Antigraivty static frictional force on the walls of confined granular columns*

We measure the static frictional force exerted by a granular material on the side wall of a vertical tube as a function of the filling height. We consider a cylindrical tube of diameter D = 5.2 cm filled to heights up to 6D , which is smaller than the Janssen screening length. We use either glass beads (4 mm monodisperse spherical beads) or sand (0.5 mm polydisperse quasi-spherical particles). The grains are added in discrete steps in such a way that the top of the column remains flat, with a fixed mass per step and a fixed waiting time between steps. For sand, the vertical force on the tube remains zero up to a height of approximately 2D, then increases linearly in the downward direction. Surprisingly, for glass beads the force on the container rises in the upward direction up to a turning height h ~ 2D, above which it declines linearly. The variation in frictional force during a waiting period between steps becomes large for heights near h. Discrete element method simulations of frictional spheres show qualitative agreement with experiments.