On the Hydrodynamic Boundary Condition for Superfluid Flow

We discuss the hydrodynamic boundary condition for a superfluid moving tangentially to a rough surface. Specifically, we argue that the scattering of quantum fluctuations off surface roughness affects the nature of the boundary condition, and that this has important consequences including a new theorized critical speed and the presence of normal fluid at any nonzero speed, even if the boundary is held at zero temperature. This hydrodynamic boundary condition is relevant not only for superfluid helium experiments but also for experiments with trapped dilute Bose-Einstein condensates, in particular those involving atomic waveguides near surfaces.