Abstract

Activity changes the physics of phase separation qualitatively and quantitatively. This talk will focus on two topics where, although big changes might be expected in principle, it turns out that activity can be dealt with by modifying passive theories without fundamentally changing outcomes. The first is nucleation in the phase separation of a conserved scalar fieldm describing e.g. the density of active particles. Here the main role of activity is to renormalize the interfacial tension in an effective free energy description for a growing spherical droplet. This description exploits the very high symmetry of the problem and the method is unlikely to generalize to more complex cases (e.g. nucleation of an active nematic). The second topic is the low-wavevector behaviour of the equal time correlator S(q) in systems undergoing phase separation. In the scaling limit, order-parameter conservation ensures S(q) vanishes at low q—which is the definition of hyperuniformity. However it has long been known to vanish as the fourth rather than second power of q in passive phase separation, a result recently found numerically for active models as well. One longstanding (but somewhat delicate) explanation of the passive result invokes a second, hidden, conservation law. We show that this conservation law remains intact when activity is added at lowest order, and discuss how violations arising from noise and from a non-constant mobility end up not changing the result. The role of fluid flow in these two problems will also briefly be discussed.