Experimentally measuring rolling and sliding in three-dimensional dense granular packings

TL;DR

This study uses advanced imaging and AI to measure 3D grain motions in dense granular packings, revealing the critical role of rotations and contact sliding in energy dissipation during cyclic compression.

Contribution

It introduces a novel experimental approach combining imaging and AI to quantify 3D rotations and sliding in granular materials, highlighting their importance in dissipation.

Findings

3D rotations are irreversible under cyclic compression.

Rotations and contact-point motion dominate bulk dynamics.

Most dissipation occurs in the bulk where grains rotate more than they translate.

Abstract

We experimentally measure a three-dimensional (3D) granular system's reversibility under cyclic compression. We image the grains using a refractive-index-matched fluid, then analyze the images using the artificial intelligence of variational autoencoders. These techniques allow us to track all the grains' translations and 3D rotations with accuracy sufficient to infer sliding and rolling displacements. Our observations reveal unique roles played by 3D rotational motions in granular flows. We find that rotations and contact-point motion dominate the dynamics in the bulk, far from the perturbation's source. Furthermore, we determine that 3D rotations are irreversible under cyclic compression. Consequently, contact-point sliding, which is dissipative, accumulates throughout the cycle. Using numerical simulations whose accuracy our experiment supports, we discover that much of the dissipation occurs in the bulk, where grains rotate more than they translate. Our observations suggest that the analysis of 3D rotations is needed for understanding granular materials' unique and powerful ability to absorb and dissipate energy.