Adhesive Loose Packings of Small Particles

TL;DR

This paper investigates how adhesion affects the packing density and structure of small spherical particles, revealing a universal regime describable by a statistical mechanical model and identifying key packing points.

Contribution

It introduces a dimensionless adhesion parameter and a coarse-grained ensemble approach to describe adhesive packings, extending the understanding of jammed matter.

Findings

Universal adhesive packing regime identified for $Ad>1$

Equation of state for adhesive packings derived

Maximal loose packing at $Z=2$, $\,\phi=1/8$

Abstract

We explore adhesive loose packings of dry small spherical particles of micrometer size using 3D discrete-element simulations with adhesive contact mechanics. A dimensionless adhesion parameter ($Ad$) successfully combines the effects of particle velocities, sizes and the work of adhesion, identifying a universal regime of adhesive packings for $Ad>1$. The structural properties of the packings in this regime are well described by an ensemble approach based on a coarse-grained volume function that includes correlations between bulk and contact spheres. Our theoretical and numerical results predict: (i) An equation of state for adhesive loose packings that appears as a continuation from the frictionless random close packing (RCP) point in the jamming phase diagram; (ii) The existence of a maximal loose packing point at the coordination number $Z=2$ and packing fraction $\phi=1/2^{3}$. Our results highlight that adhesion leads to a universal packing regime at packing fractions much smaller than the random loose packing, which can be described within a statistical mechanical framework. We present a general phase diagram of jammed matter comprising frictionless, frictional, adhesive as well as non-spherical particles, providing a classification of packings in terms of their continuation from the spherical frictionless RCP.