Slow dynamics and aging of a confined granular flow

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. The wall roughness is much lower than the typical grain size. For monodisperse assemblies this study evidences at low velocities (1<V<100μm/s) a stiffening behavior, i.e., the stress necessary to obtain a steady-state velocity increases roughly logarithmically with velocity. On the other hand, at very low driving velocity (V<1μm/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 an attempt 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 element numerical simulations to compare experimental results with 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.