Quantitative understanding of sheared colloidal rods and the effect of length and stiffness

High-aspect-ratio colloidal rods are becoming increasingly important in a wide range of technological applications and products. In biology they constitute the frame of the cytoskeleton, in the form of F-actin and micro-tubular networks, while amyloids are responsible for e.g. Alzheimer disease. The mechanical response of complex fluids containing rod-like colloids is hugely affected by the shape of the particle, though a direct relation has not been identified so far. The key to a bottom up understanding is to identify the role of rod morphology on the microscopic structural response to flow, underlying the macroscopic mechanical response. Here, we use a library of monodisperse bio-engineering viruses with variable length and stiffness, for which we determine the exact relation between structural and mechanical response by a combination of rheology and Small Angle Neutron Scattering, resolving the orientational ordering of rod-like viruses in the flow-gradient and the flow-vorticity plane [1]. This approach allowed us to determine quantitatively the length dependence of the zero-shear viscosity [3] and shear thing behavior, using a revised version of the theory developed by Doi, Edwards, and Kuzuu to rationalize the flow behavior. Furthermore, we identified the effect of flexibility which diminishes viscosity at low shear rate and enhances it at high shear rates. The elongational viscosity of stiff rods obeys theoretical predictions, while it diminishes with flexibility [2]. Thus, this work establishes a fundament for understanding the non-linear flow behavior of more complex rod-like systems, which we demonstrate for ideal bi-disperse systems