A reverse nonlocal effect of flow supressing fluidization in dense granular flow

Nonlocal effects in granular materials typically diffuse particle fluctuations, disrupting interparticle contact networks and enhancing fluidization. In this study, we experimentally identify a reverse nonlocal effect that suppresses fluidization in granular flows. An innovative vertical chute flow channel filled with photoelastic disks and equipped with moving toothed belt walls was established to generate Poiseuille and Couette-Poiseuille flows with equal and unequal wall shear speeds, respectively. Flow images were recorded with a high-speed camera and analyzed using particle tracking velocimetry and the coarse-graining method to obtain granular continuum quantities. The Poiseuille flow data are captured by existing nonlocal rheological laws based on flow-enhancing fluidization. However, the Couette-Poiseuille flow data reveal bifurcated rheological curves indicating less fluidization near the slow shear wall compared to the fast shear wall. Through statistical analysis of photoelastic stress, the bistable rheology is found to result from a self-interruption of the yielding process due to nonlocal stress fluctuations. Incorporating a normalized stress drop into the existing rheological laws effectively accounts for the reverse nonlocal effect. These findings underscore the role of stress fluctuations in developing unified rheological laws.