UBC Theses and Dissertations
Some dynamic properties of liquid helium Chopra, Kasturi Lal
This thesis describes three independent investigations of the dynamic properties of the superfluid He II film flow and the viscous flow of liquid helium in acoustic streams generated by the gradient of the radiation pressure occuring due to attenuation of sound propagating in liquid helium. The non-isothermal He II film flow induced gravitationally is studied for the covered (Cu-Ni cover of the wire-filled-tube superleak intact) and the uncovered film (Cu-Ni cover peeled off) flowing through a superleak when subjected to a thermal plus gravitational potential at a point midway between the two transfer points. It is found that the critical transfer rate of the covered film is not affected by the nature and magnitude of this potential. For the case of uncovered film warmed in the middle, in contrast to that of the covered film, the transfer can be reversed by a suitable thermal potential, the maximum rate being the same in either direction. In the case of cooling by direct pumping over the uncovered film, however, film flow stops either way; in the light of other results, we have given an explanation to this controversial observation in terms of thinning down of the film to its non-superfluid layers, a conclusion possibly equivalent to a shift of λ-point for thinner films. A closed glass capsule, sealed off at room temperature with 750 psig of He gas, sufficient to provide enough liquid at helium temperatures, is used to measure the gravitationally induced transfer rates of He II film at temperatures between 0.3°K and the λ-point. Like Ambler and Kurti, we find that the transfer rate rises by 25% above the flat minimum near 1°K though, in contrast to the steady rise observed by these authors, our results indicate slight flattening near 0.3°K. This confirms that, like thermal properties, the film flow property of He II also undergoes a radical change below 1°K. A technique is developed whereby fine particles of a suitable mixture of solid H₂ plus D₂ are suspended in liquid helium. A number of experiments are suggested to study flow properties of liquid He visually by using these particles as indicators. We have used these particles as indicators in acoustic streaming experiments designed to measure the ratio of the second to the first coefficient of viscosity. These two coefficients occur in the expression for absorption coefficient of sound and are calculated by Khalatnikov for He II. The streaming is observed to be turbulent to the lowest possible ultrasonic intensity. Thus, such determination is not feasible. The streaming in He II is found to obey the classical equation of turbulence. It is independent of temperature and shows no anomaly at or above the λ-point, possibly due to complete absorption of sound in the turbulent medium.
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