Rheological investigations of ferrofluids with a shear stress controlled rheometer.

The appearance of field- and shear-dependent changes of viscosity-the magnetoviscous effect-is correlated to the formation of chains and structures of magnetic nanoparticles. Moreover, the formation of these structures leads to the appearance of viscoelastic effects or other non-Newtonian features in ferrofluids in the presence of a magnetic field. In order to describe these phenomena, different theoretical approaches have been developed which explain the mechanism of these effects with different assumptions. One point in which these models differ, and which has to be clarified, is the appearance of yield stress and its dependence on magnetic field strength. With this aim, a stress controlled rheometer has been designed to prove the existence of this very small field-dependent yield stress for ferrofluids. The results presented here show a dependence of the yield stress on the magnetic field strength as well as on the interparticle interaction and particle size distribution. Finally, yield stress experiments have been performed for different geometries of the shear cell in order to get more information about the microstructure formed by the magnetic particles.