Flow-induced microstructure and rheology of semidilute suspensions of rodlike Brownian particles

Flow of nondilute elongated particle suspensions produces complex microstructure and rheology due to coupling of single-particle orientations with anisotropic interparticle interactions. Current theories to describe this coupling are lacking except in the cases of asymptotically large aspect ratios. We address this gap through a combined experimental and theoretical study of the flow-induced microstructure and rheology of semidilute cellulose nanocrystal dispersions. We show that current theories for nondilute rodlike Brownian dispersions significantly underpredict the viscosity and degree of shear thinning for these fluids, and that these differences are likely due to flow-induced changes in the radial distribution of interparticle separations (or its Fourier equivalent, the structure factor) for particles with a finite aspect ratio. We, therefore, develop and apply a scattering analysis that isolates contributions due to the anisotropic interparticle structure from those due to single-particle orientation through an extended form of the random phase approximation (RPA), allowing estimation of a mean-field parameter describing the strength of interparticle interactions due to flow. The analysis is validated and applied to rheoscattering measurements in steady-state simple shear flow and in complex, Lagrangian unsteady deformation histories. Ultimately, we distinguish two regimes of behavior—one in which the flow-induced microstructure can be accurately predicted using interactions observed at equilibrium, and another in which the mean-field interaction strength is modified by flow. We, thus, propose that the extended RPA model could be used as a widely applicable tool to infer and model nonequilibrium structures and interactions of anisotropic materials from scattering measurements in flow and other orienting fields.