Magnetorheology of submicron diameter iron microwires dispersed in silicone oil

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 mum and 7.6 +/- 5.1 mum 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 mum 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 mum 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.