Active matter is material driven out of equilibrium by the local dissipation of work. Well-studied examples include microparticles self-propelled by chemical reactions occurring on their surfaces, or swimming bacteria. Informational active matter is a non-equilibrium material driven by information processing on board each constituent. Swarms of living systems are driven by a mix of information processing and ‘traditional’ activity. Here, we create a minimal model of informational active matter from a hard-disk gas with non-constant diameters. This is a many-body analogue of ratcheting systems such as the fluctuating gear-and-pawl.

In this video, red particles change their diameter between collisions with each other or other passive blue particles. Therefore diameter changes do no work on the gas.

Measurements of environmental fluctuations are used to rectify many-body dynamics into coherent motion, a process that does not change the energy of the gas, only the entropy.

Below, particles are dropped and bounce chaotically, yet their final location is controlled only by manipulating their diameters. This pattern was formed without any work exerted by the particles on each other or the environment.