Mechanical properties of granular media, including snow, investigated by a low-frequency forced torsion pendulum

The oscillating probe of a low-frequency forced torsion pendulum is immersed into various granular media, such as natural sand, glass beads, and granular snow. A first layer of particles is in general solidly bound to the probe surface. The principle of operation and a rheological model are presented. The measured dynamic moduli systematically show a peak of the loss factor and a step in the absolute modulus. The effect of moisture-induced aging in glass beads of small size and the effect of sintering of ice grains in snow are investigated. The response of the pendulum is determined by the long-range statistical properties of force chains opposing the rotation of the pendulum, and by the tribological processes that take place at the grain contacts.