Abstract
Two-dimensional cluster–cluster aggregation is studied when clusters move both diffusively
and sediment with a size-dependent velocity. Sedimentation breaks the rotational
symmetry and the ensuing clusters are not self-similar fractals: the mean cluster width
perpendicular to the field direction grows faster than the height. The mean width exhibits
power-law scaling with respect to the cluster size, 
, lx = 0.61 ± 0.01, but the mean height does not. The clusters tend to become elongated in the
sedimentation direction and the ratio of the single-particle sedimentation velocity to
single-particle diffusivity controls the degree of orientation. These results are obtained
using a simulation method, which becomes more efficient the larger the moving clusters are.