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.
@article{arxiv.0911.4635,
title = {Flow-induced Agitations Create a Granular Fluid},
author = {Kiri Nichol and Alexey Zanin and Renaud Bastien and Elie Wandersman and Martin van Hecke},
journal= {arXiv preprint arXiv:0911.4635},
year = {2015}
}