Dissipative flow in the superfluid helium film in the temperature region 1.63K to 0.01K

Experiments on helium film flow over a stainless steel beaker rim were carried out in the temperature region 1.63K to 11mK. No previous measurements have been made below 35mK. The results of other studies of film flow below 1K are mutually conflicting; this is thought to originate from poor temperature stability, inadequate vibration isolation and contamination of the substrate. Careful attention was paid to these points in the design and construction of a demagnetisation cryostat, full details of which are given. Flow experiments from initial level differences of ~8mm confirmed the existence of a range of metastable transfer rates at all temperatures. The variation of the mean rate with rim height suggests that the film profile should be calculated using a van der Waals' exponent, n, of 2.85 ± .25. The low temperature increase in transfer rate reported by others was observed, but did not extend below 1K. Instead, the mean transfer rate was approximately constant from 1K to 400mK, in direct conflict with the predictions of thermal fluctuation theories of superfluid dissipation. Below 250mK the results corroborated closely those of Crum et al in that the transfer rate fell sharply with decreasing temperature. Discrepancies arose between runs below 60mK, some showing the transfer rate to decrease monotonically with temperature down to 20mK whilst others indicated a levelling out of the transfer rate, followed by a slight increase at the lowest temperatures. These effects were thought to originate from changes in the concentration of the ³He impurity in the film. Steady state driven flow experiments at level differences of