We investigate the magnetorheological (MR) properties of suspensions containing iron microwires with 260 nm diameter and two distinct length distributions of 5.4 ± 5.2 μm and 7.6 ± 5.1 μm suspended in silicone oil (0.45 Pa s). The rheological properties of these fluids were determined using a parallel plate rheometer equipped with a variable strength electromagnet. The shear stress was measured as a function of shear rate for increasing applied magnetic fields. These results were modeled using the Bingham-plastic constitutive model to determine the apparent yield stress and viscosity as a function of increasing volume fraction and length of microwires. At a saturated magnetic flux density, the yield stress using the 5.4 μm microwires was found to be 0.65, 2.23, and 4.76 kPa for the 2, 4, and 6 vol% suspensions, respectively. For the 7.6 μm wires, the yield stress increases to 8.2 kPa for the 6 vol% suspension. Compared with conventional MR fluids employing spherical particles, the degree of settling is markedly decreased in the microwire-based fluids. At 6 vol%, conventional fluids display appreciable settling whereas the microwire-based fluids display no discernable settling. Moreover, the rod-shaped microwires are shown to increase the yield stress of the fluids and enhance the MR performance.

A dimorphic magnetorheological fluid with improved oxidation and chemical stability under oscillatory shear M Sedlacik, V Pavlinek, R Vyroubal et al. Dimorphic magnetorheological fluids: exploiting partial substitution of microspheres bynanowires G T Ngatu, N M Wereley, J O Karli et al. The enhanced MR performance of dimorphic MR suspensions containing either magnetic rods or their non-magnetic analogues

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 μm . The properties of these wires were compared to commercial nickel carbonyl spheres (1–10 μm 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.