Weak Agitations, Zero Yield Stress and Smooth Flows in Granular Media

A novel state of granular matter, a so-called stationary granular fluid, which combines a solid-like appearance with a fluid-like response to external stresses has recently been created in our lab. The trick is to locally shear and stir a layer of sand and probe its mechanical response in far-away regions where the material remains stationary. In contrast to its solid-like appearance, we observed that an intruder placed into these far-away zones sinks into the sand. Our present data suggests that the yielding stress is zero in these stationary zones, just like in a simple liquid -- in complete disagreement with the classical view of granular media where (stationary or flowing) appearance and yield strength are directly linked.

We believe that this behavior is caused by tiny agitations of the grains, which lead to large fluctuations of the contact forces --- deformations on the order of nanometers are sufficient to cause order one changes in contact force for millimetric glass beads. Consequences for the theory of slow grain flows and preliminary experiments in which the propagation of these agitations are manipulated will be presented.