Hysteresis in η/s for QFTs dual to spherical black holes

We define and compute the (analog) shear viscosity to entropy density ratio η~ / s for the QFTs dual to spherical AdS black holes both in Einstein and Gauss–Bonnet gravity in five spacetime dimensions. Although in this case, owing to the lack of translational symmetry of the background, η~ does not have the usual hydrodynamic meaning, it can be still interpreted as the rate of entropy production due to a strain. At large and small temperatures it is found that η~ / s is a monotonic increasing function of the temperature. In particular, at large temperatures it approaches a constant value, whereas at small temperatures, when the black hole has a regular, stable extremal limit, η~ / s goes to zero with scaling law behavior. Whenever the phase diagram of the black hole has a Van der Waals-like behavior, i.e. it is characterized by the presence of two stable states (small and large black holes), connected by a meta-stable region (intermediate black holes), the system evolution must occur through the meta-stable region- and temperature-dependent hysteresis of η~ / s is generated by non-equilibrium thermodynamics.