STRUCTURE EVOLUTION IN A PARAMAGNETIC LATEX SUSPENSION

We present the aggregation of a model system of superparamagnetic latex particles. We measure the kinetics of the nonequilibrium aggregation process. At low volume fraction, when the magnetic field is applied, particles experience an attractive dipolar interaction and form linear chains by aggregating tip-to-tip parallel to the applied field. We find that the aggregation is transport limited and the mean cluster size increases with the square root of time. The long range dipolar interactions modify the flux in such a way that we must derive a characteristic time scale accounting for the ballistic trajectories experienced by aggregating dipoles. At surface fractions above a few percent, lateral interactions alter the aggregation mechanism; a sideways coalescence of the chains similar to a "zippering" motion is observed. This phenomenon results in a thickening of the chains to create fibers of particles and is eventually responsible for the crosslinking of chains. We present a new observation of a fully interconnected fibrous structure in concentrated suspensions. © 1992.