Slow dynamics and aging of a confined granular flow

TL;DR

This study investigates the slow flow, aging, and stick-slip behavior of confined granular materials, revealing velocity-dependent stiffening, hysteresis, humidity effects, and new dynamical regimes through experiments, modeling, and simulations.

Contribution

It provides new insights into the rheology and transition phenomena of granular flows under confinement, combining experimental, theoretical, and numerical approaches.

Findings

Logarithmic increase of stress with velocity in steady flow

Discontinuous, hysteretic transition to stick-slip at very low velocities

Humidity significantly influences granular flow behavior

Abstract

We present experimental results on slow flow properties of a granular assembly confined in a vertical column and driven upwards at a constant velocity V. For monodisperse assemblies this study evidences at low velocities ($1<V<100 \mu m/s$) a stiffening behaviour i.e. the stress necessary to obtain a steady sate velocity increases roughly logarithmically with velocity. On the other hand, at very low driving velocity ($V<1 \mu m/s$), we evidence a discontinuous and hysteretic transition to a stick-slip regime characterized by a strong divergence of the maximal blockage force when the velocity goes to zero. We show that all this phenomenology is strongly influenced by surrounding humidity. We also present a tentative to establish a link between the granular rheology and the solid friction forces between the wall and the grains. We base our discussions on a simple theoretical model and independent grain/wall tribology measurements. We also use finite elements numerical simulations to confront experimental results to isotropic elasticity. A second system made of polydisperse assemblies of glass beads is investigated. We emphasize the onset of a new dynamical behavior, i.e. the large distribution of blockage forces evidenced in the stick-slip regime.