Effect of self-propulsion on equilibrium clustering

TL;DR

This study uses simulations and an equilibrium model to explore how self-propulsion influences clustering in colloidal suspensions, revealing non-monotonic cluster size behavior with activity and distinguishing it from motility-induced clustering.

Contribution

It introduces an equilibrium model that incorporates activity-dependent attraction and repulsion, explaining cluster size variations in self-propelled colloids.

Findings

Cluster size increases then decreases with activity at low densities.

Clustering remains stable under self-propulsion.

Model semi-quantitatively matches simulation results.

Abstract

In equilibrium, colloidal suspensions governed by short-range attractive and long-range repulsive interactions form thermodynamically stable clusters. Using Brownian dynamics computer simulations, we investigate how this equilibrium clustering is affected when such particles are self-propelled. We find that the clustering process is stable under self-propulsion. For the range of interaction parameters studied and at low particle density, the cluster size increases with the speed of self-propulsion (activity) and for higher activity the cluster size decreases, showing a non-monotonic variation of cluster size with activity. This clustering behaviour is distinct from the pure kinetic (or motility-induced) clustering of self-propelling particles which is observed at significantly higher activities and densities. We present an equilibrium model incorporating the effect of activity as activity-induced attraction and repulsion by imposing that the strength of these interactions depend on activity superlinearly. The model explains the cluster size dependence of activity obtained from simulations semi-quantitatively. Our predictions are verifiable in experiments on interacting synthetic colloidal microswimmers