Scaling between Viscosity and Hydrodynamic / Magnetic Forces in Magnetic Fluids*

The aim of this work is the investigation of the magnetorheological behavior, under both simple steadyand oscillatory-shear flow regimes, of fluids composed by micron-sized iron particles (average diameter 930 ± 330 nm) dispersed in silicone oil. Magnetic fields ranging from 279 A/m (0.35 mT) to 1727 A/m (2.17 mT) were applied to the suspensions. The effect of silica nanoparticles as stabilizer of the suspensions has also been considered. The study has been made by the scaling between the viscosity of the suspension and the ratio of hydrodynamic to magnetic forces acting on the dispersed particles, given by the dimensionless Mason number (Mn), and interpreted in terms of the chainlike model taken from the theory of Martin and Anderson (J. Chem. Phys. 104 (1996) 4814-4827). The model is quite well accomplished for iron suspensions of different (20 % and 30 %) volume fraction without any stabilizing agent. The presence of added silica nanoparticles in the suspension hinders the formation of regular iron structures induced by the magnetic field, especially at the lowest applied magnetic fields. Thus the model becomes not applicable to these cases. Viscometry has been shown to be more adequate than oscillometry for scaling the viscous properties of magnetorheological suspensions with microscopic interparticle forces in terms of Mn number.