to the presence of a hydrated surface layer constituted of We investigated the mass and size distributions of aggregates hydrophilic groups (hydroxyls, carboxylate and sulfate formed under low shear (14 s 01 ) by collisions between hydrated groups). Aggregate aging was found to strongly determine colloids of micrometric size under marginal stability conditions. the colloid stability and the present paper aimed at determi-The mass c(n) and size c(d) distributions determined by particle nation of the aggregate and suspension characteristics when counting and laser diffractometry respectively were found to scale the colloids sustained a permanent stress under very low with n 0 t and d 0 a . Aggregation/fragmentation processes rapidly shear. Aggregates of micrometric size were sheared in a established a quasi dynamic equilibrium situation where, however, Couette flow system and, periodically small samples of the a prevailing aggregation and fragmentation process was successuspension were carefully collected and analyzed for particle sively determined to occur. Under marginal stability conditions, the particle concentration and ionic strength differently induced mass and size distribution determinations.
the aggregate formation. In 0.15 M NaCl suspension, below a Aggregate suspensions are characterized by their mass threshold latex concentration, an increase in the particle concenand size distributions and by their average masses and sizes. tration induced an increase of the aggregate average masses while Moreover, the variation of the average suspension characterabove this value, a decay of the average masses was determined. istics as a function of the aggregation time was found to In 0.075 M NaCl suspensions, where the influence of the van der determine the aggregation mode in perikinetic processes. Waals attraction was slightly reduced by the effect of the electrical From numerical studies, the number and weight average repulsion, an increase of the particle concentration induced a demasses were determined to increase like a power law of the cay of the aggregate average masses. Preliminary aging of the aggregation time, the scaling exponent being equal to 1 for aggregates over a critical time was assumed to be required for the diffusion-limited aggregation processes, while the two establishment of cohesive interparticle links. Correlation between exponents w and z (with w/z ร
0.5) were found to characterthe reduced mass and size distributions unequivocally led to the fractal dimension f ร
a/t of the aggregates. In 0.15 M NaCl ize the growth of the number and weight average masses suspensions the fractal dimension was determined to be 1.4 while respectively (4-7). Similar power laws were determined in 0.075 M NaCl suspensions, aging and restructuration led to experimentally for very small colloids and latex particles of aggregates of fractal dimension 1.8. These relatively low values micronic size aggregating in the presence of floculating of the fractal dimension were interpreted by the existence of a agents of low molecular weight (8, 9). The mass distribution preferential orientation of the aggregates in the low shear flow. were determined to be self-similar so that, at each time, the แญง 1998 Academic Press suspension characteristics were represented by a master