Arrested states in persistent active matter: Gelation without attraction

Carl Merrigan,Bulbul Chakraborty,Nimrod Segall,Kabir Ramola,Yair Shokef,Rakesh Chatterjee

doi:10.1103/physrevresearch.2.013260

Carl Merrigan, Bulbul Chakraborty + Show 4 more

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https://doi.org/10.1103/physrevresearch.2.013260

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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.

Highlights

Active matter, composed of particles that convert ambient energy to directed motion, has emerged as an important class of nonequilibrium systems, with examples ranging from bacterial suspensions to synthetic colloids
We explore phase separation and kinetic arrest in a model active colloidal system consisting of self-propelled, hard-core particles with nonconvex shapes
A striking feature of the arrested and nonarrested states at high activities is the appearance of “voids.” We show below that the appearance of these voids is a manifestation of an extreme form of motility-induced phase separation (MIPS) for these nonrotating particles that can be understood from asymmetric simple exclusion process (ASEP) dynamics

Summary

INTRODUCTION

Active matter, composed of particles that convert ambient energy to directed motion, has emerged as an important class of nonequilibrium systems, with examples ranging from bacterial suspensions to synthetic colloids. Recent work on an extreme limit of ABPs with long persistence time of their self-propulsion direction has revealed fluctuations in the dense limit that are qualitatively different from those at short persistence times [28] In this limit of long yet finite persistence times, clustering and heterogeneous dynamics analogous to passive gels have been observed [29], lending further credence to the idea that activity can act as an effective attractive interaction. V, we present a coarse-grained model of the dynamics that leads to a prediction of the density profile We compare these results to the density profiles obtained in our numerical simulations in Sec. VI, and construct a nonequilibrium phase diagram that delineates states based on the density profiles of the arrested states. The appendixes provide further details of the passive, glassy dynamics and discussion of finite-size effects

MODEL AND SIMULATIONS

DYNAMICAL HETEROGENEITY AND ACTIVITY-INDUCED AGING

COARSE-GRAINED DENSITY PROFILES AND HYDROSTATIC LENGTHSCALE

NONEQUILIBRIUM PHASE DIAGRAM

EFFECT OF FINITE ROTATION RATE

VIII. DISCUSSION