Onset of irreversibility in cyclic shear of granular packings.

We investigate the onset of irreversibility in a dense granular medium subjected to cyclic shear in a split-bottom geometry. To probe the micro- and mesoscale, we image bead trajectories in three dimensions throughout a series of shear strain oscillations. Although beads lose and regain contact with neighbors during a cycle, the global topology of the contact network exhibits reversible properties for low oscillation amplitudes. With increasing reversal amplitude, a transition to an irreversible diffusive regime occurs.

From frictional to viscous behavior: three-dimensional imaging and rheology of gravitational suspensions.

We probe the three-dimensional flow structure and rheology of gravitational (nondensity matched) suspensions for a range of driving rates in a split-bottom geometry. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, and confirm recent theoretical modeling on the rheology of split-bottom flows. For faster driving, the flow behavior is shown to be consistent with the rheological behavior predicted by the recently developed "inertial number" approaches for suspension flows.

Correlation between particle motion and Voronoi-cell-shape fluctuations during the compaction of granular matter.

We track particle motions in a granular material subjected to compaction using a laser scattering-based imaging method where compaction is achieved through thermal cycling. Particle displacements in this jammed fluid correlate strongly with rearrangements of the Voronoi cells defining the local environment about the particles, similar to previous observations of Rahman on cooled liquids. Our observations provide further evidence of commonalities between particle dynamics in granular matter close to jamming and supercooled liquids.