Abstract
The colloidal aggregation problem coupled with sedimentation is stratified, in the sense that the structural and dynamical quantities describing the aggregates depend on the depth at which they are measured. In this work we present the first computer simulation with particles of colloidal aggregation coupled with sedimentation, for which the clusters in the simulation box represent those clusters inside a layer at a fixed depth and of arbitrary thickness in the confinement prism. It would then be possible to compare the results with an eventual validation experiment, in which an aggregating sample is sipped out with a pipette at a fixed depth and subjected to further studies, or with a light scattering experiment in which the laser beam is focused also at a fixed depth in the prism. We confirm the acceleration of the aggregation rate followed by a slowing-down, compared with an aggregating system driven purely by diffusion. We also confirm the appearance of a "sweeping scaling regime" for which the large clusters when drifting downwards sweep smaller ones, which in turn occlude the holes and cavities of these large clusters, increasing in this way their fractal dimension. However, as the large clusters continue to grow for very big depths, and also for medium size clusters at high sedimentation strengths, we have found that the anisotropy of these clusters makes the radius of gyration not to scale with size —becoming impossible to define a fractal dimension— and the clusters become non–self-similar, as found recently by some other authors.