Upward penetration through a granular medium

We measure the force needed to push a flat plunger upwards through a granular medium. The plunger begins flush with the base of the grains' container, and we focus upon the force necessary to initiate motion.~The data show that this break-out force increases monotonically with plunger diameter and pile height as expected.~In contrast to previous measurements of the force needed for vertical penetration from above and of the horizontal drag force, this break-out force has a strong dependence on the diameter of beads making up the pile.~The nature of this bead size dependence can be altered by using different methods to form the grain pile.~Implications for the relevant force chain network will be discussed.~Research supported by NASA grant NAG3-2384 and the NSF REU program.