STEADY SHEAR VISCOSITY OF WEAKLY AGGREGATING POLYSTYRENE LATEX DISPERSIONS

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(f) 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 less-than-or-equal-to d(f) less-than-or-equal-to 2.3. Hence, combining the steady-shear viscosity measurements with the microrheological interpretation one obtains a tool for probing the structure of weakly aggregating dispersions.