Nonreciprocity, condensation and chirality in active granular systems

Abstract

We present a journey into the rich non-equilibrium phenomena of active granular systems, a versatile model for studying “living” materials. Our exploration begins with the discovery of non-reciprocal, medium-mediated attraction between motile particles. We show that particles generate reorienting strain fields, leading to an effective chase-and-capture dynamics, which we account for with a coarse-grained theory. Next, we demonstrate a striking form of condensation where a minuscule population of active rods self-assemble into a moving interface, capable of transporting and organizing a vast majority of passive beads. The investigation then turns to particle chirality, where we discover robust “skipping orbits” at hard boundaries. This pronounced edge-hugging provides a simple, high-fidelity mechanism for sorting that is effective even at the single- particle level. Finally, we show how a chiral minority can act as a tunable bias to induce and steer a larger, disordered achiral flock, establishing a new route for designing reconfigurable active materials.