Initial Rate of Flocculation of Magnetic Dispersions in an Applied Magnetic Field

Initial flocculation rates for aqueous polydisperse (diameters between 5 and 23 nm with a mean of 12 nm) maghemite (γ-Fe2O3) particles were determined in situ using small-angle light scattering. In dilute dispersions, we found that the measured rate was 36% higher in the presence of high applied magnetic fields than it was in the absence of any applied field. Fuchs' model of the kinetics of Brownian flocculation, using on an orientation-averaged magnetic interaction between particles, predicted less than a 2% increase in the flocculation rate at high magnetic fields for monodisperse 12 nm particles. Although the percentage increase was larger for larger particle sizes, even the largest size observed in our maghemite sample led to a predicted increase of less than 10%, which was well below the 36% increase observed. Our estimates suggest that the Brownian rotation of the particles was not fast enough, compared to translation, to achieve an equilibrium distribution of orientations during flocculation. When, instead of orientation averaging for a given separation distance, the maximum magnetic attraction for two dipoles aligned end to end was used, Fuchs' theory predicted a greater effect of magnetic field than observed, thus indicating the observations were within theoretical bounds.