The effect of particle shape in the small amplitude oscillatory shear behavior of magnetorheological (MR) fluids is investigated from zero magnetic field strengths up to 800 kA/m. Two types of MR fluids are studied: the first system is prepared with spherical particles and a second system is prepared with rodlike particles. Both types of particles are fabricated following practically the same precipitation technique and have the same intrinsic magnetic and crystallographic properties. Furthermore, the distribution of sphere diameters is very similar to that of rod thicknesses. Rod-based MR fluids show an enhanced MR performance under oscillatory shear in the viscoelastic linear regime. A lower magnetic field strength is needed for the structuration of the colloid and, once saturation is fully achieved, a larger storage modulus is observed. Existing sphere- and rod-based models usually underestimate experimental results regarding the magnetic field strength and particle volume fraction dependences of both storage modulus and yield stress. A simple model is proposed here to explain the behavior of microrod-based MR fluids at low, medium and saturating magnetic fields in the viscoelastic linear regime in terms of magnetic interaction forces between particles. These results are further completed with rheomicroscopic and dynamic yield stress observations.
We report the fabrication of highly crystalline magnetite particles with a uniform morphology and a relatively uniform average size of 54 nm. Particles were fabricated by oxidation of Fe(OH) 2 by nitrate in basic aqueous media. Characterization of the particles by means of transmission electron microscopy, Raman spectroscopy, X-ray and electron diffraction, and magnetometry showed that they were octahedral magnetic monodomains of highly stoichiometric magnetite (unit formula was estimated to be Fe 2.985 O 4 ), with room-temperature ferrimagnetic behavior and a saturation magnetization of 81.6 emu/g.
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