Force dynamics in weakly vibrated granular packings.

Variations in the oscillatory force F(b) on the bottom of a rigid, grain filled column, reveal rich granular dynamics when the column is vertically vibrated with an acceleration amplitude significantly less than the gravitational acceleration at the earth's surface. Large changes in F(b) occur even though the maximum relative motion of the container bottom with respect to the wall is less than 2 nm. For previously unshaken packings or high frequencies, F(b)'s dynamics are dominated by grain motion. For moderate driving conditions in already shaken samples, grain motion is virtually absent, but F(b) nevertheless exhibits strongly nonlinear and hysteretic behavior, evidencing a granular regime dominated by nontrivial force-network dynamics.