Role of sedimentation and buoyancy on the kinetics of diffusion limited colloidal aggregation

The effect of the small difference in densities between dispersion medium and polymer particles on the kinetics of fast or diffusion-limited cluster aggregation (DLCA) has been investigated experimentally using small-angle light scattering (SALS). The density of the dispersion medium was tuned by varying the volume fraction of D2O from 0 to 80% so as to investigate the effect of both aggregates sedimentation and buoyancy. It is found that the time evolution of the average radius of gyration, <R-g> determined by SALS, initially follows the usual DLCA kinetics, i.e., a power law. However, when <R-g> reaches a certain value, the growth of <Rg> starts to accelerate. The onset of such acceleration shifts to larger <R-g> values for smaller density differences between the dispersion medium and the colloidal particles, thus indicating that the acceleration results from the sedimentation or buoyancy of the aggregates that reach a certain size. This is confirmed by the measured dependence of the scattering intensity at zero angle and obscuration on <R-g>, which deviate from the theoretical predictions when <R-g> reaches a certain value. On the other hand, the effect of the small difference in densities on the structure of aggregates is insignificant, as indicated by the fact that the values of the fractal dimension, d(f), estimated from the average structure factors measured in the presence or absence of sedimentation (or buoyancy), are practically identical. Moreover, four different techniques for estimating df have been proposed and compared, and a substantially small value for d(f) (=1.63) has been observed in all investigated DLCA systems