| The stability of magnetic particles in an applied magnetic field strongly affects the properties of video tapes, floppy disks, ferrofluids, and other particulate magnetic materials. We have measured the initial flocculation rate of magnetic dispersions as a function of the applied magnetic field strength, and compared the experimental results with theoretical and simulation-based predictions of the flocculation rate in order to study the effect of magnetic forces on colloid stability. To more precisely determine the initial flocculation rates we have developed an improved small-angle light scattering technique which measures the scattering intensity of a flocculating dispersion of colloidal particles in situ, eliminating the need for troublesome sampling and dilution of the flocculating dispersions, as in previous techniques. At the small scattering angle of 2{dollar}spcirc{dollar}, the effect of floc shape on the scattering intensity is negligible, simplifying data analysis. Using this technique we have measured the rapid flocculation rate of aqueous, spherical maghemite ({dollar}gamma{dollar}-Fe{dollar}sb2{dollar}O{dollar}sb3{dollar}) particles, with a mean diameter of 12 nm, as a function of applied magnetic field strength. In dilute dispersions, the measured flocculation rate increased by 36% at high magnetic field strengths over the rate in zero applied field. The magnetic field increases the flocculation rate by orienting the particles with each other and thereby increasing their magnetic dipole-dipole interaction. The theory, based on an orientation-averaged magnetic interaction, predicts less than a 10% increase in flocculation rate at high magnetic fields, indicating that the orientation average may not accurately represent the dipole-dipole interactions during rapid flocculation. Theory also predicts that the flocculation rate is far more sensitive to the size and magnetization of the particles than to the applied magnetic field strength. We have also developed a computer program which simulates the flocculation of spherical magnetic particles: the program calculates both the translational and rotational trajectories of each particle, including the stochastic contributions of Brownian motion. |
