Rheology of a magnetorheological fluid

We report on experimental investigations of the rheological behavior of aqueous magnetic suspensions. The suspended particles are monodisperse colloidal polystyrene spheres which contain magnetic Fe2O3-grains. In the absence of a magnetic field these suspensions behave as Newtonian fluids, whereas under the influence of a magnetic field due to the formation of an ordered structure the apparent viscosity of the suspension increases up to three orders of magnitude and they clearly exhibit non-Newtonian properties, such as shear thinning and yield stress. The apparent viscosity depends on the magnetic field according to eta similar to H-2 Delta. Increasing the volume fraction of the particles in the range of 0.014 < phi < 0.12 results in a linear increase in apparent viscosity and yield stress. Both apparent viscosity and yield stress depend also on the particle size as first measurements on particles with diameters ranging from 0.5 mu m to 1.0 mu m clearly show. All measurements were carried out with a rotation viscometer using the cone-plate configuration so that the applied shear rate was well defined.