INTER-CLUSTER SCALING IN 2-DIMENSIONAL COLLOIDAL AGGREGATION

Cluster-cluster aggregation has been studied experimentally in two-dim ensional colloidal systems of moderately high area fraction. In the di ffusion limited regime the system spontaneously exhibits ordering beyo nd the scale of the fractal aggregates, which persists into the gelled state. This ordering arises from an effective inter-cluster repulsion due to the mutually exclusive depletion zones surrounding each cluste r. Various metrical and topological properties of the system indicate that this long-range order is stationary, apart from the inherent chan ge of scale as the clusters grow. Topological correlations show that t he inter-cluster self-organization is compatible with maximum entropy: the non-equilibrium system behaves as if it were in statistical equil ibrium, and no physical forces are involved in the long-range order. F or the reaction-limited case no such inter-cluster ordering is evident at any stage, the system is nonstationary, and the aggregation is gov erned by specific physical forces.