Visualization of displacement fields in a sheared granular system

We study experimentally the motion of particles in a wet granular system at the onset of flow. The suspension is density matched and subjected to a uniform confining pressure and uniform shear. Trajectories of the individual particles are visualized in three dimensions using laser sheet imaging. These trajectories reveal a flow field with pressure-dependent dilation and superdiffusive motions around the mean flow. Correlations in the particle displacements are analyzed as a function of applied confining pressure and applied shear. We find correlations with a pressure-dependant correlation length at small strains.