Flow-induced Agitations Create a Granular Fluid

TL;DR

This study demonstrates that localized stirring in a granular medium induces a fluid-like state in distant regions, revealing non-local rheological behavior where agitation propagates and influences the material's response.

Contribution

It provides experimental evidence that localized flow can fluidize granular materials remotely, highlighting non-local effects in granular rheology.

Findings

Distant regions behave like a liquid with sinking and floating probes

Drag forces scale linearly with velocity in quiescent zones

Agitations propagate beyond the stirred region, affecting rheology

Abstract

We fluidize a granular medium through localized stirring and probe the mechanical response of quiescent regions far away from the main flow. In these regions the material behaves like a liquid: high-density probes sink, low-density probes float at the depth given by Archimedes' law, and drag forces on moving probes scale linearly with the velocity. The fluid-like character of the material is set by agitations generated in the stirred region, suggesting a non-local rheology: the relation between applied stress and observed strain rate in one location depends on the strain rate in another location.