Collective Dynamics in Chiral Granular Gases

We discuss the dynamics of chiral granular matter activated by mechanical vibrations on an oscillated substrate. Each specimen is disk shaped with tilted legs which are shown to induce clockwise or anticlockwise rotation when viewed from above depending on their tilt angles and the vibration strength. The grains are 3D printed and identical to within manufacturing errors.  Global rotation states are observed with angular frequencies which depend on the system size and the area fraction of the near identical granular matter below the maximum packing fraction. Above a critical area fraction, the individual grains orbit about the center in a circular arena while preserving their relative radial position. Whereas transitions between well-defined orbits are observed below this critical fraction. The orbital speed is also observed to change with area fraction with linear dependence with radial distance at higher area fractions, and sub-linear increase at lower area fractions. We shall discuss the observed decreasing orbital speeds with system size in terms of a minimal model which accounts for the pair-wise interaction of the grains during collisions. The effect of changing friction between grains and boundary walls will be further discussed.