Critical angle of wet sandpiles

Area of Science:

• Physics of granular materials
• Surface science
• Colloid science

Background:

• The maximum stable angle of a sandpile (angle of repose) is a key property of granular materials.
• Adding liquids can induce cohesion between grains, altering bulk properties.
• Understanding the role of liquid properties and grain surface characteristics is crucial.

Purpose of the Study:

• To investigate how the addition of liquid affects the maximum stable angle of a sandpile.
• To determine the relationship between liquid volume fraction, surface tension, and sandpile stability.
• To explore the micro-scale mechanisms of liquid-induced cohesion in granular systems.

Main Methods:

• Measurement of the maximum stable angle, theta(c), of sandpiles with varying liquid volume fractions, phi.
• Systematic variation of liquid properties, including surface tension.
• Microscopic analysis (electron and fluorescence microscopy) of grain surfaces.

Main Results:

• The angle of repose, theta(c), increases with liquid volume fraction, phi.
• tan theta(c) showed no clear scaling with surface tension at low phi, but did at higher phi.
• Microscopy revealed liquid menisci formation at grain asperities and contact points.

Conclusions:

• Liquid-induced cohesion in sandpiles is a multi-stage process involving surface asperities and intergrain contacts.
• A surface roughness theory accurately describes the cohesive limit of sandpile stability.
• The findings support a model where liquid bridges form initially at surface irregularities before contributing to bulk cohesion.