Complex fluids containing low concentrations of slender colloidal rods can display a high viscosity, while little flow is needed to thin the fluid. This feature makes slender rods essential constituents in industrial applications and biology. Though this behaviour strongly depends on the rod-length, so far no direct relation could be identified. We employ a library of filamentous viruses to study the effect of rod size and flexibility on the zero-shear viscosity and shear-thinning behaviour. Rheology and small angle neutron scattering data are compared to a revised version of the standard theory for ideally stiff rods, which incorporates a complete shear-induced dilation of the confinement. While the earlier predicted length-independent pre-factor of the restricted rotational diffusion coefficient is confirmed by varying the length and concentration of the rods, the revised theory correctly predicts the shear thinning behaviour as well as the underlying orientational order. These results can be directly applied to understand the manifold systems based on rod-like colloids and design new materials.
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