Edge states, pairing, and sorting of motile chiral particles.

We present experiments on chiral active polar particles, realized as vibrated granular rods, revealing evidence for "skipping orbits" at hard boundaries. These classical 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)0031-900710.1103/PhysRevLett.125.238003]. 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.