Continuum hydrodynamic interactions and diffusion

Abstract

The Oseen tensor is not an adequate characterization for the contribution of hydrodynamic interactions to the homodyne (’’self‐beat’’) spectrum of light scattered from a dilute macromolecule solution. Physical constraints on the possible form of the true cross diffusion tensor D^T(R), which describes hydrodynamic interactions in macromolecule solutions, indicate that hydrodynamic interactions do not by themselves contribute to the homodyne spectrum. However, if the macromolecules in a solution interact directly as well as hydrodynamically, the direct and hydrodynamic interactions couple to contribute jointly to the mutual diffusion coefficient of the solute molecules, and thence to the spectrum of the quasielastically scattered light. Tracer diffusion techniques may be used to study the cross‐diffusion tensor. Comparison with the available experimental data indicates that the joint contribution of direct and coherent hydrodynamic interactions to diffusion need not be large, even in extremely concentrated solutions.