Bidisperse Magnetorheological Fluids using Fe Particles at Nanometer and Micron Scale

Conventional magnetorheological (MR) fluids are suspensions of micron-sized particles in a hydraulic or silicone oil carrier fluid. Recently, research has been conducted on the advantages of using bidisperse 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 the nanoparticles fill pores created between the larger particles, thereby reducing fluid transport during creeping flow. 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 (wt%) of the nanometer-sized particles is increased relative to the weight percent (wt%) of micron-sized 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 wt% of iron (Fe) particles. This study investigates the effect of increasing the wt% of 30 nm (nominal) Fe particles relative to 30 mm (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 the Bingham-plastic rheological model to the measured flow curves of these MR fluids is also presented.