Abstract

Granular mechanics codes use macroscopic laws to describe the damping of rolling and twisting motion in granular ensembles. We employ molecular dynamics simulation of amorphous Lennard–Jones grains to explore the applicability of these laws for nm-sized particles. We find the adhesive force to be linear in the intergrain attraction, as in the macroscopic theory. However, the damping torque of rolling motion is strongly superlinear in the intergrain attraction. This is caused by the strong increase of the ‘lever arm’ responsible for the damping torque—characterizing the asymmetry of the adhesive neck during rolling motion—with the surface energy of the grains. Also the damping torque of twisting motion follows the macroscopic theory based on sliding friction, which predicts the torque to increase whit the cube of the contact radius; here the dynamic increase of the contact radius with angular velocity is taken into account.

Highlights

  • Granular mechanics codes use macroscopic laws to describe the damping of rolling and twisting motion in granular ensembles

  • The atomistic nature of nanosized particle contacts is of relevance in several areas of science and technology, including tribology and cluster deposition ­technology[1]

  • There may be several ways to calculate fadh in molecular dynamics; we chose an approach that is in the spirit of our determination of the damping torques

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Summary

Introduction

Granular mechanics codes use macroscopic laws to describe the damping of rolling and twisting motion in granular ensembles. Particle contacts play a fundamental role in all aspects of granular ­mechanics[9] This field uses modeling and simulation to describe the dynamics of granular materials and requires rules to model the interaction of grains. Dominik and T­ ielens[13] provide an overview over these elementary mechanical contact forces and torques, as they are used in several granular mechanics models and collision c­ odes[10,14,15,16,17] and for the analysis of collision experiments between spherical g­ rains[18,19,20,21,22] These models are primarily obtained from macroscopic contact mechanics based on continuum equations. As dissipative processes—damping torques in the case of rotation—enter these characteristic energies, a critical evaluation of the atomistic nature of these processes is important for these collision parameters

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