Boundary effects of Hall viscosity

In this talk, we examine in detail the ambiguity of viscosity coefficients in two dimensions, where different components of the dissipative and non-dissipative viscosity tensor correspond to physically identical effects in the bulk—redundant viscosities give rise to the same bulk viscous force and can be shifted into one another by adding divergenceless "contact terms" to the stress tensor of the system. Considering fluid flow in systems with a boundary, we are able to distinguish between the redundant viscosity components, and interpret the contact terms as a specific choice of stress boundary conditions. We examine the waves on the surface of a chiral fluid to probe this phenomenon, finding that the dispersion of these surface waves is sensitive to the contact terms, and contextualize our results with recent experiments on chiral active fluids. Lastly, we interpret the quantum Hall fluid through the lens of our framework, showing that the boundary term one adds to the Wen-Zee action to render it gauge invariant on systems with a boundary can be interpreted as a contact term (or as modified stress boundary conditions).