Influence of humidity on granular packings with moving walls

TL;DR

This study experimentally investigates how varying relative humidity affects the apparent mass, bead motion, and packing density in a granular material within a moving tube, highlighting capillary forces' role.

Contribution

It demonstrates the humidity-dependent behavior of granular packings and validates Janssen's model across different humidity levels, revealing new insights into capillary effects.

Findings

Apparent mass increases over time at high humidity levels.

Bead motion is more significant at higher humidity.

Janssen's model remains valid across humidity variations.

Abstract

A significant dependence on the relative humidity H for the apparent mass (Mapp) measured at the bottom of a granular packing inside a vertical tube in relative motion is demonstrated experimentally. While the predictions of Janssen's model are verified for all values of H investigated (25%< H <80%), Mapp increases with time towards a limiting value at high relative humidities (H>60%) but remains constant at lower ones (H=25%). The corresponding Janssen length is nearly independent of the tube velocity for H>60% but decreases markedly for H=25%. Other differences are observed on the motion of individual beads in the packing. For H=25%, they are almost motionless while the mean particle fraction of the packing remains constant; for H>60% the bead motion is much more significant and the mean particle fraction decreases. The dependence of these results on the bead diameter and their interpretation in terms of the influence of capillary forces are discussed.