Flocking behaviour in particles

Microscopic artificial entities, known as Janus particles, when invested with a form of sight, exhibit spontaneous flocking and crowding behaviour similar to that shown by birds, fish and humans.

The finding, the result of experiments conducted by a team led by physicist François Lavergne from University of Konstanz, Germany, has implications for understanding crowd behaviour, and for the development of autonomous robots.

Janus particles, created more than two decades ago, are items typically only a couple of hundred nanometres across that are coated with two materials producing opposing effects. For instance, one half might attract water, while the other repels it.

For this work, Lavergne and colleagues applied a substance that imbued Janus particles with a version of sight – a way to detect other Janus particles – within particular narrow or broad fields.

The Janus entities are members of a class of materials known as active Brownian particles (APBs), of great interest to roboticists and physicists alike. APBs are small enough to behave like molecules trapped in gas or liquid – that is, when not subject to any external forces they undergo Brownian motion, the sort of random drifting and colliding that ensures that after a given period of time they are randomly distributed through the space available to them.

However, they are also large enough to respond in predictable ways when force is applied. Earlier research had shown that APBs will move out of Brownian drifting and form instead into densely packed clusters when outside influence is applied.

Lavergne’s team, however, wondered if similar behaviour would emerge if the particles were located in “entirely empty surroundings” and left to operate autonomously.

Hence, the specially designed Janus particles were let loose with just two properties: the ability to detect other particles within pre-defined fields of view, and the ability to move towards them when they did so.

The goal, the researchers explain in a paper published in the journal Science, was to achieve a “delicate balance of repulsive, aligning, and attractive interactions”.

They were successful, seeing group formation emerge as a result of only two basic abilities. The way in which the Janus particles clumped together was dependent on the relationship between their field of view and the force with which they could move once a fellow particle was detected.

“For narrow fields of view, individuals gathered into cohesive non-polarised groups without requiring active reorientations,” the researchers report.

“For wider fields of view, cohesion could be achieved by lowering the response threshold. We expect this motility-induced cohesion mechanism to be relevant not only for the self-organisation of living systems, but also for the design of robust and scalable autonomous systems.”

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