Far-field hydrodynamic interactions destroy motility-induced phase separations.

Hydrodynamic interactions (HI) strongly influence the collective behavior of microswimmers, such as motility-induced phase separation (MIPS). We study the collective dynamics of so-called squirmers using theoretical analysis and Active Fast Stokesian Dynamics simulations. Using the simplest neutral squirmers (`stealth swimmers'), we show that collisions among swimmers induce pusher-like dipolar flows that thermalize the translational motion and generally destroy MIPS. To illuminate the role of confining boundaries, we consider no-slip walls and stress-free liquid-liquid interfaces. We found that for 2D translational and rotational dynamics, collision-induced dipoles prohibit global phase separation, yet finite size crystalline clusters, i.e., microphase separations appear when swimmers are confined in thin films. For swimmers' motion towards or away from the confining boundary, we show that additional HI arises from the image flows reflecting off the boundary, further modifying the collective pattern formation.