Rheological modeling of fractal and dense suspensions

The development of a rheological model for aggregated suspensions is necessarily based upon a suitable characterization of the structure of the disperse phase and of the structural modifications produced by a deformation or a velocity field. The disperse phases of real aggregate particle suspensions, both dilute and concentrated, may present a wide variety of structures, which can be conveniently characterized by using the concepts of fractal geometry. In the present paper we formulate a rheological model able to correlate the structural processes induced by shear sflow conditions and the consequent shear dependence of viscosity with the shear stress changes experienced by the suspension. The flow curves calculated from the model, both for dense and fractal aggregates, closely resemble those observed for real colloidal and non-colloidal suspensions. The model appears particularly advantageous in describing the transition from shear thinning to plastic behavior, which usually occurs with increasing volume fraction or aggregation of the disperse phase. The role played by the aggregation state of the disperse phase become predominant in the low shear stress range, where aggregates may be composed of many particles, and, consequently, where the fractal dimensionality D becomes an important parameter in determining the compactness of the aggregate structure and the rheological behavior of concentrated suspensions. The validity of the proposed model is checked further through an analysis of experimental viscosity data relative to two series of epoxy-acrylic systems, containing titanium dioxide and aluminum silicate at different disperse phase concentrations.