The hydrodynamic behavior of fractal aggregates.

Measurements of the radius of gyration R sub G and the hydrodynamic radius R sub H of colloidal silica aggregates are reported. These aggregates have fractal geometry and R sub H is proportional to R sub G for 500angstroms = R sub H =7000angstroms, with a ratio R sub H/R sub G = 0.72 +-.02. The results are compared with predictions for macro- molecules of various shapes. The proportionality of both radii can be understood using the pair correlation function of fractal objects and hydrodynamic interactions on the Oseen level. The value of the ratio remains to be explained.