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Shear flow dynamics of reversible fractal clusters in dense suspensions were investigated by ultrasound scattering to study shear disruption processes of Rayleigh aggregates and further examine the effective mean field approximation used in the microrheological models. Within the framework of a fractal flocculation process, the concept of variance in local particle volume fraction is used to derive an original expression of the ultrasonic backscattering cross-section for a dense distribution of correlated Rayleigh fractal clusters. Considering the scaling laws for shear break-up of reversible aggregates in concentrated suspensions, a rheo-acoustical model in the Rayleigh scattering regime is proposed to describe the shear stress dependence of the low frequency backscattering coefficient with physical parameters. The anisotropic scattering effects from flocs of size larger than ultrasound wavelength are further discussed. In a second part, we report flow dependent changes of the ultrasonic backscattering coefficient in a plane–plane flow geometry to analyze shear break-up processes of hardened or deformable red cell aggregates in dextran solutions. Rheo-acoustical experiments were examined on the basis of the rheo-acoustial model and the effective medium approximation. The ability of ultrasound scattering to determine the critical shear stress and to give quantitative information on particle surface adhesive energy is analyzed. [Copyright &y& Elsevier]