A microrheological model of aggregating dispersions is proposed in which the shear stress is estimated as the sum of hydrodynamic and structural parts. The former is attributed to the hydrodynamic cores of fractal aggregates, which behave as a suspension of impermeable spheres. The latter accounts for the forces transmitted by chains of particles linking neighboring aggregates into a transient network. To calculate the structural part the concept of fractal aggregation is incorporated into a transient network theory, to account for the creation and breakup of chains of colloidal particles connecting the aggregates. Rigid and soft chains are distinguished. The former have multiply connected backbones which deform as contorted elastic rods, while the latter have at least one soft junction and deform without elastic resistance until fully loaded. The contribution of the soft chains to the stress tensor is neglected. The calculations treat two different mechanisms for the evolution of rigid chains: a purely mechanical one, which corresponds to a shear-controlled structure built up in flow, and a thermal mechanism, which pertains to a quasiequilibrium structure undisturbed by shear. We calculate steady-shear viscosities in the former case and viscoelastic functions in the latter. The model can be fitted satisfactorily to the experimental results for a well-characterized polystyrene latex dispersion with physically acceptable parameters.

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In order to describe the steady shear behavior of weakly aggregating dispersions a microrheological model is developed. This model combines the concept of fractal aggregation in shear flow with the transient network modeling originally developed for polymer dynamics. The former accounts for the hydrodynamic stress in the aggregates, while the latter is employed to calculate the stress component arising from the forces transmitted by chains of particles combining the neighboring aggregates into a network. The contribution of this network is shown to be significant for steady shear behavior at low volume fractions of solid and low shear rates. The validity of the model is tested by fitting it to experimental data on a well characterized weakly aggregating polystyrene latex dispersion. In this way a number of relevant parameters, among which the fractal dimension d 1 of the aggregates and the capture efficiency of particle chains belonging to neighboring aggregates, are determined. The best agreement of the calculations with the experiments is found at small capture efficiencies and rather high fractal dimensions, i.e., 2.0

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