Probing the interior dynamics of shear thickening colloidal suspensions using confocal line scans

Under high stresses, shear thickening suspensions transition from low viscosity to high viscosity or shear jammed states. While both the bulk rheology and the microscopic driving forces of such transitions are well-studied, the bridge between the microscopic and macroscopic scales is incomplete. Measurements at the boundaries indicate that dynamic high stress regions propagate through shear thickening suspensions of colloidal spheres, but the means of propagation within the suspension interior was inaccessible due to imaging limitations. Here we use line scans on a confocal microscope to visualize particle flows in a sheared suspension, increasing our temporal resolution by two orders of magnitude. These line scans reveal that high stress regions are correlated with significant changes in particle speed, density, and order that extend many particle diameters deep into the suspension.