Aggregation kinetics of paramagnetic colloidal particles

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Abstract
Magnetic‐field‐induced chain formation of superparamagnetic latex particles is investigated via optical microscopy and digital image analysis. Our experiment is unique in that the aggregation process takes place in the bulk, where particles are free to diffuse in three dimensions, rather than on a surface. We find that the mean chain size 〈s(t)〉 has a power‐law dependence on time, 〈s(t)〉∼tz, as predicted by the Smoluchowski equation and three‐dimensional simulations of diffusion‐limited aggregation of oriented particles. The value of the exponent z is found to have a weak inverse dependence on particle volume fraction φ and dimensionless dipole strength λ. We propose a new model for a characteristic time scale proportional to 1/λ and 1/φ. Comparison with experimental observations shows this scaling to be very effective but to break down suddenly above a critical volume fraction.

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