Ultrasound scattering from concentrated suspensions of aggregated red cells in shear flow

Ultrasound scattering technique is used to investigate dynamics of reversible fractal aggregates in dense suspensions and analyze shear break-up processes of Rayleigh fractal clusters. On the basis of an homogenous fractal flocculation and the hybrid scattering model in the Rayleigh scattering regime, a first order expression of the ultrasound scattering cross-sectional area per unit of volume (backscattering coefficient) is derived for a dense distribution of correlated Rayleigh fractal clusters. From the scaling laws for shear break-up of reversible aggregates in concentrated suspensions, a rheo-acoustical study is then proposed to describe the shear stress dependence of the low frequency scattered power per unit of volume. In a second part, experimental flow dependent changes of the ultrasound backscattering coefficient in a plane-plane flow geometry were reported to analyze shear break-up processes of hardened or deformable red cell aggregates in polymer solution (neutral dextran polymer). Rheo-acoustical experiments were examined within the framework of the effective mean field approximation and the proposed rheo-acoustical model. The ability of ultrasound scattering to determine the critical disaggregation shear stress inducing a complete disaggregation and to give quantitative information on particle surface adhesive energy are finally analyzed.