Odd Viscodiffusive Fluids

TL;DR

This paper develops a theory of odd viscodiffusive fluids, revealing how chiral fluids exhibit unique cross-coupled viscous and diffusive transport phenomena, with implications for experimental measurement and biological systems.

Contribution

It introduces the concept of odd viscodiffusive fluids, derives their transport relations, and proposes experimental models and biological examples, expanding understanding of chiral fluid dynamics.

Findings

Transport coefficients derived from microscopic fluctuations.

Chiral liquids naturally exhibit viscodiffusive couplings.

Predictions made for bacterial suspensions in shear flows.

Abstract

We introduce a theory of "odd viscodiffusive fluids," which exhibit three-dimensional odd transport phenomena through the coupling of viscous and diffusive transport. In these fluids, diffusive fluxes may arise from orthogonal velocity gradients and, reciprocally, stresses may arise from concentration gradients. We examine microscopic fluctuations using the recently proposed "flux hypothesis" to derive Green-Kubo and reciprocal relations for the governing transport coefficients. These relations suggest that only parity symmetry, and not time-reversal symmetry, must be broken at the microscopic scale to observe these couplings. Chiral liquids, whether passive or active, are therefore a natural choice as viscodiffusive fluids. We then introduce two analytically tractable model systems, namely a generator and a corresponding reciprocal engine, which illustrate the nature of viscodiffusive cross-coupling in chiral matter and enable the experimental measurement of the novel transport coefficients. Finally, we make the case for chiral bacterial suspensions to be odd viscodiffusive fluids, and use our theory to predict the behaviors exhibited in prior experimental microfluidic studies involving bacterial migration in response to shearing flows.