Collective motion in ferrofluids

Abstract

Collective motion in a ferrofluid is discussed on the basis of ferrohydrodynamics. Simple examples elucidate the effect of a collection of spinning suspended particles on Stokes flow. Spin viscosity emerges as a crucial transport coefficient. In the case of a uniform rotating magnetic field applied to a ferrofluid in a cylinder or a spherical cavity the collective fluid motion is driven by the curl of the torque density, which differs from zero only in a thin boundary layer with thickness proportional to the square root of the spin viscosity.