfect sphere, we can extract the anisotropy in the interfacial free energies,” said author Bingqing Cheng. “This means if one knows the anisotropy in the interfacial free energies, one can predict the average shape of the nucleus using classical theory.”

Typically, the classical theory of nucleation is regarded to be inaccurate due to the differences between its predictions of a smooth and spherical nucleus and the faceted shapes seen in simulations. However, the present work bridges that gap.

“This study reconciles the dependency regarding the shape of nuclei between classical theory and simulations and suggests the applicability of classical theory extend beyond what people may think previously,” Cheng said.

She noted their framework can be extended to other systems in order to obtain a more complete description of nucleation.

Source: “Classical nucleation theory predicts the shape of the nucleus in homogeneous solidification,” by Bingqing Cheng, Michele Ceriotti, and Gareth A. Tribello, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5134461 .