Electrokinetic and viscoelastic properties of magnetorheological suspensions of cobalt ferrite

Magnetorheological suspensions are among the most promising colloidal systems from the point of view of their many potential technological applications. Most studies on these systems have, however, been performed using a non-polar liquid as dispersion medium. In this work we consider aqueous suspensions of synthetic, spherical cobalt ferrite particles. From electrophoretic mobility determinations, we found an isoelectric point (i.e.p.) at pHi.e.p.∼6–7. Viscosity determinations demonstrated that for all pH values the suspensions showed a shear-thinning behavior, but that the viscosity was maximum for pH values close to pHi.e.p.. A good correlation was found between the zeta (ζ) potential and fitting parameters of experimental data to the Krieger or Cross equations. Creep-recovery determinations were also performed on the aqueous ferrite suspensions, and the effect of a small applied magnetic field (B=2.5 mT) was analyzed. These experiments clearly demonstrated that the elastic response (instantaneous deformation) of the systems was more significant in the presence of the field. This agrees with earlier results (both in aqueous and non-aqueous suspensions) indicating the formation of long-range structures of particles aggregated by the interaction between the induced magnetic moments of the solids. As a further confirmation of this behavior, oscillatory tests were also performed. The presence of the magnetic field give raise to larger values of the elastic and viscous moduli, the structuring provoked by magnetic field makes the systems more difficult to deform and also less prone to flow under a given shear stress.