Dynamics of Stress, Velocity and Density Fluctuations in a Shear-thickening Suspension

Dense particulate suspensions demonstrate a significant increase in average viscosity beyond a material-specific critical shear stress. Utilizing direct measurements of spatially resolved boundary stresses during the thickening phase in a suspension of monodisperse silica microspheres, we observe the presence of regions of well-defined dynamic localized high-stress which intermittently manifest at stress levels exceeding the critical threshold. When subjected to higher applied stresses, a persistent cluster of localized high-stress regions develops, propagating in the direction of flow at a velocity nearly matching that of the top boundary. These prominent localized stress fluctuations coincide with a rapid increase in particle speed, the loss of positional order, and a decrease in particle concentration near the boundary. Notably, we observe that particle speeds during these events are independent of depth, suggesting collective motion as a shear-jammed aggregate. However, between consecutive high-speed regions, particle velocities exhibit nearly affine flow, indicating that these jammed regions propagate as density waves with continual particle exchange and only modest non-affine particle transport.