Conventional magnetorheological (MR) fluids are suspensions of micron sized particles in a hydraulic or silicone oil carrier fluid. Recently, research has been conducted into the advantages of using bidisperse MR fluids, which are mixtures of two different powder sizes in the MR suspension. The MR fluids investigated here use a mixture of conventional micron sized particles and nanometer sized particles. The settling rate of such bidisperse fluids using nanometer sized particles is reduced because thermal convection and Van der Waals forces experienced by the nanometer sized particles compete favorably with gravitational forces. This reduction in the settling rate comes at a cost of a reduction in the maximum yield stress that can be manifested by such an MR fluid at its saturation magnetization. There is a measurable and predictable variation in rheological properties as the weight percent of the nanometer sized particles is increased relative to the weight percent of micron size particles, while maintaining a constant solids loading in the MR fluid samples. All bidisperse fluids tested in this study have a solids loading of 60 weight% (wt%) of Fe particles. This study investigates the effect of increasing the weight percent of 30 nanometer (nominal) Fe particles relative to 30 micron (nominal) Fe particles on rheological characteristics such as yield stress and postyield viscosity. The goal of this study is to find an optimal composition of the bidisperse fluid that provides the best combination of high yield stress and low settling rate based on empirical measurements. The applicability of rheological models, such as the Bingham-plastic and the Hershel Buckley models, to the measured flow curves of these MR fluids is also presented.

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