Disruption of structural order in sheared dense suspensions due to frictional constraints

Dense suspensions exhibit an array of interesting non-Newtonian rheological properties. For practical applications, it is crucial to understand how those properties arise from the microstructure, which depends on both macroscopic forcing and microscopic constraints. We probe this connection with large-scale simulations of 100,000 suspended particles undergoing shear. Using varied particle interactions and flow conditions, we explore both shear-thinning and shear-thickening regimes, while analyzing the microstructural characteristics of both. We observe shear-thinning behavior that is coupled to ordering for lubricated contacts, where hexagonal crystal layers with chain-like defects can slide unhindered with the flow. In contrast, upon introducing frictional constraints to sliding motion between particles, the ordering is disrupted and shear thickening becomes apparent. In both cases, we compute the individual particle contributions to the stress to identify the stress-carrying structures that form under shear. The shear stress is found to be highly localized in the defects for the ordered suspension, whereas the introduction of friction substantially changes the nature of the stress localization.