Measuring the rotational viscosity of ferrofluids without shear flow

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