The spherical Fe3O4 with 300 nm in diameter was synthesized by typical thermal decomposition of Fe (III) organometallic precursor in polyol and polyacrylic acid. Fe-nanorods were prepared by reducing Fe (III) nitrate in presence of polyol-hydrazine-CTAB. Morphology and magnetic characterization of the nanoparticles were performed by FESEM, XRD and VSM studies. We performed detailed non-linear magnetorheological properties of three MR fluids (10 vol-%) containing isotropic Fe3O4 and anisotropic Fe-nanorods under both small and large amplitude oscillatory flow. The MR samples demonstrated strong magnetorheological Payne effect i.e. rapid stress relaxation under increasing deformation and uniform magnetic field beyond linear viscoelastic region (LVR), which was not studied in detail before in case of bidisperse MR fluids. We have also shown that stress softening was more pronounced for MR fluids with higher anisotropic contents, in contrast to isotropic MR fluid. The onset strains for LVR to non-linear region transition for anisotropic fluids were much lower than that of isotropic spherical nanoparticle containing fluid. The stronger MR response for nanorod-containing MR fluids can be explained in terms of enhanced field-induced structuration.
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