We present experimental results showing that colloidal silica spheres can be induced to aggregate either slowly or very rapidly. The slow process always yields clusters with a fractal dimensionality ${\mathrm{d}}_{\mathrm{f}}$=2.08±0.05, but the rapid process can produce clusters with either ${\mathrm{d}}_{\mathrm{f}}$=1.75±0.05 or ${\mathrm{d}}_{\mathrm{f}}$=2.08±0.05. However, clusters with ${\mathrm{d}}_{\mathrm{f}}$=1.75 are always observed to restructure so as to yield ${\mathrm{d}}_{\mathrm{f}}$=2.08±0.05.

• Received 28 October 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.738