Variation in granular frictional resistance across nine orders of magnitude in shear velocity

Determining the shear-velocity dependence of dry granular friction can provide insight into the controlling variables in a dry granular friction law. Scattered laboratory results suggest that granular friction is greatly affected by shear-velocity (v), but shear experiments over the large range of naturally occurring shear-velocities are lacking. Herein we examined the shear velocity dependence of dry friction for three granular materials, quartz sand, glass beads and fluorspar, across nine orders of magnitude of shear velocity (10^-8 m/s - 2 m/s). Within this range, granular friction exhibited four regimes, following a broad approximate "m" shape including two velocity-strengthening and two velocity-weakening regimes, and we discussed the possible physical mechanisms of each regime. This shear velocity dependence appeared to be universal for all particle types, shapes, sizes, and for all normal stresses over the tested range. We also found that ultra-high frequency vibration as grain surfaces were scoured by micro-chips formed by spalling at high shear velocities, creating ~20 µm diameter impact pits on particle surfaces. This study provided laboratory laws of a friction-velocity (μ-v) model for granular materials.