Anomalous density dependence of static friction in sand

We measured experimentally the static friction force ${\mathit{F}}_{\mathit{s}}$ on the surface of a glass rod immersed in dry sand. We observed that ${\mathit{F}}_{\mathit{s}}$ is extremely sensitive to the closeness of packing of grains. A linear increase of the grain density yields to an exponentially increasing friction force. We also report on the periodicity of ${\mathit{F}}_{\mathit{s}}$ during gradual pulling out of the rod. Our observations demonstrate the central role of grain bridges and arches in the macroscopic properties of granular packings. © 1996 The American Physical Society.