Proc. Natl Acad. Sci. USA 113, 12919–12924 (2016)

Quite apart from the stampeding herds that have captured the imagination of the active-matter community, there lies a wealth of non-equilibrium phenomena in systems of particles that don't move anywhere at all — or so Benjamin van Zuiden and co-workers found, when they set out to predict the behaviour of an ensemble of self-spinning dimers. Immobile, and yet actively rotating, the dimers experienced a range of unique non-equilibrium steady states that looked just like crystals, liquids and glasses — but were driven by entirely different forces.

Using simulations and theory, the team examined a system of active spinners with orientation-dependent repulsive interactions. They found that when the density was low, the dimers self-organized into a triangular lattice and phase-locked their orientations into a pattern that varied periodically. When the density increased, the crystal melted as the active torques competed with the interactions. Emergent edge currents heralded a non-equilibrium transition to the active spinner liquid, and disappeared again when the system arrested at higher densities.