Oral: From Chaos to Clusters: The Impact of Flow on Motility-Induced Phase Separation

Motility-induced phase separation (MIPS) describes how self-propelling particles phase-separate due to their motility, a phenomenon observed in many natural systems. Many motile organisms thrive in fluid environments, and chaotic background flows often act as mixing agents. This raises the question: how does background flow affect the phase separation of motile organisms? To explore this, we study active Brownian particles (ABPs) in a periodic four-roll-mill flow using numerical simulations. Without flow, the system undergoes MIPS. We maintain a packing fraction of 0.7 and investigate the effect of flow on phase separation, referred to as flow-induced phase separation (FIPS). We examine the role of the Peclet number (Pe), which measures the ratio of self-propulsion to rotational diffusion, on cluster size distribution (CSD) and giant number fluctuations in both MIPS and FIPS regimes. In both cases, cluster size increases with Pe but is smaller with flow. The CSD exhibits a power-law decay, with the exponent initially decreasing before stabilizing as Pe rises. The decay is steeper without flow. Number fluctuations scale as a power law with mean particle number, with flow reducing the scaling exponent. Overall, flow reduces cluster sizes and steepens the decay of fluctuations compared to systems without flow.