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.

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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

This work investigates the settling response of bi-disperse silicone oil-based magnetorheological fluids with varying solids loading (content) of Fe magnetic particles at three ratios of nanoparticles to micron size particles. The solid loading (content) of Fe magnetic particles in silicone oil was 50 or 60% by weight with nanoparticle fractions of 0, 10, or 15% by weight of the total solids loading. Sedimentation experiments are conducted using Z-axis translating laser light scattering at ambient temperature. Sedimentation velocities are determined from clear layer separation and functional particulate transport by creeping flow. Settling rates of 1—3 μm/s have been observed in the dispersions without nanoparticles. Viscosity testing shows lower yield stress with nanoparticle inclusion and the typical rise in shear stress with increasing shear rate and applied magnetic field using both the linear Bingham and the non-linear Hershel—Bulkley models. Adding nanoparticles also reduced sedimentation velocity by roughly two orders of magnitude from those without nanoparticles. The nanoparticles act as plugs or otherwise interact in forming soft gel particulates leading to a denser sedimentation front yielding fewer percolation holes through which the fluid can permeate. Either way, the addition of nanoparticles leads to a more effective stabilization mechanism for the dispersion.

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