Influence of particle shape on the properties of magnetorheological fluids

We present studies of the rheological properties and dispersion stability of MR fluids as a function of particle shape by comparing fluids made with uniform nickel spheres to those employing nickel microwires suspended in silicone oil. The wires were fabricated using template-based electrodeposition having diameters in the range 300 +/- 30 nm and lengths in the range of 5 - 25 mu m. The properties of these wires were compared to commercial nickel carbonyl spheres (1-10 mu m dia.). Qualitatively, the off-state (field off) viscosity of fluids containing only wires was found to be substantially greater than those fluids that contain only spherical particles. Rheological measurements of the on-state viscosities were conducted using a custom rheometer equipped with an electromagnet capable of magnetic fields up to 0.6 T. Placed in this field, we observed yield stresses of 1.88 +/- 0.23 and 1.86 +/- 0.26 kPa for 7.6 volume percent of pure nickel spheres and microwires, respectively. The fluids containing spherical nickel particles tended to settle rather quickly (< 20 minutes), while those containing only wires remained suspended even after several months.