Microphase separation in confined swimmers.

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 show that the collective behaviors depend not only on the squirming modes but also strongly on the boundary conditions for the flow in the system, as well as their hydrodynamic ensembles. Specifically, we show that ``stealth'' swimmers at fixed swim velocity exhibit micro-phase separations when the flow is confined by either no-slip walls or stress-free liquid-liquid interfaces. Under strong confinement, the difference between ``pushers" and ``pullers'' vanishes and they also show similar micro-phase separations. These results help to reconcile previous theoretical and numerical investigations. From the simulations, we assemble a comprehensive phase diagram of active squirmers. We present a simple theory to rationalize the HI-induced micro-phase separation behavior.