Motility-induced phase separation in the presence of hydrodynamic interactions*

Free of the constraints of equilibrium statistical physics, active matter systems exhibit a variety of unexpected phenomena. Their origin lies in detailed balance being broken by self-propulsion and interactions between active particles at the microscopic level. Such systems can often be classified as either 'dry' or 'wet' active matter when dominated by friction with their surroundings and long-ranged hydrodynamic interactions (HI), respectively. In dry active matter, an archetypal example is given by the motility-induced phase separation, while in wet active matter, the same role is played by bacterial turbulence-largescale collective motion of dilute suspension of motile organisms.

In this talk we consider a system that incorporates elements of both dry and wet active matter. We study collective motion in a 2D layer of model microswimmers suspended in a viscous fluid. In the absence of HI, we observe motility-induced phase separation, while the presence of 2D HI only trivially changes the phase diagram of such systems. However, when the microswimmer layer is suspended in a 3D bulk fluid, we find that the growth of motility-induced clusters is arrested, leading to microphase separation. We discuss the mechanism of such arrest and explain the origin of the emergent lengthscale.