Emergent Kelvin waves in chiral active matter

TL;DR

This paper develops a hydrodynamic theory for far-from-equilibrium chiral active fluids, revealing spontaneous boundary currents and chiral Kelvin waves, thus advancing understanding of non-equilibrium emergent phenomena in active matter.

Contribution

The authors derive first-principles hydrodynamic equations for chiral active fluids, explaining spontaneous boundary currents and the existence of chiral Kelvin waves without external rotation.

Findings

Spontaneous boundary currents analogous to oceanic coastal currents.

Existence of acoustic chiral Kelvin waves in confined chiral active matter.

Transport coefficients estimated and compared favorably with forced flow data.

Abstract

The phenomenological equations of hydrodynamics describe emergent behavior in many body systems. Their forms and the associated phenomena are well established when the quiescent state of the system is one of thermodynamic equilibrium, yet away from equilibrium relatively little is firmly established. Here, we deduce directly from first principles the hydrodynamic equations for a system far from equilibrium, a chiral active fluid in which both parity and time-reversal symmetries are broken. With our theory, we rationalize the emergence of a spontaneous boundary current in the confined fluid, a feature forbidden at equilibrium, which allows us to extract estimates of transport coefficients that we favorably compare to forced flows. The hydrodynamic solution reveals that the boundary current is analogous to a quasigeostrophic coastal current, a well known phenomenon in oceanography. Such currents are conjugate to a class of chiral waves called Kelvin waves. Motivated by this analogy, we demonstrate that an acoustic chiral Kelvin wave mode also exists in confined chiral active matter in the absence of an imposed rotation, originating from the spontaneous emergence of a Coriolis-like parameter in the bound modes of a chiral fluid.