Activating granular creep with vibrations and heat

Under low stresses or prolonged shear, granular materials become jammed, where we expect motion to cease. We have recently observed persistent, indefinite deformation in an experimental granular heap under only the action of gravity. These dynamics showed hallmarks of of creep in a relaxing glass: power-law aging, quadrupolar spatial correlations, and heterogeneous zones of deformation. This hints at a regime in which activated creeping motion may be sustained below the global yield. Motivated by similarity of relaxing sandpile and relaxing glass, we seek to activate creep by external disturbances and observe the resultant states. To do so, we introduce controlled vibrations and heat to an otherwise quiescent granular heap, and measure the resulting creep strains with spatially-resolved diffusive wave spectroscopy (DWS). In response to gentle taps, creep deformation is distributed throughout the pile, manifest as discrete zones of plastic activity. As time progresses, deformation within the bulk diminishes and becomes confined to a thin layer at the free surface. Increasing the vibration strength causes this layer to span the system size, at which point a steady hydrodynamic-like flow appears at the free surface. Heating the grains produces small displacements due to thermal expansion and contraction. In this case, the phenomenology is similar to vibrations, albeit with a clear distinction: elastic strains are observed in the compacted core of the pile. We identify critical amplitudes of vibration required for system-spanning failure that vary as a function of the age of the pile. These results are geophysically relevant, as soils are heated and cooled daily, and experience ambient vibrations provided by the environment.