Critical Velocities in 3 He– 4 He Mixtures

Mutual friction between 3 He and 4 He II plays an important role in the hydrodynamics of 3 He– 4 He mixtures. A close analogy exists with mutual friction between the normal and the superfluid components in pure 4 He II. In pure 4 He II, the flow properties are characterised by the appearance of critical velocities below which no mutual friction occurs. In this paper we report on the first observation of non-zero critical velocities in 3 He– 4 He mixtures in flow channels consisting of bundles of (about 100) parallel tubes. At the critical velocity the temperature rise at the exit of the channel was on the order of a few millikelvin. Both continuous and hysteretic transitions were observed. The experimental results show that the product of the critical velocities and the diameter is in good approximation constant.