Effect of the background flow on motility-induced phase separation

Area of Science:

• Soft matter physics
• Non-equilibrium statistical mechanics
• Complex fluids

Background:

• Active Brownian particles (ABPs) with soft-repulsive interactions show motility-induced phase separation (MIPS) in the absence of flow.
• Understanding the interplay between self-propulsion and external flow is crucial for controlling active matter systems.

Purpose of the Study:

• To investigate the combined effects of motility-induced phase separation and flow-induced mixing in active Brownian particles.
• To identify and characterize distinct regimes of particle distribution under shear flow.

Main Methods:

• Simulations of active Brownian particles subjected to a four-roll-mill flow.
• Introduction of dimensionless parameters: scaled time (τ) and scaled speed (V).
• Analysis of drift velocity, diffusivity, mean-squared displacement, giant number fluctuations, radial distribution function, and cluster-size distribution.

Main Results:

• Three distinct regimes of ABP distribution were identified based on scaled speed (V) and scaled time (τ).
• At low V, flow dominates, leading to a homogeneous mixture.
• At high V, motility dominates, resulting in MIPS.
• An intermediate regime (V ~ 1) shows a transition dependent on τ: homogeneous mixing for τ < 1 and a novel flow-induced phase separation (FIPS) for τ > 1.

Conclusions:

• The study reveals a rich phase diagram for active Brownian particles in shear flow, controlled by motility and flow parameters.
• A new phenomenon, flow-induced phase separation (FIPS), is discovered, driven by the interplay of flow topology, particle motility, and size.
• The findings provide insights into the fundamental behavior of active matter under external flow conditions.