# Odd viscosity in chiral active fluids

## TL;DR

This paper explores how chiral active fluids, composed of self-spinning particles, exhibit odd viscosity due to broken symmetries, leading to unique flow behaviors and potential applications in microscopic machinery.

## Contribution

It demonstrates the emergence of odd viscosity in chiral active fluids and analyzes its effects on flow and density modulations, highlighting novel out-of-equilibrium phenomena.

## Key findings

- Chiral active fluids break parity and time-reversal symmetry.
- Odd viscosity couples pressure and vorticity, affecting flow patterns.
- Transverse flow response suggests applications in microscopic machines.

## Abstract

Chiral active fluids are materials composed of self-spinning rotors that continuously inject energy and angular momentum at the microscale. Out-of-equilibrium fluids with active-rotor constituents have been experimentally realized using nanoscale biomolecular motors, microscale active colloids, or macroscale driven chiral grains. Here, we show how such chiral active fluids break both parity and time-reversal symmetries in their steady states, giving rise to a dissipationless linear-response coefficient called odd viscosity in their constitutive relations. Odd viscosity couples pressure and vorticity leading, for example, to density modulations within a vortex profile. Moreover, chiral active fluids flow in the direction transverse to applied compression as in shock propagation experiments. We envision that this collective transverse response may be exploited to design self-assembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active-rotors fluids.