Loose packings of frictional spheres

TL;DR

This study investigates how the Stokes number influences the formation of loose packings of frictional spheres during sequential deposition, revealing that kinetic energy dissipation controls the packing limit rather than static stability.

Contribution

It demonstrates that the Stokes number governs the loose packing limit in frictional spheres, linking dynamic deposition conditions to packing density.

Findings

Loose packing volume fraction depends on the Stokes number.

The Stokes number threshold corresponds to full kinetic energy dissipation.

Interparticle friction influences the random loose packing volume fraction.

Abstract

We have produced loose packings of cohesionless, frictional spheres by sequential deposition of highly-spherical, monodisperse particles through a fluid. By varying the properties of the fluid and the particles, we have identified the Stokes number (St) - rather than the buoyancy of the particles in the fluid - as the parameter controlling the approach to the loose packing limit. The loose packing limit is attained at a threshold value of St at which the kinetic energy of a particle impinging on the packing is fully dissipated by the fluid. Thus, for cohesionless particles, the dynamics of the deposition process, rather than the stability of the static packing, defines the random loose packing limit. We have made direct measurements of the interparticle friction in the fluid, and present an experimental measurement of the loose packing volume fraction, \phi_{RLP}, as a function of the friction coefficient \mu_s.