RHEOLOGY OF WEAKLY FLOCCULATED SUSPENSIONS OF RIGID PARTICLES .1.

A rheological law for hard spheres in purely hydrodynamic interaction is used to describe,the steady state viscosity of weakly aggregated su spensions of rigid particles. The shear viscosity only involves the vo lume fraction and the maximum packing concentration of particles. Part icle aggregation influences the parameters of the reference viscosity law. Within the framework of fractal aggregation, we introduce the vol ume fraction of aggregates and we derive the equilibrium mean radius o f clusters from an effective medium approximation. The proposed rheolo gical equation is close to the phenomenological Casson equation for so ft clusters of fractal dimensionality D = 2. In a second part, we pres ent rheo-optical experiments for studying the break-up of red cell agg regates in a shear flow and for determining the critical disaggregatio n shear stress of the flowing suspension mainly representative of the surface adhesive energy between particles. The proposed microrheologic al model well describes viscometric data in the low shear regime and a llows information about the shear induced restructuration and the life time of clusters.