Nature | Editorial



Follow the crowd

The behaviour of millions of minuscule beads reveals some secrets of collective motion.

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Their claimed wisdom is disputed, but no one should doubt the ability of crowds to make collective decisions. Flocks of starlings twist in unison like smoke swirls in a summer sky, and shoals of fish tack and veer as if in response to electric shocks. Locusts swarm and herds of humans can head in very unwise directions indeed. Even simple bacteria show collective behaviour.

Individuals in each of these systems have very different abilities to communicate with each other, to actively pass on information about their intended actions, so why does collective behaviour across all scales look so similar? Is there some unknown sensation that allows the individuals that comprise such seemingly intelligent crowds to steer; some distant wisdom? Although such behaviour is easy to observe, it has proved hard to capture in simple physical models. If we could master it, the information that this might yield could help engineers to develop swarming robots and design safer crowd-control measures.

On page 95 of this issue, researchers in France report that they have induced collective motion in millions of tiny plastic beads. The miniature spheres, they say, can sense the orientation of their rolling neighbours and adjust their own actions accordingly. In this way, the scientists can encourage the beads to follow the crowd, simply by pouring more of them into the system.

The scientists — Denis Bartolo and his colleagues — squeezed a conducting liquid suspension of the beads into a miniature racetrack sandwiched between two glass plates, and watched what happened when they applied an electric field. An electrohydrodynamic curiosity called Quincke rotation causes the beads to twitch and then start to roll. At first, they head off at constant speed but in all directions. Then, as more beads are added and their number passes a critical point, the individuals form a crowd and their individual motion coalesces into coherent movement in a unified direction — just like that of a flock of birds. This happens, the scientists say, because the rolling spheres can sense the orientation of their neighbours through simple hydrodynamic and electrostatic interactions.

From plastic balls to intelligent dust: there could be interesting implications here for work that aims to harness self-propelled and swarming microparticles, for example, to diagnose disease or improve communications. The writer Mark Twain said: “Whenever you find yourself on the side of the majority, it is time to pause and reflect.” But sometimes, the majority really does rule.

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