Dissipation in the flow of 4He films over a coarse alumina substrate

We report in depth measurements of the dissipation-induced chemical-potential gradient for a flowing 4He film. Both temperature and film-thickness gradients as a result of dissipation were recorded with the use of third-sound time-of-flight techniques. This allowed us to account for changes in the flow velocity as a result of the film thinning and record the total change in the chemical potential across the dissipation region. In addition we used a relatively coarse substrate surface to search for vortex-pinning effects. Our results are consistent with flow-induced vortex depairing as the cause of dissipation, as seen by others