Fractal dimension of large aggregates under different flocculation conditions

The two-dimensional fractal dimension (D_f) of large aggregates of kaolin (>540μm) during the shear flocculation process for kaolin solution was investigated using non-intrusive in situ image-based acquisition system. Separate experiments were also carried out for three different sized sub-ranges of large aggregates (0.540-1.125mm; 1.125-1.750mm; 1.750-2.375mm). Digital images were taken at a frequency of 10Hz for 10s for each different pairs of gradients of velocity (G_f) of 20 and 60s^-1 and flocculation times of 2; 3; 4; 5; 10; 20; 30; 60; 120 and 180min. For the same conditions, particle size distribution (PSD) was also determined. Under the investigated conditions, the lowest G_f produced the greatest D_f (1.69) at a flocculation time of 30min for the whole range of aggregates. Also, the evolution of the longest length of aggregate (l) and D_f with time, showed that the dynamic steady-state was reached at different times for each shear rate and l ranges. However, D_f varied for each size sub-range (ca. 1.1 to 1.8). Finally, the behavior of the aggregate structure may be understood by the predominance of different aggregation mechanisms such as cluster-cluster for G_f of 60s^-1 and particle-cluster for G_f of 20s^-1.