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From the formation of animal flocks to the emergence of coordinate motion inbacterial swarms, at all scales populations of motile organisms displaycoherent collective motion. This consistent behavior strongly contrasts withthe difference in communication abilities between the individuals. Guided bythis universal feature, physicists have proposed that solely alignment rules atthe individual level could account for the emergence of unidirectional motionat the group level. This hypothesis has been supported by agent-basedsimulations. However, more complex collective behaviors have beensystematically found in experiments including the formation of vortices,fluctuating swarms, clustering and swirling. All these model systemspredominantly rely on actual collisions to display collective motion. As aresult, the potential local alignment rules are entangled with more complex,often unknown, interactions. The large-scale behavior of the populationstherefore depends on these uncontrolled microscopic couplings. Here, wedemonstrate a new phase of active matter. We reveal that dilute populations ofmillions of colloidal rollers self-organize to achieve coherent motion along aunique direction, with very few density and velocity fluctuations. Identifyingthe microscopic interactions between the rollers allows a theoreticaldescription of this polar-liquid state. Comparison of the theory withexperiment suggests that hydrodynamic interactions promote the emergence ofcollective motion either in the form of a single macroscopic flock at lowdensities, or in that of a homogenous polar phase at higher densities.Furthermore, hydrodynamics protects the polar-liquid state from the giantdensity fluctuations. Our experiments demonstrate that genuine physicalinteractions at the individual level are sufficient to set homogeneous activepopulations into stable directed motion.

Emergence of macroscopic directed motion in populations of motile colloids