Maximum angle of stability of a wet granular pile

Anyone who has built a sandcastle recognizes that adding liquid to the sand grains increases the overall stability. However, measurements of the stability in wet granular materials often conflict with theory and with each other(1-7). The friction-based Mohr-Coulomb model(3,8) distinguishes between granular friction and interparticle friction, but uses the former without providing a physical mechanism. A frictionless model for the geometric stability of dry particles on the surface of a pile(2) is in excellent agreement with experiment. However, the same model applied to wet grains overestimates the stability and predicts no dependence on system size. Here we take a frictionless liquid-bridge model and perform a stability analysis within the pile. We reproduce our experimentally observed dependence of the stability angle on system size, particle size and surface tension. Furthermore, we account for past discrepancies in experimental reports by showing that sidewalls can significantly increase the stability of granular material.