Phase Separation in Flocks, Herds and Swarms (Polar Active Fluids)

Speaker: Maxx Miller (University of Oregon)

Date: Monday May 5, 2025

Time: 4:00 – 5:00 pm

Location: 472 Willamette Hall (IFS Seminar Room)

Abstract: “Flocking” is the collective coherent motion of a large number of self-propelled entities (e.g., flocking of birds, schooling of fish, swarming of bees). Just as simple equilibrium fluids are described at large length and time scales by the Navier-Stokes equations, flocks also have a hydrodynamic description, known as the Toner-Tu equations.  Recently, we have extended these equations to account for strong attractive interactions between Flockers (which can arise from, e.g., autochemotaxis). We find that sufficiently strong attractive interactions cause the flock to phase separate into high- and low-density bands oriented parallel to the direction of mean-flock motion. This banding is reminiscent of ant trails, and the phase separation of the flock is in many ways analogous to equilibrium phase separation of simple fluids (i.e., the liquid-gas transition). However,  we have shown using the dynamical renormalization group that phase separation in flocks belong to a novel universality class, different from that of phase separation in simple equilibrium fluids.

Host: Jayson Paulose