On the Hydrodynamic Boundary Condition for Superfluid Flow

TL;DR

This paper explores how surface roughness influences the boundary conditions of superfluid flow, revealing effects like a new critical speed and persistent normal fluid presence at any nonzero speed, impacting superfluid and Bose-Einstein condensate experiments.

Contribution

It introduces a novel perspective on the hydrodynamic boundary condition for superfluids, emphasizing the role of quantum fluctuation scattering off surface roughness.

Findings

Surface roughness affects superfluid boundary conditions.

A new critical speed for superfluid flow is proposed.

Normal fluid presence persists at any nonzero speed.

Abstract

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.