Aggregation and Breakup of Particles in a Shear Flow.

An experimental study was carried out to investigate aggregation and breakup of particles in a Couette flow system with the inner cylinder rotating at constant speed. Experiments were conducted with monodisperse and polydisperse suspensions at different particle volume concentrations. Depending on both the shear stress and the particle concentration, three different regimes were found. In the first regime, when the concentration is less than a critical value, the final diameter of the aggregate is independent of concentration and depends only on the shear. When the concentration is higher, two new regimes are identified, with the final diameter of the aggregate depending on both shear stress and particle concentration. Also, the transition between these two regimes corresponds to the transition from laminar to turbulent flow. In addition, the final diameter of the aggregates in the turbulent flow regime is independent of the size of the primary particles and results to be is controlled by the Kolmogorov length scale. On the contrary, in the laminar flow regime the final diameter of the aggregates shows a light tendency to decrease as the diameter of the primary particles increases.