The angular acceleration of liquid helium II

Abstract
Direct measurements have been made of the torques required to produce angular acceleration and retardation of liquid helium II contained in a can filled with closely spaced disks at a temperature of 1.27°K. The acceleration and retardation processes are not equivalent, and it is suggested that this asymmetry is due to the presence of vortex lines in the uniformly rotating superfluid. It is found that the forces producing superfluid acceleration are dependent on the past history of the helium and are greatly enhanced if the disk surfaces are roughened; furthermore, the superfluid flow shows a strong tendency to be macroscopically irrotational during retardation. These properties suggest that the forces are associated with turbulence in the superfluid, and it is found that a consistent description of the results can be given in terms of a mutual friction due to such turbulence. Values of the mutual friction appropriate to a steady flow are deduced and found to be in good agreement with values deduced from the results of other isothermal flow experiments; they can be represented, above a critical velocity, by a modification of the law introduced by Gorter & Mellink (1949), namely Fsn = A p sp n with A=23+/-2 posie -1 . V 0 =-0.6 +/- 0.2 c, s -1 Below the critical velocity a persistent current with a lifetime of at least 25 min has been observed. In the last section quantitative results on the growth and decay of turbulence in a non-equilibrium situation are presented, and laws describing the growth and decay processes are formulated.

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