Fluctuations in Granular Media Role of Frustration and Disorder Analogy with Spin Glasses and Neural Networks

Experiments on motions and deformations of non-cohesive granular materials are characterized by important fluctuations both in time (sudden breaks, avalanches, earthquakes) and space (strain localizations, yield surfaces). The conventional wisdom, based on the standard tendency to homogeneize, has been to dismiss them as experimental noise. Here, we suggest that these fluctuations observed both at large scales and at the grain scale constitute an important intrinsic signature of the mechanics of granular media. An analogy with spin glasses and neural networks, based on the existence of grain packing disorder and grain rotation "frustration", shows that discontinuities in time and space are intimately related. The qualitative properties of the experimentally observed "intrinsic" and "sample to sample" fluctuations are rationalized within the proposed analogy.