Active nonreciprocal attraction between motile particles in an elastic medium

TL;DR

This study demonstrates that self-propelled particles in an elastic medium exhibit nonreciprocal attraction due to elastic strains, with experiments and simulations confirming the theoretical model's predictions.

Contribution

The paper introduces a coarse-grained theory explaining nonreciprocal attraction between motile particles in elastic media, supported by experimental and simulation evidence.

Findings

Particles turn and move towards each other in elastic media.

Elastic strains create nonreciprocal sensing and pursuit behaviors.

Displacement fields follow a |x|^{-1/2} decay tail.

Abstract

We show from experiments and simulations on vibration-activated granular matter that self-propelled polar rods in an elastic medium on a substrate turn and move towards each other. We account for this effective attraction through a coarse-grained theory of a motile particle as a moving point-force density that creates elastic strains in the medium that reorient other particles. Our measurements confirm qualitatively the predicted features of the distortions created by the rods, including the $|x|^{-1/2}$ tail of the trailing displacement field and non-reciprocal sensing and pursuit. A discrepancy between the magnitudes of displacements along and transverse to the direction of motion remains. Our theory should be of relevance to the interaction of motile cells in the extracellular matrix or in a supported layer of gel or tissue.