The Connection between Biaxial Orientation and Shear Thinning for Quasi-Ideal Rods

Abstract

The complete orientational ordering tensor of quasi-ideal colloidal rods is obtainedas a function of shear rate by performing rheo-SANS (rheology with small angle neutronscattering) measurements on isotropic fd-virus suspensions in the two relevant scatteringplanes, the flow-gradient (1-2) and the flow-vorticity (1-3) plane. Microscopic ordering canbe identified as the origin of the observed shear thinning. A qualitative description of therheological response by Smoluchowski, as well as Doi–Edwards–Kuzuu theory is possible,as we obtain a master curve for different concentrations, scaling the shear rate with theapparent collective rotational diffusion coefficient. However, the observation suggests that theinterdependence of ordering and shear thinning at small shear rates is stronger than predicted.The extracted zero-shear viscosity matches the concentration dependence of the self-diffusion ofrods in semi-dilute solutions, while the director tilts close towards the flow direction already atvery low shear rates. In contrast, we observe a smaller dependence on the shear rate in the overallordering at high shear rates, as well as an ever-increasing biaxiality