  We analyze reversibility of both displacements and rotations of spherical
grains in three-dimensional compression experiments. 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, analyze
reversibility of rotations. We observe that for moderate compression
amplitudes, up to one bead diameter, the translational displacements of the
beads after each cycle become mostly reversible after an initial transient. By
contrast, granular rotations are largely irreversible. We find a weak
correlation between translational and rotational displacements, indicating that
rotational reversibility depends on more subtle changes in contact
distributions and contact forces between grains compared with displacement
reversibility.
