Frequency Dependence of the Critical Velocity of a Sphere Oscillating in   Superfluid Helium-4
  It is shown that the critical velocity of a small sphere oscillating in superfluid helium increases with the square root of the oscillation frequency. This behavior can be described by a simple dimensional argument. The size of the sphere and the temperature of the superfluid are found to have no or only very little effect. Surface properties of the sphere and remanent vorticity may have an influence but have not been under systematic investigation in these measurements 
R. H\"anninen and W. Schoepe