Anomalous density dependence of static friction in sand

TL;DR

This study experimentally investigates how static friction in dry sand varies with grain packing density, revealing exponential growth and periodicity effects linked to grain bridges and arches.

Contribution

It introduces the observation of exponential friction increase and periodicity in static friction related to grain structures, highlighting the role of bridges and arches.

Findings

Static friction force increases exponentially with grain density.

Friction exhibits a novel periodicity during rod extraction.

Grain bridges and arches are central to granular packing behavior.

Abstract

We measured experimentally the static friction force $F_s$ on the surface of a glass rod immersed in dry sand. We observed that $F_s$ 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 a novel periodicity of $F_s$ 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.