Critical velocities in 3He-4He mixtures

Mutual friction between 3He and 4He II plays an important role in the hydrodynamics of 3He–4He mixtures. A close analogy exists with mutual friction between the normal and the superfluid components in pure 4He II. In pure 4He 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 3He–4He 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