Film Transfer in Helium II: III - Influence of Radiation and Impurities

Transfer of the unsaturated helium II film

The critical transfer rate of the unsaturated helium II film has been measured on surfaces of glass and german silver by a heat conduction method. It is found that a considerable reduction of the transfer rate occurs when the vapour pressure over the film is decreased only slightly below the saturation value. At a given percentage of the saturation pressure, there exists a critical temperature above which film flow will not take place. This critical temperature is shown to be sharply defined, and to decrease with decreasing percentage satura­tion. At the full saturation pressure the transfer rate is markedly different on the various surfaces used, but as the vapour pressure over the film decreases the flow rate tends to become the same for all surfaces. The critical temperature for onset of superfluidity is also independent of the substrate. The temperature dependence of the transfer rate is different from that for the saturated film, but very similar to the variation found in the flow of liquid through channels of width less than that of the saturated film.

Superflow of helium II through narrow slits

The flow of liquid helium II has been investigated under gradients of pressure and temperature in slits of 1μdiameter. Besides the flow rate, the heat current and the pressure difference at the ends of the slit, the pressure at an intermediate point within the slit has been determined. It was found that for superflow the entire drop in pressure and temperature occurs at the narrowest place in the slit. In the remainder of the slit mass flow takes place under effectively zero gradient of pressure or temperature. The experiments also indicate the existence of a critical flow rate beyond which frictional dissipation makes its appearance. The critical rate was determined by four different criteria which yielded consistent results. The temperature dependence of the critical rate is similar to that observed in the helium II film. With flow under a temperature gradient and for higher flow rates the hydrostatic pressure within the slit was found to drop below that at the ends and an explanation for this effect has been suggested. Some experiments with wider slits have shown that in these even for small velocities the transport is a mixture of superflow and normal flow which renders the phenomena very complex.

The thickness of the saturated helium film above and below the λ -point

The properties of the static helium film covering a vertical, polished metal surface, at heights between 0∙4 and 1∙6 cm above bulk liquid and from 1∙2 to 3∙8°K, have been reinvestigated using an improved form of apparatus based on the optical method of Burge & Jackson (1951). The original calibration curve of the instrument was found to be incorrect by about 30%, and an entirely new one has been constructed in which the only assumption is that the refractive indices of film and bulk liquid are the same. At 2∙05°K the film thickness d is given by d = k / H 1/ z , where H is the height above bulk liquid, k = 3∙15 x 10 –6 cm and z = 2∙3. The value of z increases to about 2∙6 as the temperature falls to 1∙32°K , but at temperatures below 2∙05°K it varies slightly with height. The value of k varies little between 1∙2 and 3∙8°K, and it has been shown that the film is essentially the same above and below the λ -point. This confirms the theories of the film which are based primarily on van der Waals’s forces of attraction, and it seems clear that the existence of the film is not connected with the λ -phenomenon. Below the λ -point a thin layer of solid air on the metal surface increases the static film thickness enormously. Above the λ -point this phenomenon does not occur, but extreme temperature homogeneity is necessary in order that the film may form. The differences between these results and those of other investigations of the film are discussed.