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. [References: 18]
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