Hydrodynamic interactions induce microphase separation in active systems

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 the 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, respectively. Manifestations of broken detailed balance often comprise novel phases that are absent in equilibrium. 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' - large-scale collective motion of a dilute suspension of motile organisms. In this talk we introduce a model that simultaneously includes long-range hydrodynamic interactions between microswimmers and microscopic ingredients necessary for the formation of motility-induced clusters. We demonstrate that the model yields a variety of new phases. Most importantly, we find that the growth of motility-induced clusters is arrested by hydrodynamic interactions leading to microphase separation. We discuss its mechanism and propose a phase diagram for such systems.