Morphology-resolved stress contributions in sheared wet granular materials

TL;DR

This study uses advanced imaging to analyze how liquid morphology affects stress distribution and shear behavior in wet granular materials, revealing that simple capillary bridges dominate cohesion.

Contribution

It provides a morphology-resolved analysis linking liquid structures at the grain scale to macroscopic rheological properties without adjustable parameters.

Findings

Capillary bridges account for 85% of total capillary pressure at .05 liquid-to-solid ratio.

Shear localization causes higher-order liquid morphologies to accumulate near the shear zone boundary.

Incorporating morphology-resolved capillary pressure reproduces macroscopic friction behavior.

Abstract

Three-dimensional X-ray microtomography, coupled to rheometric measurements, enables a morphology-resolved reconstruction of capillary stresses at the grain scale in unsaturated wet granular materials. Liquid domains are automatically classified into capillary bridges, dimers, trimers, and larger clusters, and their spatial organization is tracked as a function of shear deformation and liquid content. We show that shear localization governs the redistribution of the liquid phase: capillary bridges remain uniformly distributed throughout the sample, while higher-order morphologies accumulate preferentially near the lower boundary of the shear-zone through a shear-driven coalescence mechanism. Despite this spatial localization, simple two-grain bridges generate the dominant contribution to the isotropic capillary pressure, accounting for nearly 85\% of the total at liquid-to-solid volume ratio $\epsilon = 0.05$, whereas more complex liquid clusters contribute only weakly to the overall cohesion. Incorporating the morphology-resolved capillary pressure into an effective-stress framework qualitatively reproduces the macroscopic friction coefficient across the full range of investigated liquid contents, without adjustable parameters. These results establish a predictive micro--macro link between liquid morphology and the rheology of wet granular materials.