Maximum Angle of Stability of a Wet Granular Pile

TL;DR

This paper investigates the maximum stability angle of wet granular piles, reconciling experimental observations with a geometric, frictionless model that accounts for system size, particle size, surface tension, and wall effects.

Contribution

It introduces a geometric stability model for wet granular piles that explains experimental results and the influence of sidewalls, addressing previous theoretical and experimental discrepancies.

Findings

Stability angle depends on system size, particle size, and surface tension.

Sidewalls can significantly increase pile stability.

The model aligns well with experimental data.

Abstract

Anyone who has built a sandcastle recognizes that the addition of liquid to granular materials increases their stability. However, measurements of this increased stability often conflict with theory and with each other [1-7]. A friction-based Mohr-Coulomb model has been developed [3,8]. However, it distinguishes between granular friction and inter-particle friction, and uses the former without providing a physical mechanism. Albert, {\em et al.} [2] analyzed the geometric stability of grains on a pile's surface. The frictionless model for dry particles is in excellent agreement with experiment. But, their model for wet grains overestimates stability and predicts no dependence on system size. Using the frictionless model and performing stability analysis within the pile, we reproduce the dependence of the stability angle on system size, particle size, and surface tension observed in our experiments. Additionally, we account for past discrepancies in experimental reports by showing that sidewalls can significantly increase the stability of granular material.