Out-of-bounds hydrodynamics in anisotropic Dirac fluids

We study the hydrodynamic behavior in two-dimensional, interacting electronic systems with merging Dirac points at charge neutrality. The dispersion along one crystallographic direction is Dirac-like, while it is Newtonian-like in the orthogonal direction. As a result, the electrical conductivity is metallic in one and insulating in the other direction. One element of the shear viscosity tensor violates the famous lower bound for the shear viscosity to entropy density ratio,an effect that can be probed via anisotropic thermal Hagen- Poiseuille flow.