Strain-transport superposition in shear-thinning dense non-Brownian suspensions

Abstract: Shear thinning in dense non-Brownian suspensions is often attributed to shear-induced microstructural evolution, including changes in alignment, anisotropy, and near-contact statistics, yet how these changes influence particle-scale dynamics remains unclear. Using particle-resolved simulations of dense suspensions that shear thin through diverse microscopic mechanisms, including short-range attraction, repulsion, and load-dependent friction, we show that the magnitude of nonaffine particle velocities is controlled solely by the imposed shear rate, independent of coordination number, structural anisotropy, and interaction details. In contrast, macroscopic stress and viscosity remain strongly sensitive to the underlying interactions. When mean-squared displacements transverse to the flow are rescaled by accumulated strain and the nonaffine velocity variance, all data collapse onto a single master curve, revealing strain-controlled transport with a robust ballistic-to-diffusive crossover. These results demonstrate a fundamental decoupling between particle-scale kinematics and macroscopic rheology and identify nonaffine velocity fluctuations as the emergent dynamical scale governing shear-driven transport.