Effective Interactions in Active Brownian Suspensions

TL;DR

This paper develops a first-principles microscopic theory for active colloids that explains motility-induced phase separation (MIPS) and predicts how activity modifies phase behavior, aligning with simulations and experiments.

Contribution

It introduces a parameter-free, microscopic theory that elucidates how activity generates effective many-body interactions leading to MIPS.

Findings

The theory predicts phase separation in repulsive systems matching simulations.

Moderate activity suppresses phase separation in attractive systems.

High activity can induce reentrant clustering, consistent with experiments.

Abstract

Active colloids exhibit persistent motion, which can lead to motility-induced phase separation (MIPS). However, there currently exists no microscopic theory to account for this phenomenon. We report a first-principles theory, free of fit parameters, for active spherical colloids, which shows explicitly how an effective many-body interaction potential is generated by activity and how this can rationalize MIPS. For a passively repulsive system the theory predicts phase separation and pair correlations in quantitative agreement with simulation. For an attractive system the theory shows that phase separation becomes suppressed by moderate activity, consistent with recent experiments and simulations, and suggests a mechanism for reentrant cluster formation at high activity.