The effects of dispersed phase saturation magnetization and applied magnetic fields on the rheological properties of magnetorheological (MR) fluids are described. MR fluids based on two different grades of carbonyl iron powder with different average particle size, 7-9 µm (grade A) and 2 µm (grade B), were prepared. Vibrating sample magnetometer measurements showed that the saturation magnetization values were 2.03 and 1.89 T for grades A and B, respectively. Rheological measurements were conducted for 33 and 40 vol% grade A and grade B based MR fluids with a specially built double Couette strain rate controlled rheometer at flux densities ranging from 0.2 to ∼0.8 T. The yield stresses of 33 and 40 vol% grade A were 100 ± 3 and 124 ± 3 kPa, respectively at 0.8 ± 0.1 T. The yield stress values of MR fluids based on finer particles (grade B) were consistently smaller. For example, the yield stresses for 33 and 40 vol% grade B based MR fluid were 80 ± 8 and 102 ± 2 kPa, respectively at 0.8 ± 0.1 T. The yield stresses at the flux density approaching magnetic saturation in particles (B ∼ 0.8T) were found to increase quadratically with the saturation magnetization (µ 0 M s ) of the dispersed magnetic phase. This is in good agreement with the analytical models of uniformly saturated particle chains developed by Ginder and co-workers. The results presented here show that the decrease in yield stress for finer particle based MR fluids is due to the relatively smaller magnetization of the finer particles.

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The influence of the remnant magnetization of the soft magnetic particulates, used as a dispersed phase, on the redispersibility of magnetorheological (MR) fluids is discussed. Calculations of the magnetic interaction energy showed that for 33-vol% MR fluids based on particles of iron (∼6 m), manganese zinc ferrite (∼2.3 m), and nickel zinc ferrite (∼2.1 m), the ratios of the magnetic interaction energy to the thermal energy were 161,000, 6400, and 3900, respectively. These calculations showed that even the seemingly small levels of remnant magnetization, associated with particulates employed in MR fluids, introduced significant dipole-dipole interparticle interactions. It is proposed that this interaction causes most MR fluids to show a tendency for "cake formation," which makes it difficult to redisperse these fluids. Our modeling presented here also suggests practical strategies to enhance the redispersibility of MR fluids.

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Thermal conductivity of magnetorheological suspensions synthesized with iron powder and silicone oil is experimentally investigated for varying particle volume fractions (5, 20, and 40 vol%) of two different grades of iron (Fe) and magnetic field strengths. In order to determine the temperature range at which the thermal conductivity of MR fluids is more effective for different heat transfer applications, the experiments are done for three different temperature intervals in three different temperature ranges: from −20 to 0  ° C, from 0 to 50  ° C, and from 50 to 100 ° C. In this study, ISO 8301 ‘Thermal insulation—determination of steady state thermal resistance and related properties—heat flow meter apparatus’ is used. The thermal conductivity of the MR fluids shows an increase with increasing magnetic field and volume fraction in the temperature intervals from 0 to 50 ° C and from 50 to 100 ° C. In particular, there is a substantial enhancement in the thermal conductivity for the 50–100 ° C temperature interval (enhancement ratio by almost 134% for 40SM at H = 150 G). However, the thermal conductivity shows a decrease in the lower temperature interval from −20 to 0 ° C (a decrease by 42% for 40SM at 150 G), which could be due the effect of the thermal conductivity of silicone oil at lower temperatures. Although the heat transfer coefficient is higher for higher particle concentrations, the percentage increase is more pronounced for lower particle concentrations, especially in the 0 to 50 ° C temperature interval (for the 40SM sample at 150 G 18% an enhancement for a 20 K temperature difference is observed, whereas for the 20% MR fluid sample, the enhancement is 34%).