A new study finds that piles of sand grains, even when undisturbed, are in constant motion. These experimental results challenge existing theories in both geology and physics about how soils and other types of disordered materials behave.

Most people only become aware of soil movement on hillsides when soil suddenly loses its rigidity, a phenomenon known as yield. «Say that you have soil on a hillside. Then, if there’s an earthquake or it rains, this material that’s apparently solid becomes a liquid,» says principal investigator Douglas Jerolmack of Penn. «The prevailing framework treats this failure as if it’s a crack breaking. The reason that’s problematic is because you’re modeling the material by a solid mechanical criterion, but you’re modeling it at the point at which it becomes a liquid, so there’s an inherent contradiction.»

Such a model implies that, below yield the soil is a solid and therefore should not flow, but soil slowly and persistently «flows» below its yield point in a process known as creep. The prevailing geological explanation for soil creep is that it is caused by physical or biological disturbances, such as freeze-thaw cycles, fallen trees, or burrowing animals, that act to move soil.

In this study, lead author and Penn Ph.D. candidate Nakul S. Deshpande was interested in observing individual sand particles at rest which, based on existing theories, should be entirely immobile. «Researchers have built models by presuming certain behaviors of the soil grains in creep, but no one had actually just directly observed what the grains do,» says Deshpande.

To do this, Deshpande set up a series of seemingly simple experiments, creating sand piles in small plexiglass boxes on top of a vibration isolation worktable. He then used a laser light scattering technique called diffusing-wave spectroscopy, which is sensitive to very small grain movements. «The experiments are technically challenging,» Deshpande says about this work. «Pushing the technique to this resolution is not yet common in physics, and the approach doesn’t have a precedent in geosciences or geomorphology.»

Deshpande and Jerolmack also worked with long-time collaborator Paulo Arratia, who runs the Penn Complex Fluids Lab, to connect their data with frameworks from physics, materials science, and engineering to find analogous systems and theories that could help explain their results. Vanderbilt’s David Furbish, who uses statistical physics to study how particle motions influence large-scale landscape changes, provided explanation for why previous models were physically inadequate and inconsistent with what the researchers had found.

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Materials provided by University of Pennsylvania. Original written by Erica K. Brockmeier. Note: Content may be edited for style and length.