| Summary: | The work reported in this thesis divides naturally into two parts. The first concerns the development of a technique for isolating the supra-surface film of Helium II from the bulk liquid and the Tap our pressure over it, and the study of properties of the film in equilibrium with the gas phase only. The second part of the thesis gives the results of a general investigation of high transport rates in the film. A solution of the problem of isolating the film was seen to depend upon the devising of a practical form of super-fluid filter. Capillary leaks through metal tubes containing closely packed wires, such as those used by Allen and Misener (J. F. Allen and A. D. Misener, Nature, 141, 75, 1938) in their work on the viscosity of the bulk liquid were found to be suitable. The construction and testing of such wire-tube filters is described. The performance data are tabulated for a typical one, consisting in this case of two tubes in parallel, each containing 900 wires of #47 Eureka in a 5 cm. long cupronickel tube, initially .157" outside diameter, .072" inside diameter, drawn down finally to .104" outside diameter. Representative values of the amounts of gas, N.T.P., passed by this filter into a vacuum from one atmosphere of helium were: .01 cc./min. at 290°K; .06 cc./min. at 90°K; .45 cc./min. at 20°K; 7 cc./min. at 4.3°K. As the pressure is reduced in the helium temperature region, the amount of gas passed diminishes correspondingly, and at 4 cm. Hg., 2.2°K, it becomes .07 cc./min. N.T.P. Below the λ-point, the filters pass no measureable quantity of gas when they are not dipping into liquid. The total transport circumference provided by the above combination of two leaks in parallel is 30 cm., giving a maximum possible film flow corresponding to 110 cc. gas/min., N.T.P. It is pointed cut that the filters can be easily incorporated into metal apparatus, and an example is given of their application to the study of the vapour pressure over the film, and also the effect on the transport rate of lowering the vapour pressure over the film. It is found that the vapour pressure in equilibrium with the isolated film is the same as that of the bulk liquid at the same temperature. Above a critical rate of pumping, apparently determined by the incidental film flow up an auxiliary tube, and the evaporation loss of the particular arrangement used, the pressure over the isolated film suddenly decreases, and transport stops. It is shown that at the same time there is a sudden fall of temperature at the transport surface, and apparently a thermomechanical effect in the film which prevents it from moving into the container being pumped. It seems possible that the film is still present but immobilized, and if this is the case, oppositely directed thermomechanical forces along the path do not cancel each other, but act independently. A variety of geometrical arrangements hare been investigated for the occurrence of high transfer rates. It has been found that the transport through a small hole in a surface can be several times larger than that calculated from the perimeter of the hole and the rate established by Daunt and Mendelssohn (J. G. Daunt and K. Mendelssohn, Proc. Roy. Soc. A, 170, 423, 1939) for flow over cylindrical beakers. A number of such holes hare been tested, and it has been found that there is sometimes a strong dependence of the transport rate on the height of the hole above the upper liquid level, and on the shape of the inside surface of the beaker. Experiments have been made similar to those of K. R. Atkins (Nature, 161, 925, 1948), who reported high transfer rates in capillaries ending in bulbs, and under the influence of the temperature gradients. The results of Atkins have not been verified, and a probable explanation, based upon the interpretation of his geometrical arrangement, has been given for the apparently high rates observed in the second of his experiments. Some possible explanations for the other discrepancies are considered, and it is recognized that hitherto unexplored experimental factors may be involved. Experiments have been made to eliminate certain possibilities, one in particular being the formation of thick films under certain conditions. This has led incidentally to an observation of a striking example of the high conductivity of helium as a free liquid, apart from the presence of solids on which subsurface could be carried. A summary of the experimental background of the field investigated, and some of the theoretical interpretations, is given in two chapters at the beginning of the thesis.
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