Collective Hall current in chiral active fluids: Coupling of phase and mass transport through traveling bands

TL;DR

This paper demonstrates the emergence of stable traveling bands in active fluids of circle swimmers, which induce a collective Hall effect through coupling of phase and mass transport, revealing novel transport phenomena.

Contribution

It introduces non-dispersive soliton solutions in active fluids that generate a collective Hall effect, a novel transport property not seen in passive systems.

Findings

Traveling bands are stable and non-dispersive in simulations.

These bands induce a bulk particle current perpendicular to propagation.

A transition occurs from traveling bands to synchronized states with polar clusters.

Abstract

Active fluids composed of constituents that are constantly driven away from thermal equilibrium can support spontaneous currents and can be engineered to have unconventional transport properties. Here we report the emergence of (meta-)stable traveling bands in computer simulations of aligning circle swimmers. These bands are different from polar flocks and we show that they can be understood as non-dispersive soliton solutions of the underlying non-linear hydrodynamic equations with constant celerity (phase propagation speed) that is much larger than the propulsion speed. In contrast to solitons in passive media, these bands can induce a bulk particle current with a component perpendicular to the propagation direction, thus constituting a collective Hall (or Magnus) effect. Traveling bands require sufficiently small orbits and undergo a discontinuous transition into a synchronized state with transient polar clusters for large orbital radii.