Magnetoviscosity of Paraffin-Based Barium Ferrite Ferrofluid

The ferrofluids synthesized from platelet magnets in liquid media are quite interesting due to their shape-induced magnetic response. In this paper, the magnetoviscosity of barium ferrite (BaM) ferrofluid is studied to understand the magnetic response of BaM platelet particles. The platelet particles were synthesized by the hydrothermal method and coated with oleic acid. The X-ray diffraction data confirm the platelets consisting of BaM phase. The field-emission scanning electron microscopy and transmission electron microscopy micrographs show hexagonal platelets of 3-5 nm thickness and 50-250 nm in size. The magnetization versus magnetic field plot shows hard ferromagnetic behavior of the nanoparticles. The ferrofluid was synthesized by dispersing coated platelets in paraffin. The flow curves of the ferrofluid exhibit shear thinning with power law behavior in the presence of magnetic field. The yield stress values are determined by extrapolating shear stress versus shear rate plots to zero shear rate at various applied magnetic field values using Herschel-Bulkley equation. The magnetoviscosity, η, of the ferrofluid as a function of magnetic field is investigated at various shear rates. The η value increases with the increase in magnetic field, reaches a maximum value and then saturates at higher fields. This trend is due to the alignment of platelet-shaped magnetic particles along the direction of magnetic field at different shear rates. With the increase in magnetic field, the interaction between the magnetic nanoparticles and their arrangement becomes stronger leading to stacked plate chain formation. This shows that the variation of magnetoviscosity as a function of shear rate can be controlled by competition between the flow field and the magnetic field.