STRUCTURE EVOLUTION IN A PARAMAGNETIC LATEX SUSPENSION

We present the aggregation of a model system of superparamagnetic late x particles. We measure the kinetics of the nonequilibrium aggregation process. At low concentration, when a uniform and constant magnetic f ield is applied, particles experience an attractive dipolar interactio n and form linear chains by aggregating tip to tip parallel to the app lied field. We find that the aggregation is transport limited and the mean cluster size increases with the square root of time. At higher co ncentrations, lateral interactions alter the aggregation mechanism; a sideways coalescence of the chains similar to a ''zippering'' motion i s observed. This phenomenon results in a thickening of the chains to c reate fibers of particles and is eventually responsible for the crossl inking of chains. In a rotating magnetic field, at low concentrations, linear aggregates are observed. Their size decreases with an increasi ng rotation rate. At higher concentrations, circular aggregates are fo rmed.