RESUMO
Orthokinetic aggregation of colloids trapped at the air-liquid interface was studied by direct imaging in a couette cell. This method allowed us to follow the temporal evolution of both the cluster-mass distribution and the cluster structure at a shear rate where Brownian aggregation is suppressed. The interactions between the monodisperse latex particles floating at the air-liquid interface were controlled either by varying the electrolyte concentration or by creating a bidisperse system through the addition of small particles. The results show that the clusters in all of the systems are characterized by a high fractal dimension, indicating that the clusters are rearranged and densified by the shear. Kinetic analysis suggests that aggregation of monodisperse systems mainly proceeds through homogeneous aggregation, i.e., large clusters sticking to other large clusters. The bidisperse system, finally, with a size ratio around 10, favored a more heterogeneous aggregation among small and large clusters throughout the aggregation process; a slightly lower fractal dimension was observed compared to the strongly aggregated monodisperse system. Copyright 1999 Academic Press.
RESUMO
Aggregation and cluster formation of a two-dimensional colloidal system consisting of 2-µm alkoxylated silica particles trapped at the air-toluene interface have been studied. This novel system allows particle interactions to be controlled by varying the length of the grafted alkyl chains; simple estimates suggest particle bond strengths of 15 and 30 kT for the octadecyl- and octyl-coated system, respectively. Video-enhanced microscopy and image analysis enabled a simultaneously study of kinetics and structure of the ensemble of clusters in situ. The octyl system displayed a DLCA-like structure, D(f) approximately 1.45, whereas the octadecyl system resulted in a more dense structure, D(f) approximately 1.55. The temporal evolution of the cluster-mass distribution displayed a transition point between regimes of slower and faster aggregation for both systems, which was interpreted as a transition from DLCA to convection-limited cluster aggregation (CLCA). Copyright 1999 Academic Press.
