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 concentration, when a uniform and constant 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. At higher concentrations, 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. In a rotating magnetic field, at low concentrations, linear aggregates are observed. Their size decreases with an increasing rotation rate. At higher concentrations, circular aggregates are formed.