Can we ignore hydrodynamic interactions in semi-dilute suspensions of colloidal slender rods?

Although hydrodynamic interactions (HI) are well recognized for their substantial role in the rheology of emulsions and suspensions of spherical particles, their influence on elastic fibers is yet to be understood. To address this gap, we employ scaling analysis and Brownian Dynamics simulations with or without HI to compute the rod orientation distribution, tumbling frequency and stress state as a function of shear rate. Although HI do not directly contribute significantly to the stress, we find (at Peclet number > 100) they influence the rod tumbling rate and orientation distribution, and thereby increase the stress substantially. These conclusions help explain recent experimental data of rodlike suspensions.