Nested dipolar Halbach arrays for the determination of magnetorheological properties at variable magnetic field

A new apparatus for the measurement of magnetorheological properties of magnetic fluids in a commercial strain-controlled rheometer is proposed. The new setup makes use of the concept of the Halbach cylinder to generate a homogeneous dipolar magnetic field, using only permanent magnets. Two of these Halbach cylinders are combined, allowing the magnetic field to be varied by simply rotating the cylinders relatively to one another. The magnetic field achieved in the middle of the cylinders varies from nearly zero to about 0.6 T without a significant variation on the high homogeneity (ΔB/B < 2 × 10^− 2) of the resulting field. To perform the rheological experiments inside a strong magnetic field, measurement geometries (concentric cylinders and a four-bladed vane) were constructed from a non-magnetic material (polyoxymethylene). For the first experiments with the new setup, a commercial magnetic suspension of carbonyl iron particles in hydrocarbon oil was investigated. Stress growth experiments at varying shear rates in the steady shear regime, as well as strain sweep experiments at constant frequency in the dynamic oscillatory regime, were performed. The results show a shift of the linear (elastic) regime in oscillatory shear to higher strain values for increasing magnetic field. The steady shear viscosity increases with increasing magnetic field. The behaviour of the magnetic suspension was understood in the context of the ratio from the viscous to the polarisation forces, the so-called Mason number.