Pushing the glass transition towards random close packing using self-propelled hard spheres

TL;DR

This paper demonstrates that self-propulsion in hard spheres accelerates dynamics and shifts the glass transition to higher densities, enabling exploration of packings near the random close packing limit.

Contribution

It introduces a novel approach using activity to push the glass transition towards higher packing fractions in hard sphere systems.

Findings

Relaxation dynamics are significantly faster with increased activity.

The glass transition shifts to higher packing fractions as activity increases.

Potential to study dense packings near the random close packing limit.

Abstract

Although the concept of random close packing with an almost universal packing fraction of ~ 0.64 for hard spheres was introduced more than half a century ago, there are still ongoing debates. The main difficulty in searching the densest packing is that states with packing fractions beyond the glass transition at ~ 0.58 are inherently non-equilibrium systems, where the dynamics slows down with a structural relaxation time diverging with density; hence, the random close packing is inaccessible. Here we perform simulations of self-propelled hard spheres, and we find that with increasing activity the relaxation dynamics can be sped up by orders of magnitude. The glass transition shifts to higher packing fractions upon increasing the activity, allowing the study of sphere packings with fluid-like dynamics at packing fractions close to random close packing. Our study opens new possibilities of investigating dense packings and the glass transition in systems of hard particles.