The Impact of Particle Aspect Ratio, Pile Preparation Methods, and Container Confinement on the Resting Angle of Repose of Granular Piles

The resting angle of repose is a crucial parameter characterizing granular piles after an avalanche. We investigate how the angle of repose varies with particle aspect ratio and examine how container confinement and pile preparation methods impact the slope of piles made with aspheric particles. For granular piles formed by pouring grains into a cubic container with distant retaining walls, we observe a gradual decrease in the angle of repose from 30.9 ± 0.7 deg to 26.2 ± 0.8 deg as the particle aspect ratio shifts from oblate (1:4) to prolate (6:1), a result that does not agree with recent numerical simulations. Additionally, we tested piles formed in a narrow rectangular container, with a width equal to the length of the longest prolate particle. The results show that container confinement magnifies the dependence of angle of repose with aspect ratio, with a more pronounced increase for more oblate particles. However, if the pile is formed by avalanches induced by rotating the container around its face normal axis, the trend completely reverses, and the angle of repose increases for more prolate particles. Wall friction and particle entanglement emerge as the most probable factors driving these behaviors.