Ice grains grow by dissolution, ripening and grain boundary migration

TL;DR

This paper introduces a machine-learned bond-order potential model for water that enables large-scale, long-time molecular dynamics simulations, revealing detailed mechanisms of ice grain growth, dissolution, and ripening.

Contribution

The study develops a computationally efficient, accurate BOP model trained on temperature-dependent properties, allowing unprecedented simulation of ice nucleation and grain coarsening.

Findings

Captured competition among ice phases during nucleation.

Observed grain dissolution and Ostwald ripening processes.

Elucidated grain boundary migration mechanisms.

Abstract

Despite the exponential growth in computing resources and the availability of a myriad of different theoretical water models, an accurate, yet computationally efficient molecular level description of mesoscopic grain growth remains a grand challenge. The underlying phase transitions and dynamical processes in deeply supercooled systems are often rendered inaccessible due to limitations imposed by system sizes and timescales, which is further compounded by their sluggish kinetics. Here, we introduce a machine-learned, bond-order-based potential model (BOP) that more accurately describes the anomalous behavior, as well as structural and thermodynamical properties of both liquid water and ice, with at least two orders of magnitude cheaper computational cost than existing atomistic water models. In a significant departure from conventional force-field fitting, we use a multilevel evolutionary strategy that trains the BOP model against not just energetics but temperature dependent properties inferred from on-the-fly molecular dynamics simulations as well. We use the BOP model to probe the homogeneous ice nucleation and growth process by performing molecular dynamics on multi-million molecule systems for up to microsecond time scales. These massively parallel, long-time simulations naturally capture the competition between cubic, hexagonal, and stacking disordered ice phases during nucleation and early stages of growth leading to the formation of nanometer sized grains. Subsequently, we elucidate the hitherto elusive mechanism of grain coarsening of the crystallized mixed ice phases, which occur through grain dissolution and Ostwald ripening followed by grain boundary migration.