Effect of the background flow on the motility induced phase separation

TL;DR

This study explores how background flow influences motility-induced phase separation in active Brownian particles, revealing a new flow-induced phase separation regime and mapping out different behavioral phases based on flow parameters.

Contribution

It introduces a novel phase, flow-induced phase separation (FIPS), and characterizes the interplay between flow and motility in active particle systems.

Findings

Flow dominates at low velocities, leading to homogeneous mixtures.

High velocities restore motility-induced phase separation.

Intermediate flow regimes exhibit a new FIPS phase depending on system parameters.

Abstract

We simulate active Brownian particles (ABPs) with soft-repulsive interactions subjected to a four-roll-mill flow. In the absence of flow, this system exhibits motility-induced phase separation (MIPS). To investigate the interplay between MIPS and flow-induced mixing, we introduce dimensionless parameters: a scaled time, $\tau$, and a scaled velocity, ${\rm v}$, characterizing the ratio of ABP to fluid time and velocity scales, respectively. The parameter space defined by $\tau$ and ${\rm v}$ reveals three distinct ABP distribution regimes. At low velocities ${\rm v} \ll 1$, flow dominates, leading to a homogeneous mixture. Conversely, at high velocities ${\rm v} \gg 1$, motility prevails, resulting in MIPS. In the intermediate regime (${\rm v} \sim 1$), the system's behavior depends on $\tau$. For $\tau <1$, a moderately mixed homogeneous phase emerges, while for $\tau >1$, a novel phase, termed flow-induced phase separation (FIPS), arises due to the combined effects of flow topology and ABP motility and size. To characterize these phases, we analyze drift velocity, diffusivity, mean-squared displacement, giant number fluctuations, radial distribution function, and cluster-size distribution.