Progressive friction mobilization and enhanced Janssen's screening in confined granular rafts

TL;DR

This paper investigates how friction mobilizes progressively in confined granular rafts under compression, revealing that internal stress screening develops slowly and can surpass classical Janssen's predictions due to localized compaction fronts.

Contribution

It introduces a detailed experimental analysis of stress development in granular rafts and explains enhanced screening phenomena with a new theoretical model.

Findings

Surface stress screening builds up slowly during compression.

Large aspect ratio rafts exhibit stronger screening than Janssen's model predicts.

Localized compaction fronts explain the enhanced screening effect.

Abstract

Confined two-dimensional assemblies of floating particles, known as granular rafts, are prone to develop a highly nonlinear response under compression. Here we investigate the transition to the friction-dominated jammed state and map the gradual development of the internal stress profile with flexible pressure sensors distributed along the raft surface. Surprisingly, we observe that the surface stress screening builds up much more slowly than previously thought and that the typical screening distance later dramatically decreases. We explain this behavior in terms of progressive friction mobilization, where the full amplitude of the frictional forces is only reached after a macroscopic local displacement. At further stages of compression, rafts of large length-to-width aspect ratio experience much stronger screenings than the full mobilization limit described by the Janssen's model. We solve this paradox using a simple mathematical analysis and show that such enhanced screening can be attributed to a localized compaction front, essentially shielding the far field from compressive stresses.