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

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 with which they coexist. We then theoretically elucidate this phase behavior 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.

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Plain Language Summary

Using microscopic synthetic particles capable of self-propulsion, researchers have created a new generation of materials assembled from mobile units: active matter. In one remarkable example, micrometer-size particles move about and probe the orientation of their neighbors. When interacting, these active particles self-assemble into synthetic flocks to form spontaneously flowing liquids. Here, we show that upon increasing the local density of particles, liquid flocks collectively turn into arrested solids through a freezing transition akin to that observed when cooling water below0°C.

By combining experiments and theory, we demonstrate that polar liquids assembled from motile colloids undergo a “motility-induced solidification,” a transition generic to any flocking group in which the individuals slow down as the distance to their neighbors decreases. This transition results in the formation of a novel state of active matter: amorphous active solids, the first experimental confirmation of a theoretical prediction put forward more than a decade ago.

Amorphous active solids have a lively inner structure and continuously melt and reform at their boundaries. Although composed of particles that spend most of their time at rest, they steadily propagate through the active liquids with which they coexist, much like traffic jams along crowded highways.

Beyond the specifics of synthetic active matter, these findings should be relevant to understanding motion of robot fleets, bacteria swarms, and animal flocks in nature.