Crystallization of self-propelled hard-discs : a new scenario

TL;DR

This paper investigates how active, polar self-propelled discs crystallize and melt in a vibrated monolayer, revealing a dynamic, heterogeneous transition with persistent melting and cluster formation, challenging traditional crystal stability.

Contribution

It introduces experimental insights into the crystallization behavior of active polar discs, contrasting it with isotropic discs, and uncovers a novel self-melting transition in active matter systems.

Findings

Clusters form and melt spontaneously, leading to heterogeneous dynamics.

Large clusters span the system at high packing fractions, with a non-monotonic size distribution.

The system remains dynamically active without becoming arrested, even at high densities.

Abstract

We experimentally study the crystallization of a monolayer of vibrated discs with a built-in polar asymmetry, a model system of active liquids, and contrast it with that of vibrated isotropic discs. Increasing the packing fraction $\phi$, the quasi-continuous crystallization reported for isotropic discs is replaced by a transition, or a crossover towards a "self-melting" crystal. Increasing the packing fraction from the liquid phase, clusters of dense hexagonally-ordered packed discs spontaneously form, melt, split and merge leading to a highly intermittent and heterogeneous dynamics. The resulting steady state cluster size distribution decreases monotonically. For packing fraction larger than $\phi^*$, a few large clusters span the system size and the cluster size distribution becomes non monotonic, the transition being signed by a power-law. The system is however never dynamically arrested. The clusters permanently melt from place to place forming droplets of active liquid which rapidly propagate across the system. This state of affair remains up to the highest possible packing fraction questioning the stability of the crystal for active discs, unless at ordered close packing.