Extreme active matter at high densities

Abstract: Extreme active matter, an assembly of self-propelled particles with large persistence time $\tau_p$ and high P\'eclet number, exhibits remarkable behaviour at high densities. As $\tau_p\to 0$, the assembly undergoes a gradual slowing down of density relaxations, as one reduces the active propulsion force $f$, until at the glass transition, the relaxation times diverge. In the other limit, $\tau_p \to \infty$, the fluid jams on lowering $f$, at a critical threshold $f^*(\infty)$, with stresses concentrated along force-chains. As one moves away from this jamming threshold, the force-chains dynamically remodel, and the lifetime of the force-balanced configurations diverges as one approaches $f^*(\infty)$, by tuning $\tau_p$. In between these limits, the approach to dynamical arrest at low $f$, goes through a phase characterised by intermittency in the kinetic energy. This intermittency is a consequence of long periods of jamming followed by bursts of plastic yielding associated with Eshelby deformations, akin to the response of dense amorphous solids to an externally imposed shear. The frequency of these plastic bursts increases as one moves towards the intermittent phase-fluid boundary, where the correlated plastic events result in large scale vorticity and turbulence. Dense extreme active matter brings together the physics of glass, jamming, plasticity and turbulence, in a new state of driven classical matter.