Particle kinetics and collective dynamics in chiral gasses, fluids, and crystals

The velocity profile in an active two-dimensional chiral material---composed of centimeter-scale geared particles confined in a circular vibrating chamber---is measured for particle concentrations ranging from a dilute gas to a dense crystalline packing. In the dilute regime, the average particle velocity vanishes except within a distance of a single particle diameter of the chamber wall. At intermediate concentrations, the velocity profile develops an edge current that decays exponentially from the wall. A crystalline packing of chiral particles rotates as a solid body. These velocity profiles are well fit by a two-parameter linear model in which the transitional and rotational velocities of particles relax to match the motion of the surrounding particles. The corresponding relaxation rates increase continuously and smoothly across these phases of chiral matter and diverge at the maximum packing density.