Rate-dependent drag instability in granular materials

TL;DR

This paper explores the velocity-dependent conditions causing large force fluctuations in granular materials during drag tests, revealing a novel instability mechanism beyond classical stick-slip dynamics.

Contribution

It introduces a new understanding of drag force fluctuations in granular media, identifying a velocity-dependent instability mechanism distinct from traditional models.

Findings

Force fluctuations up to 60% observed at low velocities

Instability disappears at higher velocities and near the surface

Proposed an alternative physical mechanism for force fluctuations

Abstract

We investigate the conditions leading to large drag force fluctuations in granular materials. The study is based on a set of experimental drag tests, which involve pulling a plate vertically through a cohesionless granular material. In agreement with previous observations, drag force exhibits significant and sudden drops -up to 60%- when the plate is pulled out at low velocities. We further find that this instability vanishes at higher pullout velocities and near the surface. We empirically characterise the frequency and amplitude of these fluctuations and find that these properties are not consistent with a classical stick-slip dynamics. We therefore propose an alternative physical mechanism that can explain these force fluctuations.