# Freezing a Flock: Motility-Induced Phase Separation in Polar Active Liquids

## TL;DR

This paper combines experiments and theory to show that dense polar active matter undergoes a first-order phase transition from flocking to active solids via motility-induced phase separation, revealing a universal jamming mechanism.

## Contribution

It introduces a minimal hydrodynamic model explaining active solidification as a motility-induced phase separation in polar active liquids.

## Key findings

- Active solids nucleate and coarsen until phase separation.
- Active solidification is a first-order dynamical transition.
- Suppression of collective motion is a generic feature of dense motile units.

## Abstract

Combining model experiments and theory, we investigate the dense phases of polar active matter beyond the conventional flocking picture. We show that above a critical density flocks assembled from self-propelled colloids arrest their collective motion, lose their orientational order and form solids that actively rearrange their local structure while continuously melting and freezing at their boundaries. We establish that active solidification is a first-order dynamical transition: active solids nucleate, grow, and slowly coarsen until complete phase separation with the polar liquids they coexists with. We then theoretically elucidate this phase behaviour by introducing a minimal hydrodynamic description of dense polar flocks and show that the active solids originate from a Motility-Induced Phase Separation. We argue that the suppression of collective motion in the form of solid jams is a generic feature of flocks assembled from motile units that reduce their speed as density increases, a feature common to a broad class of active bodies, from synthetic colloids to living creatures.