Crystallisation and Polymorph Selection in Active Brownian Particles

TL;DR

This study uses numerical simulations to investigate how activity influences crystallisation and polymorph selection in active Brownian particles, revealing a shift from nucleation to spinodal regimes and a preference for FCC structures at high activity levels.

Contribution

It demonstrates how activity suppresses crystallisation, alters nucleation dynamics, and favors FCC polymorphs over HCP in active Brownian particles, extending understanding beyond passive systems.

Findings

Crystallisation is suppressed by activity and occurs at higher densities.

Transition from nucleation and growth to spinodal nucleation with increasing activity.

Activity favors FCC polymorphs by annealing HCP stacking faults.

Abstract

We explore crystallisation and polymorph selection in active Brownian particles with numerical simulation. In agreement with previous work [Wysocki $\textit{et al.}$ $\textit{Europhys. Lett.}$, $\textbf{105}$ 48004 (2014)], we find that crystallisation is suppressed by activity and occurs at higher densities with increasing $P\'{e}clet$ number ($Pe$). While the nucleation rate decreases with increasing activity, the crystal growth rate increases due to the accelerated dynamics in the melt. As a result of this competition we observe the transition from a nucleation and growth regime at high $Pe$ to "spinodal nucleation" at low $Pe$. Unlike the case of passive hard spheres, where preference for FCC over HCP polymorphs is weak, activity causes the annealing of HCP stacking faults, thus strongly favouring the FCC symmetry at high $Pe$. When freezing occurs more slowly, in the nucleation and growth regime, this tendency is much reduced and we see a trend towards the passive case of little preference for either polymorph.