Oblique Impact of Frictionless Spheres: On the Limitations of Hard Sphere Models for Granular Dynamics

TL;DR

This paper examines the limitations of frictionless hard sphere models in granular dynamics, highlighting conditions where the assumption of instantaneous collisions may not hold, especially for viscoelastic particles.

Contribution

It introduces a universal condition to evaluate the validity of hard-sphere models for viscoelastic granular particles, addressing a key assumption in event-driven molecular dynamics.

Findings

Hard-sphere models may fail for certain material and system parameters.

A universal condition is proposed to assess the validity of the hard-sphere approximation.

Implications for the accuracy of event-driven molecular dynamics simulations.

Abstract

When granular systems are modeled by frictionless hard spheres, particle-particle collisions are considered as instantaneous events. This implies that while the velocities change according to the collision rule, the positions of the particles are the same before and after such an event. We show that depending on the material and system parameters, this assumption may fail. For the case of viscoelastic particles we present a universal condition which allows to assess whether the hard-sphere modeling and, thus, event-driven Molecular Dynamics simulations are justified.