A flock of birds or a school of fish sometimes swirls into a vortex, even though no individual commands 'spin.' Such collective rotation helps the group stay together and mix better. It is usually explained by the individuals being asymmetric, like a screw, or pushing each other with a twist. But a new study finds another route.
In a computer model, particles resembling bacteria swim through a medium with passive 'dust motes.' These motes jitter randomly, and the active particles dodge them. At moderate density, such an environment acts as feedback: the runners gather into large ordered crystals. The most surprising part is that these crystals do not stay put but rotate uniformly, like a disk.
Previously, it was thought that rotation required either chiral particles or specific interactions. Here, rotation arises only for particles that maintain their direction for a long time — almost like bullets. If particles change course frequently, as in ordinary Brownian motion, crystals still form but do not rotate. So the secret lies in the 'stubbornness' of the runners.
This discovery shows that the environment is not just a backdrop but an active participant in self-organization. Perhaps this could be used to create materials with unusual responses, where rotation is switched on and off simply by tuning the medium.
In the model, runner particles with a long 'directional memory' move almost in a straight line, like bullets, and it is they that form the rotating crystals — not those that dart about chaotically.
In science fiction, swarms of nanobots often rotate in unison for defense or attack — here we see a real mechanism by which the environment can make them spin without external control.