On the reversibility of granular rotations and translations

We analyze reversibility of both displacements and rotations of spherical grains in three-dimensional compression experiments. We track grain motion during cyclical compression achieved via compressing a moving wall in a rectangular box. Using transparent acrylic beads with cylindrical holes and index matching techniques, we are not only capable of tracking displacements but also, for the first time, rotations. We observe that for compression amplitudes up to the bead diameter, the bead's translational displacements after each cycle eventually become mostly reversible. By contrast, granular rotations appear to be largely irreversible. Our results indicate a weak correlation between translational and rotational displacements, indicates that rotational reversibility depends on more subtle changes in the contact distributions and contact forces between grains compared with displacement reversibility. These experimental results are corroborated by our molecular dynamics simulations.