Electrokinetic and viscoelastic properties of magnetorheological suspensions of cobalt ferrite

Magnetorheological suspensions are among the most promising colloidal syste ms from the point of view of their many potential technological application s. Most studies on these systems have, however, been performed using a non- polar liquid as dispersion medium. In this work we consider aqueous suspens ions of synthetic, spherical cobalt ferrite particles. From electrophoretic mobility determinations, we found an isoelectric. point (i.e.p.) at pH(i.e .p.) similar to 6-7. Viscosity determinations demonstrated that for all pH values the suspensions showed a shear-thinning behavior, but that the visco sity was maximum for pH values close to pH(i.e.p.). A good correlation was found between the zeta Q potential and fitting parameters of experimental d ata to the Krieger or Cross equations. Creep-recovery determinations were a lso performed on the aqueous ferrite suspensions, and the effect of a small applied magnetic field (B = 2.5 mT) was analyzed. These experiments clearl y demonstrated that the elastic response (instantaneous deformation) of the systems was more significant in the presence of the field. This agrees wit h earlier results (both in aqueous and non-aqueous suspensions) indicating the formation of long-range structures of particles aggregated by the inter action between the induced magnetic moments of the solids. As a further con firmation of this behavior, oscillatory tests were also performed. The pres ence of the magnetic field give raise to larger values of the elastic and v iscous moduli, the structuring provoked by magnetic field makes the systems more difficult to deform and also less prone to flow under a given shear s tress. (C) 2001 Elsevier Science B.V. All rights reserved.