Motivated by recent light scattering experiments by Wiltzius, we discuss the various factors affecting the hydrodynamic radius RH 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 Brinkman. For spherically symmetric aggregates this model leads to values of RH 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 discripancies, and argue that they might be due to assymmetry 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.
