Arrested States in Persistent Active Matter: Gelation without Attraction

TL;DR

This paper investigates how activity in self-propelled colloids with nonconvex shapes leads to phase separation and kinetic arrest, creating gel and glass states without attractive interactions, using simulations and analytical models.

Contribution

It introduces a model linking activity to attraction, revealing gelation and glassy states in active matter with nonconvex particles, supported by analytical and numerical analysis.

Findings

Active particles exhibit phase separation similar to attractive colloids.

Nonconvex shapes facilitate jamming and inhibit rotations.

Coarse-grained models predict density profiles of arrested states.

Abstract

We explore phase separation and kinetic arrest in a model active colloidal system consisting of self-propelled, hard-core particles with nonconvex shapes. The passive limit of the model, namely cross-shaped particles on a square lattice, exhibits a first-order transition from a fluid phase to a solid phase with increasing density. Quenches into the two-phase coexistence region exhibit an aging regime. The nonconvex shape of the particles eases jamming in the passive system and leads to strong inhibition of rotations of the active particles. Using numerical simulations and analytical modeling, we quantify the nonequilibrium phase behavior as a function of density and activity. If we view activity as the analog of attraction strength, the phase diagram exhibits strong similarities to that of attractive colloids, exhibiting both aging, glassy states and gel-like arrested states. The two types of dynamically arrested states, glasses and gels, are distinguished by the appearance of density heterogenities in the latter. In the infinitely persistent limit, we show that a coarse-grained model based on the asymmetric exclusion process quantitatively predicts the density profiles of the gel states. The predictions remain qualitatively valid for finite rotation rates. Using these results, we classify the activity-driven phases and identify the boundaries separating them.