Populations of millions of colloidal rolling particles are shown to self-organize to achieve coherent motion; comparison between experiment and theory based on the microscopic interactions between these ‘rollers’ suggests that hydrodynamic interactions promote the emergence of the collective motion. Collective motion can be observed in the natural world at all scales, from flocking birds to schooling fish and swarming bacteria, but it is difficult to capture such behaviour in simple physical models. Artificial 'active matter' systems that show collective behaviour usually rely on collisions, making the description of interactions complex. Denis Bartolo and colleagues have now developed a unique experimental system consisting of self-propelled rolling spheres that self-organize and move in one direction in a crowd of millions. The spheres 'sense' each other via straightforward hydrodynamic interactions so that all parameters can be easily calculated and tuned. This work demonstrates that genuine physical interactions at the individual level are sufficient to set homogeneous active populations into stable directed motion. The system could be used to model natural collective motion and to design new self-organized materials and swarming microrobots. From the formation of animal flocks to the emergence of coordinated motion in bacterial swarms, populations of motile organisms at all scales display coherent collective motion. This consistent behaviour strongly contrasts with the difference in communication abilities between the individuals. On the basis of this universal feature, it has been proposed that alignment rules at the individual level could solely account for the emergence of unidirectional motion at the group level1,2,3,4. This hypothesis has been supported by agent-based simulations1,5,6. However, more complex collective behaviours have been systematically found in experiments, including the formation of vortices7,8,9, fluctuating swarms7,10, clustering11,12 and swirling13,14,15,16. All these (living and man-made) model systems (bacteria9,10,16, biofilaments and molecular motors7,8,13, shaken grains14,15 and reactive colloids11,12) predominantly rely on actual collisions to generate collective motion. As a result, the potential local alignment rules are entangled with more complex, and often unknown, interactions. The large-scale behaviour of the populations therefore strongly depends on these uncontrolled microscopic couplings, which are extremely challenging to measure and describe theoretically. Here we report that dilute populations of millions of colloidal rolling particles self-organize to achieve coherent motion in a unique direction, with very few density and velocity fluctuations. Quantitatively identifying the microscopic interactions between the rollers allows a theoretical description of this polar-liquid state. Comparison of the theory with experiment suggests that hydrodynamic interactions promote the emergence of collective motion either in the form of a single macroscopic ‘flock’, at low densities, or in that of a homogenous polar phase, at higher densities. Furthermore, hydrodynamics protects the polar-liquid state from the giant density fluctuations that were hitherto considered the hallmark of populations of self-propelled particles2,3,17. Our experiments demonstrate that genuine physical interactions at the individual level are sufficient to set homogeneous active populations into stable directed motion.

Emergence of macroscopic directed motion in populations of motile colloids