Vortex Structures inside Shear Bands in Sands

We show experimental evidence of vortices emanating from collapsed force chains in shear bands in real sands. Digital image correlation (DIC)-based local displacement analyses have been conducted on overlapping particle clusters to provide grain-scale displacement measurements of sand specimens undergoing plane strain compression. Spatially varying systematic patterns in kinematic data fields have been found within shear bands, defining a coordinated collapse event of the force chains that initially comprise the shear band upon formation. We have identified co-rotating vortices or "circulation cells" within the shear band by subtracting an overlaid linearly varying shear band displacement field from the observed non-homogeneous shear band displacement field. These vortex structures correlate precisely with previously observed kinematic patterns. We discuss the fate of these vortices and note differences in the vortex patterns observed in our sands from those observed by others in simulations on "ideal" granular materials.