Measuring the rotational viscosity of ferrofluids without shear flow

The shear free solid-body rotation of a ferrofluid in a magnetic field experiences a damping due to internal rotational friction. A device based on the principle of a torsional pendulum is presented. It allows us to perform direct and precise measurements of the field dependence of the genuine rotational viscosity without a disturbing shear flow. Experimental results for a moderately concentrated ferrofluid compare reasonably well with theoretical models for monodisperse ferrofluids of noninteracting rigid dipoles.