Superfluid drag in helium II

[摘要] The drag on a 1. 21 millimeter diameter sphere due to flowing superfluid helium II was measured at 1.6°K, 1.8°K, 2.0°K and 2.13°K in a "superfluid wind tunnel" (test region 3 cm long and 1 cm diameter) in which 500Å Millipore was used as the barrier to normal component flow for the first experiments, and 100 Å Millipore for the later ones.The superfluid flow through the tunnel was much steadier than it was in similar tunnels used in previous experiments.The velocity of the superfluid was calculated from a measurement of the loss rate of liquid helium in a standpipe at the entrance to the tunnel. (This design eliminated uncertainty in the corrections for evaporation.)The test sphere was mounted 1.3 cm from the axis of a quartz torsion fiber assembly, on which was also mounted a magnetic dipole. The drag was then measured using a null technique in which the torque due to the drag on the sphere in the flowing superfluid was balanced by the torque on the dipole due to an externally applied magnetic field. After corrections were made for a backflow of normal component which leaked through the Millipore, the results were as follows: Within the experimental error, zero drag was often observed for superfluid velocities up to 0.21 cm/ sec at 1.6°K, 0.48 cm/sec at 1.8°K, 1.1 cm/sec at 2.0°K and 4.4 cm/sec at 2.13°K. (These velocities do not necessarily represent superfluid critical velocities, but they are probably limiting velocities imposed by the measurement technique.) At 2.13°K, only zero drag was observed, but at the other three temperatures, drag was also observed with steady values between zero and a drag comparable to that which would be produced by an ordinary, low viscosity liquid having the density and velocity of the superfluid. These results are qualitatively consistent with the Onsager-Feynman model of quantized vortices, but the velocities at which zero drag was observed are much larger than quantitative predictions based on this model.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Superfluid drag, helium II, Physics