Snapshots of (L-R) Swarm, Rotating Cluster, Polar Cluster and Disordered Cluster phases.

Plastic particles show decreasing organization from left to right, ranging from orderly swarm (far left) to random movement (far right). Credit: H. Karani et al./Phys. Rev. Lett.

Physics

From chaos to order: ‘microswimmers’ form a swarm

Researchers wrangle tiny plastic spheres into imitating the synchronized swimming of a group of microbes.

Tiny plastic beads that are wandering aimlessly through water can spontaneously form organized swarms and clusters — just like swimming bacteria.

Petia Vlahovska and her colleagues at Northwestern University in Evanston, Illinois, sought to artificially replicate bacterial swarming using microscopic polystyrene spheres. The researchers placed these microswimmers in oil and subjected them to pulses from an electric field. That created a polarized charge on the spheres’ surfaces, causing the microswimmers to suddenly spin and propel themselves through the fluid.

If the pulses were widely spaced enough for the spheres to depolarize, they moved in random directions and bunched into disorganized clusters. But when the team shortened the time between pulses, the lingering charge had a ‘memory’ effect on the bead. This resulted in more organized clusters that aligned with each other and eventually began rotating.

Shortening the pulses’ duration resulted in a new behaviour: the formation of one continuous, roiling swarm of beads. This tunability could allow the particles to be used to test theories of collective dynamics, the authors write.