Motion of micron-size particles in turbulent helium II

The surprising result of a recent experiment in turbulent helium II is that micron-size tracer particles move with about half the speed of the imposed normal fluid. We develop a theory of the interaction of small spheres with quantized vortices and predict that the particles' slip velocity (resulting from the balance of buoyancy and Stokes drag forces) must be corrected by an amount which is proportional to the normal fluid velocity, in quantitative agreement with the observations.