Emergence of macroscopic directed motion in populations of motile colloids
Top Cited Papers
- 6 November 2013
- journal article
- Published by Springer Science and Business Media LLC in Nature
- Vol. 503 (7474), 95-98
- https://doi.org/10.1038/nature12673
Abstract
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 level. This hypothesis has been supported by agent-based simulations. However, more complex collective behaviours have been systematically found in experiments, including the formation of vortices, fluctuating swarms, clustering and swirling. All these (living and man-made) model systems (bacteria, biofilaments and molecular motors, shaken grains and reactive colloids) 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 particles. Our experiments demonstrate that genuine physical interactions at the individual level are sufficient to set homogeneous active populations into stable directed motion.Keywords
This publication has 27 references indexed in Scilit:
- Hydrodynamics of soft active matterReviews of Modern Physics, 2013
- Confinement Stabilizes a Bacterial Suspension into a Spiral VortexPhysical Review Letters, 2013
- Collective motionPhysics Reports, 2012
- Large-scale vortex lattice emerging from collectively moving microtubulesNature, 2012
- Polar patterns of driven filamentsNature, 2010
- Collective motion and density fluctuations in bacterial coloniesProceedings of the National Academy of Sciences of the United States of America, 2010
- From Disorder to Order in Marching LocustsScience, 2006
- Hydrodynamics and phases of flocksAnnals of Physics, 2005
- Onset of Collective and Cohesive MotionPhysical Review Letters, 2004
- Novel Type of Phase Transition in a System of Self-Driven ParticlesPhysical Review Letters, 1995