Effects of hydrodynamic interactions in Motility-Induced Phase Separations.

Hydrodynamic interactions (HI) strongly influence the collective behavior of microswimmers, such as motility-induced phase separation (MIPS). We systematically study the collective dynamics of so-called squirmers using Active Fast Stokesian Dynamics simulations. We focus on ``stealth'' swimmers with 2D orientations but with fully 3D flow fields. We discuss different swimming modes, or ``hydrodynamic ensembles'', such as fixed swim force or fixed swim velocity, and show that the collective behaviors depend not only on the squirming modes but also strongly on both the hydrodynamic ensemble and the boundary conditions for the flow in the system. Specifically, we show that ``stealth'' swimmers at fixed swim velocity only exhibit crystalline clusters when the flow is confined by either no-slip walls or stress-free liquid-liquid interfaces. These results help to reconcile previous theoretical and numerical investigations. From the simulations, we assemble a comprehensive phase diagram of active squirmers.