Extreme Active Matter at High Densities

Extreme active matter, consisting of self-propelled particles characterized by large persistence time τ_p and high Péclet number, exhibits remarkable behavior at high densities. In the limit τ_p → ∞, the fluid jams as the self-propulsion force f is decreased below a critical value f^*(∞). The system is stuck at a force-balanced configuration for f < f^*(∞), with stresses concentrated along force chains. For large but finite τ_p, the approach to dynamical arrest at low f goes through a phase characterized by intermittency in kinetic and potential energy. This intermittency is a consequence of long periods of jamming separated by bursts of plastic yielding associated with Eshelby deformations akin to those found in the response of dense granular materials to an externally imposed shear. In the vicinity of the boundary between the intermittent and "normal" fluid phases, correlated plastic events result in large-scale vorticity and turbulent motion. Thus, dense extreme active matter brings together the physics of glass, jamming, plasticity and turbulence, in a new state of driven classical matter.