The limit of macroscopic homogeneous ice nucleation at the nanoscale

TL;DR

This study uses molecular simulations to explore how nanoscale confinement affects homogeneous ice nucleation, finding that nucleation rates are largely bulk-like in thin films and that confinement effects are minimal.

Contribution

It provides new insights into the influence of nanoscale confinement on ice nucleation rates, clarifying discrepancies in previous literature.

Findings

Nucleation remains bulk-like in films just large enough for a critical nucleus.

Physical boundaries do not significantly alter nucleation rates.

Confinement effects on nucleation are minimal at current experimental scales.

Abstract

Nucleation in small volumes of water has garnered renewed interest due to the relevance of pore condensation and freezing under conditions of low partial pressures of water, such as in the upper troposphere. Molecular simulations can in principle provide insight on this process at the molecular scale that is challenging to achieve experimentally. However, there are discrepancies in the literature as to whether the rate in confined systems is enhanced or suppressed relative to bulk water at the same temperature and pressure. In this study, we investigate the extent to which the size of the critical nucleus and the rate at which it grows in thin films of water are affected by the thickness of the film. Our results suggest that nucleation remains bulk-like in films that are barely large enough accommodate a critical nucleus. This conclusion seems robust to the presence of physical confining boundaries. We also discuss the difficulties in unambiguously determining homogeneous nucleation rates in nanoscale systems, owing to the challenges in defining the volume. Our results suggest any impact on a film's thickness on the rate is largely inconsequential for present day experiments.