Symmetry-breaking motility of an active hinge in a crowded channel

A recent experiment [K. Son, Y. Choe, E. Kwon, L. G. Rigon, Y. Baek and H.-Y. Kim, Soft Matter, 2024, 20, 2777-2788] showed that self-propelled particles confined within a circular boundary filled with granular medium spontaneously form a motile cluster that stays on the boundary. This cluster exhibits persistent (counter)clockwise motion driven by symmetry breaking, which arises from a positive feedback between the asymmetry of the cluster and those of the surrounding granular medium. To investigate this symmetry-breaking mechanism in broader contexts, we propose and analyze the dynamics of an active hinge moving through a crowded two-dimensional channel. Through extensive numerical simulations, we find that the lifetime of the hinge's motile state varies nonmonotonically with the packing fraction of the granular medium. Furthermore, we observe an abrupt transition in the configuration of passive particles that sustain hinge motility as the hinge's maximum angle relative to the channel wall increases. These findings point to the possibility of designing superstructures composed of passive granular media doped with a small number of active elements, whose dynamic modes can be switched by tuning the properties of their components.