STRUCTURE AND BREAKUP OF FLOCS SUBJECTED TO FLUID STRESSES (FRACTAL, SHEAR, AGGREGATE, COLLOID, COAGULATE)

The structure and properties of flocculated suspensions were investigated experimentally and theoretically, with emphasis on the breakup induced by fluid stresses. Small-angle light scattering was determined to be appropriate for characterization of the flocs.; Experiments investigated the breakup of flocculated 0.14 (mu)m diameter polystyrene latices due to uniform shear. The average number of particles per floc, , and radius of gyration, , varied as shear rate ('x), with x = -0.879 and -0.353, a weaker dependence than predicted by existing breakup models. Comparison of the two yields (PROPORTIONAL) ('D), where D = 2.48 ((TBOND)fractal dimensionality), indicating a nonuniform structure, conflicting with the uniform porosity assumptions of the models. Other methods for determining the fractal dimensionality yielded 1.6 (LESSTHEQ) D (LESSTHEQ) 2.5, depending on the shear history of the samples. The effect of increasing electrolyte concentration was slight weakening of the flocs for NaCl < 0.04 M followed by significant strengthening above 0.04 M; DLVO theory successfully correlated this trend.; The effect of a converging flow on the breakup of flocs was investigated experimentally, indicating (PROPORTIONAL) strain rate ('-1.0). ('D), however, varied little, possibly due to a change of floc shape from predominantly spherical to elongated. A numerical analysis of the flow indicated higher stresses near the corners of the orifice than at the centerline.; The elastic properties of volume-filling flocculated networks were investigated, with emphasis on the effect of (phi), the volume fraction, on G', the storage modulus. We found G' (PROPORTIONAL) (phi)('n), with n = 2.53 for freshly flocculated samples and n = 4.43 for aged samples. This suggests the formation, as (phi) increases, of additional force-bearing links between the chains comprising the elastic backbone of the network.; An improved model for instantaneous floc breakup was developed which considers a spatial variation of the local volume fraction within the floc. The experimentally observed relationship of (PROPORTIONAL) (gamma)('-0.353) was correlated for D = 2.48 by assuming G' (PROPORTIONAL) (phi)('4.45), in agreement with the experimentally observed elastic properties of the flocculated networks (aged samples). The maximum strain at rupture was (TURN)5%, in agreement with the limiting strain for linearity in the elasticity experiments. More general theoretical results were also presented to facilitate comparison of future experiments with the model.