Poster: Mixing and demixing effect on ABPs due to 2D background flow

We investigate how the background flow affects motility-induced phase separation (MIPS) of isotropic, self-propelling particles (SPPs), a phenomenon observed in motile organisms thriving in fluid environments. Using a large number of SPPs advected by a four-roll mill flow known for effectively mixing scalar fields, we explore the dynamics of SPPs that follow active Brownian particle (ABP) behavior and exhibit MIPS at a packing fraction (ϕ >~ 0.4). For this study, we set ϕ = 0.7 and define two dimensionless quantities: scaled time (SPP to fluid time scale ratio) and scaled velocity (SPP to fluid velocity ratio). Based on these, we identify three distinct regimes. At small-scaled velocity (<<1), flow dominates and leads to a homogeneously mixed phase. At large-scale velocity (>>1), motility dominates, resulting in MIPS. At moderate scaled velocity (~1), the system can either show a moderately mixed phase or a novel regime termed flow-induced phase separation (FIPS), driven by the interplay of flow topology and SPP motility. We present a phase diagram and characterize drift velocity, diffusivity, giant number fluctuation, radial distribution function, and cluster-size distribution for deeper insights.