Swarming of micron-sized hematite cubes in a rotating magnetic field – Experiments

•Micron-sized hematite cubes form swarms under rotating magnetic field.•Swarms rotate along the field with a lower frequency.•Swarm rotation can be quantitatively explained by a theoretical model. Energy input by under-field rotation of particles drives the systems to emergent non-equilibrium states. Here we investigate the suspension of rotating magnetic cubes. Micron-sized hematite cubes are synthesized and observed microscopically. When exposed to a rotating magnetic field, they form rotating swarms that interact with each other like liquid droplets. We describe the swarming behaviour and its limits and characterize the swarm size and angular velocity dependence on the magnetic field strength and frequency. A quantitative agreement with a theoretical model is found for the angular velocity of swarms as a function of the field frequency.