# Odd viscosity in two-dimensional incompressible fluids

## TL;DR

This paper explores the observable effects of odd viscosity in two-dimensional incompressible fluids, revealing how it influences flow, forces, and bubble shapes, and proposing a measurement protocol for odd viscostance.

## Contribution

It provides the first detailed analysis of odd viscosity effects on boundary conditions and flow shapes in 2D hydrodynamics, including exact solutions for bubble deformation.

## Key findings

- Flow is independent of odd viscosity under certain boundary conditions.
- Force on a closed contour does not depend on odd viscosity.
- Bubble shape in shear flow depends explicitly on odd viscosity.

## Abstract

In this work, we present observable consequences of parity-violating odd viscosity term in incompressible 2+1D hydrodynamics. For boundary conditions depending on the velocity field (flow) alone we show that: (i) The fluid flow quantified by the velocity field is independent of odd viscosity, (ii) The force acting on a closed contour is independent of odd viscosity, and (iii) The odd viscosity part of torque on a closed contour is proportional to the rate of change of area enclosed by the contour with the proportionality constant being twice the odd viscosity. The last statement allows us to define a measurement protocol of {\it odd viscostance} in analogy to Hall resistance measurements. We also consider {\it no-stress} boundary conditions which explicitly depend on odd viscosity. A classic hydrodynamics problem with no-stress boundary conditions is that of a bubble in a planar Stokes flow. We solve this problem exactly for shear and hyperbolic flows and show that the steady-state shape of the bubble in the shear flow depends explicitly on the value of odd viscosity.