TURBULENT AND LAMINAR DRAG OF SUPERFLUID-HELIUM ON AN OSCILLATING MICROSPHERE

A magnetic sphere (radius 100 m), levitating inside a superconducting niobium capacitor, is immersed into superfluid helium. Vertical oscillations of the sphere can be excited and detected. At resonance we measure the velocity amplitude as a function of the driving force between 0.35 and 2.2 K. In the linear regime (laminar flow) the drag is given by Stokes' solution above 1.1 K and by ballistic roton and phonon drag below 0.7 K. At larger velocities we find a sharp transition to turbulent drag which varies with the square of the velocity above a temperature independent threshold velocity. © 1995 The American Physical Society.