Inversion of Magnetic Forces Between Microparticles and Its Effect on the Magnetorheology of Extremely Bidisperse Magnetic Fluids

Abstract

We investigate the interaction between micon-sized magnetizable particles dispersed in a nanodispersed ferrofluid upon application of a magnetic field. As expected, at large distances interaction between two particles is governed by the magnetic dipolar force, which is attractive if the angle between the line of centers and the field is lower than 51°. However, when the gap between the surfaces of the particles is about one diameter, two approaching particles suddenly stop instead of coming into contact at zero gap . This stopping distance is quite independent on the intensity of the applied magnetic field. On the other hand, by studying two ferrofluids with a different size distribution of the nanoparticles, we found that this equilibrium distance can be increased a lot by a small change in the average size of the ferrofluid nanoparticles. In addition, the rheological properties of these (bidisperse) MR suspensions, constituted by magnetizable microparticles dispersed in ferrofluids, are also appreciably affected by this small change in the ferrofluid particle size distribution, in agreement with the microscopic observations. Based on experimental observations of a more concentrated zone of ferrofluid between the microparticles, we demonstrate that this zone is the result of a phase condensation on the ferrofluid nanoparticles. This condensation is induced by the enhancement of magnetic field near the magnetic poles of the microparticles and especially in the zone separating which separates the micron-sized particles. This concentrated domain of ferrofluid between the microparticles exerts a repulsive magnetic force which results of its tendency to be extended in order to minimize the demagnetizing field. We show that the demagnetizing effects play a dominant role when the microparticles are clos enough. Thus, contrary to the classical results of magnetostatic of continuums, magnetic interaction between magnetizable microparticles in ferrofluids experiences inversion from attraction at large interparticle distances to repulsion when tha particles are close enough.