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Hypothesis: The viscosity of dense suspensions surges when the applied stress surpasses a material-specific critical threshold. There is growing evidence that the thickening transition involves non-uniform flow and stress with considerable spatiotemporal complexity. Nevertheless, it is anticipated that dense suspensions of calcium carbonate particles with purely repulsive interactions may not conform to this scenario, as indicated by local pressure measurements with millimeter spatial resolution. Experiment: Here we utilize Boundary Stress Microscopy (BSM), a technique capable of resolving stresses down to the micron scale, to search for evidence of stress heterogeneity. In addition, we measure the flow field at the lower boundary of the suspension where the boundary stress is measured. Findings: We find localized regions of high-stresses that are extended in the vorticity direction and propagate in the flow direction at a speed approximately half that of the rheometer’s top plate. These high-stress regions proliferate with the applied stress accounting for the increased viscosity. Furthermore, the velocity of particles at the lower boundary of the suspension shows a significant and complex nonaffine flow that accompanies regions of high-stresses. Hence, our findings demonstrate that stress and flow inhomogeneity are intrinsic characteristics of shear-thickening suspensions, regardless of the nature of interparticle interactions. 

We introduce a method for precise and accurate measurements of particle speeds in dense suspensions flowing at high rates and demonstrate the utility of the approach for revealing complex flow fluctuations during shearing in a setup that combines imaging with a confocal microscope and shearing with a rheometer. We scan the focal point in one dimension, aligned with direction of flow, producing absolute measurements of speed that are independent of suspension structure and particle shape. We compare this flow-direction line scanning approach with a complementary method we introduced previously, measuring speed using line scanning in the vorticity direction. By comparing results in various flow conditions, including shear-thinning and thickening regimes, we demonstrate the efficacy of our new approach. We find that both approaches exhibit qualitatively similar flow profiles, but a comparative analysis reveals a 15%–25% overestimation in speed measurement using vorticity line scanning, with discrepancies generated by anisotropic suspension microstructure under flow. Moreover, in the thickening regime where complex flow fields are present, both approaches capture local speed fluctuations. However, line scanning in the flow direction reveals and precisely captures stagnation and backflows, a capability not achievable with vorticity line scanning. The approach introduced here not only provides a refined technique for speed measurement in fast-flowing suspensions but also emphasizes the significance of accurate measurement techniques in advancing our understanding of flow behavior in dense suspensions, particularly in contexts where strong non-affine flows are prevalent.