Nonlinear acoustic resonance and wave-induced softening in dense granular matter through flow heterogeneities

We report a series of experiments on the softening and compaction of a dense granular pack through traveling acoustic pressure and shear waves. Softening is manifested by a reduction the traveling-wave speed, as the amplitude of the disturbance increases beyond some threshold. We explain these seemingly contradictory observations using a theoretical model, based on shear transformation zones (STZs), that directly attributes these observations to dynamical heterogeneities and slipping contacts in the granular pack. Softening is accounted for by the increase of the fraction of STZs or slipping contacts as a function of increasing strain amplitude, while compaction is explained by an Ising-like correlation between STZs in the subyield regime. In so doing, we demonstrate the fundamental connection between nonaffine granular rearrangements, mesoscopic glassy dynamics, jamming and unjamming, and matter-wave interactions.