Suppression of Sub-surface Freezing in Free-Standing Thin Films of a Coarse-grained Model of Water

TL;DR

This study uses molecular dynamics and advanced sampling to show that vapor-liquid interfaces in thin water films suppress ice nucleation, with nuclei near the interface being less stable and more aspherical than in bulk water.

Contribution

It demonstrates that vapor-liquid interfaces inhibit ice nucleation in thin water films using coarse-grained models and free energy calculations, revealing the role of nucleus shape and stability.

Findings

Vapor-liquid interfaces suppress ice nucleation near the surface.

Pre-critical nuclei near the interface are less stable than in bulk.

Nuclei near the interface tend to be more aspherical.

Abstract

Freezing in the vicinity of water-vapor interfaces is of considerable interest to a wide range of disciplines, most notably the atmospheric sciences. In this work, we use molecular dynamics and two advanced sampling techniques, forward flux sampling and umbrella sampling, to study homogeneous nucleation of ice in free-standing thin films of supercooled water. We use a coarse-grained monoatomic model of water, known as mW, and we find that in this model a vapor-liquid interface suppresses crystallization in its vicinity. This suppression occurs in the vicinity of flat interfaces where no net Laplace pressure in induced. Our free energy calculations reveal that the pre-critical crystalline nuclei that emerge near the interface are thermodynamically less stable than those that emerge in the bulk. We investigate the origin of this instability by computing the average asphericity of nuclei that form in different regions of the film, and observe that average asphericity increases closer to the interface, which is consistent with an increase in the free energy due to increased surface-to-volume ratios.