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.
Highlights
The emergence of collective motion in groups of living creatures or synthetic motile units is a well-established physical process [1,2,3,4,5,6]: Self-propelled particles move coherently along the same direction whenever velocityalignment interactions overcome orientational perturbations favoring isotropic random motion
The system reaches a stationary state thanks to a slow coarsening dynamics illustrated in Fig. 3(d), where we show the temporal evolution of the length of two macroscopic active solids and of the overall solid fraction
Combining experiments on Quincke rollers and active-matter theory, we show that the phase behavior of polar active units is controlled by a series of two dynamical transitions: a Flocking transition that transforms active gases into spontaneously flowing liquids and a motility-induced phase separation that results in the freezing of these polar fluids and the formation of active solids
Summary
The emergence of collective motion in groups of living creatures or synthetic motile units is a well-established physical process [1,2,3,4,5,6]: Self-propelled particles move coherently along the same direction whenever velocityalignment interactions overcome orientational perturbations favoring isotropic random motion This minimal picture goes back to the seminal work of Vicsek et al [7] and made it possible to elucidate the flocking dynamics of systems as diverse as bird groups, polymers surfing on motility assays, shaken grains, active colloidal fluids, and drone fleets [6,8,9,10,11,12,13]. Using numerical simulations and analytical theory, we elucidate all our experimental findings and demonstrate that the solidification of colloidal flocks provides a realization of the long-sought-after complete motility-induced phase separation (MIPS) [5,20,21,22,23,24,25,26,27,28]
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