Phase Separation in a Chiral Active Fluid of Inertial Self-Spinning Disks

We show that systematic particle rotations in a fluid composed of disk-shaped spinners can spontaneously lead to phase separation. The phenomenon arises out of a homogeneous and hydrostatic stationary state, due to a pressure feedback mechanism that increases local density fluctuations. We show how this mechanism induces phase separation, coined as rotation induced phase separation (RIPS), when the active rotation is not properly counterbalanced by translational friction. A low density phase can coexist with a dense chiral liquid due to the imbalance between pressure and stress transmitted through chiral flows when a significant momentum transfer between rotational and translational motion can be sustained. As a consequence, RIPS is expected to appear as a general nonequilibrium phenomenon in chiral fluids.