Hydrodynamic Alignment of Rodlike Macromolecules During Ultracentrifugation

The effect of intermolecular hydrodynamic interaction on the alignment of rodlike macromolecules during ultracentrifugation is presented. It is shown that the resulting effect is extremely small in magnitude, and in fact predicts a decrease in observed sedimentation coefficient with applied field, which is contrary to experimentally observed properties of the sedimentation coefficient. The intermolecular hydrodynamic interaction mechanism is thereby shown to be unimportant in explaining the origin of this phenomenon. A possible alternative mechanism involving tactoids is briefly discussed.