On the hydrodynamic radius of fractal aggregates.

Motivated by recent light scattering experiments by Wiltzius we discuss the various factors affecting the hydrodynamic radius R sub H of fractal aggregates, underlining the need for further experiments. After a critical discussion of the results of the Kirkwood-Riseman approximation for the hydrodynamic radius, we analyze the porous sphere model of Debye-Bueche and Brickman. For spherical symmetric aggregates this model leads to values of R sub H which are substantially larger than found experimentally by Wiltzius, but somewhat smaller than found in numerical simulations. We make various suggestions for the physical origin of these discrepancies, and argue that they might be due to asymmetry of the aggregates. We discuss how this suggestion can be tested experimentally with depolarized light scattering as well as with sedimentation experiments, and theoretically with the aid of computer simulations.