Thermal properties of athermal granular materials

Two and a half centuries after Coulomb's explanation of the angle of repose of a granular pile in relation to frictional slip between different layers, our understanding of yielding processes in granular materials remains incomplete. The main reason for this is that granular piles are comprised of a vast ensemble of discrete solid particles that interact through intricate molecular contact forces. Here, we explore the dynamics of a dry granular system at both the granular and molecular levels, revealing two qualitatively distinct yielding behaviors. We show that, while the friction peak associated with the granular rearrangement is independent of time, the molecular friction displays aging that is thermal in nature. We observe that a granular system subjected to a sub-critical stress can show thermally activated processes through slow creep deformations. As a result, the system under stress, without granular-level friction, is susceptible to a spontaneous failure, which may occur after a delay as long as several hundred seconds. These findings have important practical implications for understanding the yielding of granular materials under various loading conditions at different temperatures, with potential applications spanning from the processing of powdered materials in the polymer, pharmaceutical, and food industries to geotechnical engineering and geophysics.