RHEOLOGICAL MODELING OF COMPLEX FLUIDS - III - DILATANT BEHAVIOR OF STABILIZED SUSPENSIONS

A new structural model of shear-thickening ''dilatancy'' is proposed f or (strongly) stabilized disperse systems. This model is based on the effective volume fraction (EVF) concept, developed in Part I of this s eries and previously used for the rheological modeling of complex flui ds. In such a description, the latter are considered as concentrated d ispersions of basic structural units (SUs) - either small? compact clu sters or primary particles -, forming large structures at low shear. A s shear rate increases, rupturing of these large structures leads to t he shear thinning observed prior to dilatancy. The novelty of this mod el lies in assuming that, beyond the onset of dilatancy, hydrodynamic forces promote, at the expense of basic-SUs; the formation of hydrodyn amic clusters as both rheo-optical experiments and numerical simulatio ns recently demonstrated, in contradiction with the (classical) theory based on a shear induced disruption of particle layering. Dilatancy d irectly results from the increase of the EVF of the dispersion, closel y related to the increasing volume of continuous phase imprisoned insi de hydroclusters whose size grows as the shear rate: increases. Predic tions of the model are discussed in comparison with the main features observed in a large number of dilatant dispersions: especially the vol ume fraction dependences of viscosity and critical shear rates (onset of dilatancy, maximum and discontinuity in viscosity) also the effects of particle size, polydispersity and suspending fluid viscosity.