Investigations of yield stress in ferrofluids with a stress controlled rheometer

One of the most important properties of ferrofluids is the strong change of viscosity when subject to an applied magnetic field – the so called magnetoviscous effect. The rheological experiments as well as theoretical studies correlate this effect with the appearance of chain-like structures of magnetic particles due to the strong interparticle interaction in presence of a magnetic field. Furthermore, viscoelastic effects or other non-Newtonian features, like yield stress in ferrofluids, can theoretically be described with these structures under the influence of the magnetic field. In earlier experiments, when a shear rate controlled rheometer has been used, yield stress could not be investigated directly. A field dependent yield stress could only be estimated in ferrofluids. For direct yield stress investigations, a dedicated stress controlled rheometer is needed, so that the yield stress and its dependence on the magnetic field can be investigated directly. This paper presents investigations of yield stress and its dependence on the magnetic field strength as well as on volume concentration of large magnetic particles. In order to get more information about the structure formed by the particles, variation of gap thickness of the shear cell for the yield stress experiments has been used. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)