FLOC STRUCTURE AND GROWTH-KINETICS FOR RAPID SHEAR COAGULATION OF POLYSTYRENE COLLOIDS

Colloid coagulation in a flowing suspension is governed by floc structure and by the colloidal and hydrodynamic interactions between the aggregates. We have investigated both the internal structure and growth rate of rigid flocs formed by rapid coagulation in a linear shear flow, examining structure by static light scattering and measuring sizes by dynamic light scattering. Comparison of the results for sheared suspensions with Brownian coagulation data reveals a similar structure for the two modes; in both cases the flocs exhibit characteristics of a fractal with dimension d = 1.8 ± 0.1, and for both modes the average number of nearest neighbors is approximately the same. In contrast, the growth kinetics are inherently different, as expected. To model growth in a shear flow, we treat a porous floc as a body with a hydrodynamic radius which is smaller than the capture radius corresponding to floc-floc contact, and we also find it necessary to invoke a criterion for the maximum size a floc can obtain at a given shear rate. Necessary structural details are provided by the static light scattering results. We compare kinetics predicted by this model with data at several shear rates and find qualitative agreement. © 1991.