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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.

Scientists and engineers are most familiar with single-crystal or polycrystalline field-responsive or “smart” materials with responses typically occurring while the materials remain in the solid state. This issue of MRS Bulletin focuses on another class of field-responsive materials that exhibits a rapid, reversible, and tunable transition from a liquidlike, free-flowing state to a solidlike state upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior in response to an externally applied electric or magnetic field and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids, respectively. They are often described as Bingham plastics, and exhibit a strong field-dependent shear modulus and a yield stress that must be overcome to initiate gross material deformation or flow. Prototypical ER fluids consist of linear dielectric particles (such as silica, titania, and zeolites) dispersed in nonconductive liquids such as silicone oils. Homogeneous liquid-crystalline (LC) polymerbased ER fluids have also been recently reported. MR fluids are based on ferromagnetic or ferrimagnetic, magnetically nonlinear particles (e.g., iron, nickel, cobalt, and ceramic ferrites) dispersed in organic or “aqueous liquids. Unlike ER and MR fluids, ferrofluids (or magnetic fluids), which are stable dispersions of nanosized superparamagnetic particulates (~5–10 nm) of such materials as iron oxide, do not develop a yield stress on application of a magnetic field. Applications of ferrofluids are primarily in the area of sealing devices (see Rosensweig for more information). Since ferrofluids are well-known and have been extensively discussed elsewhere in the literature, they will not be treated in detail here.

A novel class of relatively stable and redispersible MR fluids based on meso-scale magnetic particles of iron or nickel zinc ferrite and nano-scale additives is described. For a flux density B ~ 1 Tesla, the iron based MR fluids exhibited yield stresses of ~100 kPa. For ferrite based fluids the yield stress values were as high as ~15 kPa at B ~ 1T. The yield stresses at the flux density required for magnetic saturation increase quadratically with the saturation magnetization of the particulate material, in good agreement with a model for the yield stress of uniformly saturated particle chains. At lower flux densities, the yield stress was generally observed to increase as B 3/2, consistent with models of the role of local saturation of the particle magnetization. The additives were found to enhance the stability and redispersibility of the MR fluids: they appeared to promote a small non-zero yield stress in the absence of a field but were found not to have a substantial effect on the field-dependent yield stresses.

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