SHEAR-INDUCED AGGREGATION AND BREAKUP OF POLYSTYRENE LATEX-PARTICLES

The structure of aggregates formed by diffusion-limited aggregation can be characterized by a fractal dimension of df ≈ 1.75. Further, fluid shear changes the trajectories of the colliding particles and applies a force on the aggregates, so that both the shear and the resulting force can lead to a different structure. Shear forces also cause breakup of the aggregates, finally leading to a balance between growth and breakup. Monodispersed polystyrene latex spheres of 2.17 μm diameter in a highly electrolyte environment were sheared in a Couette-flow system. An increasing shear rate accelerated the growth and decreased the stable size of the aggregates. The density of the aggregates in the initial growth phase was characterized by a fractal dimension of df ≈ 2.1, changing to df ≈ 2.5 in later stages of their growth. The initial fractal dimension can be explained by accelerated restructuring. A change in the particle trajectories (relative to Brownian motion) has recently been shown (1) not to alter the fractal dimension. The further increase in the fractal dimension is correlated to the fragmentation and can be explained by a selective breakup that removes preferably the most porous parts from an aggregate. © 1992.