Gabriel Redner (Brandeis University)
Abstract. Active systems are composed of self-propelled units which interact to produce a wide range of complex behaviors. In this talk I will describe a minimalist active system comprised of self-propelled spheres immersed in a fluid and confined to two dimensions, interacting through hard-core repulsion alone. I will describe an unusual type of phase transition in which the system spontaneously separates into dense and dilute phases, even in the absence of any attractive interaction. I will draw comparisons between this athermal active process and the behavior of ordinary fluids undergoing phase separation. When attraction is added to the active system, the system's phase diagram becomes reentrant as a function of activity, with phase separation at low and high activity with a single-phase fluid in between. I will introduce a kinetic model which captures the main features of the phase behavior and describes how activity can act to both suppress and induce phase separation.