Fluid Dynamics and Viscosity in Strongly Correlated Fluids

Abstract

We review the modern view of fluid dynamics as an effective low energy, long wavelength theory of many body systems at finite temperature. We introduce the concept of a nearly perfect fluid, defined by a ratio $\eta/s$ of shear viscosity to entropy density of order $\hbar/k_B$ or less. Nearly perfect fluids exhibit hydrodynamic behavior at all distances down to the microscopic length scale of the fluid. We summarize arguments that suggest that there is fundamental limit to fluidity, and we review the current experimental situation of measurements of $\eta/s$ in strongly coupled quantum fluids.Comment: 71 pages, prepared for Annual Reviews of Nuclear and Particle Science; Arxiv version contains additional endnotes and references. (Second revised version: Final version for Annual Reviews, minor corrections and additional figures, endnotes expanded and new references.