The connection between shear thinning and biaxial orientation for quasi-ideal rods

TL;DR

This study investigates how shear thinning in colloidal rods relates to their microscopic biaxial orientation, using rheo-SANS measurements to connect shear rate, ordering, and viscosity in fd-virus suspensions.

Contribution

It provides the first detailed measurement of the complete orientational ordering tensor of colloidal rods as a function of shear rate, linking microscopic ordering to rheological behavior.

Findings

Shear thinning correlates with microscopic ordering of rods.

Zero-shear viscosity matches self-diffusion concentration dependence.

Biaxiality increases with shear rate, affecting ordering.

Abstract

The complete orientational ordering tensor of quasi-ideal colloidal rods is obtained as a function of shear rate by performing rheo-SANS (rheology with small angle neutron scattering) measurements on isotropic fd-virus suspensions in the two relevant scattering planes, the flow-gradient (1-2) and the flow-vorticity (1-3) plane. Microscopic ordering can be identified as the origin of the observed shear thinning. A qualitative description of the rheological 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 the apparent collective rotational diffusion coefficient. However, the observation suggests that the interdependence 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 of rods in semi-dilute solutions, while the director tilts close towards the flow direction already at very low shear rates. In contrast, we observe a smaller dependence on the shear rate in the overall ordering at high shear rates, as well as an ever-increasing biaxiality.