Experimental analysis of coagulation of particles under low-shear flow.

The aggregation and breakup of particle flocs were investigated by monitoring the size distribution of a suspension of aggregates, with diameter d(o), under shear flow created by two mixing systems. The aggregation behavior was studied in 63 experiments under various conditions of induced shear rate and particle volume concentration for particle aggregates smaller than the Kolmogorov scale. Despite small shear rates being characteristics of natural systems, only experiments with comparatively high shear rates have been conducted to date. Because of this reason, in this study, the shear rates were chosen to mimic those found in natural systems. In the first set of experiments the aggregate size, d, was analyzed by changing the mean shear, G (ranging from 0.70 to 27.36 s(-1)) created in a tank with a grid oscillating through the whole suspension volume. In the second set of experiments, a spherical flask was placed in an orbital shaking table where G ranged from 0.45 to 2.40s(-1). In all the cases there was an increase of d at increasing G . The dependence on d was found to be identical for the particle volume concentrations investigated, phi=0.2,0.8,2,4,6,8 and 10 x 10(-5), with the stable aggregate size shifting towards aggregate growth as particle volume concentration increased. These results demonstrate that shear provided a means to keep the particle number count high for collisions to occur but it is small enough that the aggregation-breakup balance is dominated by aggregation.