Effect of friction on the angle of repose of elongated particles

Abstract

The angle of repose ($AOR$) is a key parameter in powder engineering, primarily influenced by the pouring process, the inter-particle constitutive laws and particle shape. Therefore, the $AOR$ of a bulk material is intuitively linked to its macromechanical internal friction angle. Previous studies indicate that more irregular particle shapes, such as angular or elongated particles, increase both shear strength and $AOR$. However, in granular materials with highly irregular shapes and low interparticle friction (or frictionless conditions), this trend reverses for the shear resistance. Despite these results, studies on the $AOR$ of such materials are rare and the physical mechanisms of this behavior are unclear. This study investigates the $AOR$ of dry, cohesionless granular piles composed of elongated particles using 3D DEM simulations. To represent a wide range of conditions specific to various industrial processes — including pharmaceuticals, food engineering, geotechnics, and mining — we extensively vary particle characteristics, focusing on particle elongation and interparticle friction. The particles, modeled as rounded-cap cylinders, have aspect ratios (length/diameter) ranging from 1 (spheres) to 4. For high interparticle friction ($\mu >0.2$), the $AOR$ increases systematically with elongation. However, at low friction, a critical aspect ratio of 1.5 emerges, beyond which the tendency changes and the $AOR$ decreases. We show that this counterintuitive behavior is related to the solid fraction, which depends on particle shape, and can be traced to purely geometrical characteristics such as coordination number and statistical particle orientation. Non-intuitively, the elongation of the particles does not influence the force distributions within the piles.