The theory of nondissipative flow in the helium film

The necessary conditions are examined for a film in contact with its own vapor to flow without dissipation. When the local gas is in equilibrium with the vapor of a bulk reservoir, the equilibrium thickness of the flowing film is the same as that of the film at rest. For nonsteady flow, the predicted thickness depends on a nonequilibrium condition that cannot be deduced from thermodynamic arguments. A plausible form is proposed for the needed condition. It is shown that the film may be thinner than the rest value for times large compared with apparent relaxation times of the system. The results reconcile a number of experiments which had previously appeared contradictory. In addition, the theory is consistent with measurements of the convection speed of third sound and shows why previous efforts to explain the attenuation of third sound have failed