Fractal properties of flocs formed by fluid shear and differential settling

The fractal properties of fine-grained sediment flocs formed by fluid shear and by differential settling were studied through floc porosity–size relationships. Flocs were produced from flocculation tests in a Couette-type flocculator in which fluid shear was the dominant collision mechanism for flocculation, and in a disk-type flocculator in which differential settling was the dominant collision mechanism. Floc samples were then introduced into a settling tube to measure their sizes and settling speeds by a double-exposure photographic method. The porosities of flocs were determined from the settling speed data using a porous sphere settling model. It was found that the flocs produced in the Couette-type flocculator could be regarded as fractals, with a fractal dimension ranging from 1.83 to 1.97. However, the flocs produced in the disk-type flocculator did not exhibit simple scaling behavior and could not be regarded as fractals. The differences in fractal property are due to the differences in floc formation schemes associated with the collision mechanisms. The similarities between experimentally obtained flocs and computer simulated clusters are discussed. The multistage growth model, a formation scheme for flocs formed by fluid shear, is consistent with the cluster–cluster model used in computer simulations. A formation scheme for flocs formed by differential settling is postulated; it is consistent with the particle–cluster model used in computer simulations.