Crystallisation and Polymorph Selection in Active Brownian Particles

We explore crystallisation and polymorph selection in active Brownian particles with numerical simulation. In agreement with previous work [Wysocki et al. Europhys. Lett., 105 48004 (2014)], we find that crystallisation is suppressed by activity and occurs at higher densities with increasing Péclet 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.