Edge states, pairing, and sorting of motile chiral particles

Abstract: We present experiments on chiral active polar particles, realized as vibrated granular rods, revealing the formation of robust ``skipping orbits'' at hard boundaries. These edge states exhibit a net circulation opposite to the particles' intrinsic rotation and lead to a pronounced accumulation at the boundary, stronger than for their achiral counterparts. The directed nature of these orbits provides a simple yet high-fidelity mechanism for chiral sorting -- even for solitary particles, unlike in T Barois et al., Phys. Rev. Lett. 125 , 238003 (2020). We propose a unified theoretical framework for boundary interactions of both chiral and achiral particles. In this model, an effective outward radial force, proportional to motility and chirality, explains the observed boundary-hugging. Our theory predicts, and our experiments confirm, a transition in the pairing of two particles of the same chirality, from apolar spinners to polar circle walkers, with increasing packing fraction of an ambient medium of beads.