Hydrodynamic Interactions Destroy Motility-Induced Phase Separation in Active Suspensions

Motility-induced phase separation (MIPS) is a distinctive phenomenon in active matter that arises from its inherent nonequilibrium nature. Despite recent progress in understanding MIPS in dry active systems, it has been debated whether MIPS can be observed in wet systems in which fluid-mediated hydrodynamic interactions (HIs) are present. We use theory and large-scale active fast Stokesian dynamics simulations of the so-called squirmer model to show that collision-induced pusher force dipoles, which are present even for the simplest neutral squirmers (stealth swimmers), destroy MIPS when HIs are included. Both rotational and translational HIs independently suppress phase separation: rotation by shortening a swimmer's persistence length (and thus reducing the swim pressure), and translation by a confinement-scale advective fluid flow. We further clarify that collisional dipoles between swimmers and boundaries can generate attractive flows that promote particle aggregation observed in some previous simulations and experiments. Finally, we show how to recover MIPS in fluidic environments by tuning the magnitude of the HIs through brushlike surface coatings on the active particles.