Measurements of the radius of gyration ${\mathrm{R}}_{\mathrm{G}}$ and the hydrodynamic radius ${\mathrm{R}}_{\mathrm{H}}$ of colloidal silica aggregates are reported. These aggregates have fractal geometry and ${\mathrm{R}}_{\mathrm{H}}$ is proportional to ${\mathrm{R}}_{\mathrm{G}}$ for 500 Å≤${\mathrm{R}}_{\mathrm{H}}$≤7000 Å, with a ratio ${\mathrm{R}}_{\mathrm{H}}$/${\mathrm{R}}_{\mathrm{G}}$=0.72±0.02. The results are compared with predictions for macromolecules of various shapes. The proportionality of the two radii can be understood with use of the pair correlation function of fractal objects and hydrodynamic interactions on the Oseen level. The value of the ratio remains to be explained.

• Received 5 November 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.710