Uniformly sized microspheres interacting via long-range magnetic dipolar forces are used to study diffusion-limited cluster aggregation in a plane. The results show that it is possible to scale the temporal evolution of the cluster size distribution and that there is a crossover in fractal dimension from D =1.52~0.05 to D =1.16+'0.05 in the limit of weak and strong dipolar coupling. External magnetic fields are shown to produce pronounced chaining with D approaching 1. The results compare favorably with computer simulations of aggregation of the same type of particles.

We briefly introduce a simple extension of the diff usion-limited aggregation model of Witten and Sander, the cluster-cluster aggregation model, in which all clusters diffuse and grow by sticking at their first contact. This model is particularly well adapted to describe aggregation experiments on colloids or aerosols. Three recent extensions of the model are emphasized which take into account possible restructuring, cluster polarizability or indirect aggregation by polymers. In these three cases the computer simulations are compared with experiment.

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