Active ploughing through a compressible viscoelastic fluid: Unjamming and emergent nonreciprocity

TL;DR

This paper investigates how active particles interact with a dense, compressible viscoelastic fluid, revealing a transition from trapping to ploughing behavior, and demonstrating emergent nonreciprocal interactions near the jamming-unjamming transition.

Contribution

It introduces a hydrodynamic model showing how active particles generate long-range wakes and exhibit nonreciprocal attraction, advancing understanding of active matter in complex fluids.

Findings

Active particles create long-range density wakes.

Nonreciprocal attraction emerges near jamming transition.

Active particles switch from self-trapping to ploughing with increased activity.

Abstract

A dilute suspension of active Brownian particles in a dense compressible viscoelastic fluid, forms a natural setting to study the emergence of nonreciprocity during a dynamical phase transition. At these densities, the transport of active particles is strongly influenced by the passive medium and shows a dynamical jamming transition as a function of activity and medium density. In the process, the compressible medium is actively churned up -for low activity, the active particle gets self-trapped in a spherical cavity of its own making, while for large activity, the active particle ploughs through the medium, either accompanied by a moving anisotropic wake, or leaving a porous trail. A hydrodynamic approach makes it evident that the active particle generates a long range density wake which breaks fore-aft symmetry, consistent with the simulations. Accounting for the back reaction of the compressible medium leads to (i) dynamical jamming of the active particle, and (ii) a dynamical non-reciprocal attraction between two active particles moving along the same direction, with the trailing particle catching up with the leading one in finite time. We emphasize that these nonreciprocal effects appear only when the active particles are moving and so manifest in the vicinity of the jamming-unjamming transition.