## Pushing the glass transition towards random close packing using self-propelled hard spheres    [PDF]

Ran Ni, Martien A. Cohen Stuart, Marjolein Dijkstra
Although the concept of random close packing (RCP) with an almost universal packing fraction value of $\phi_RCP \simeq 0.64$ for hard spheres was already introduced more than half a century ago [1], it is still not settled whether the RCP is characterized by a single density value or by a finite range of packing fractions [2]. The main difficulty in the search of the densest packing is that states with packing fractions beyond the glass transition at $\phi_g \approx 0.58$ are inherently non-equilibrium systems for which the dynamics slows down with a structural relaxation time that diverges with density; hence, the RCP is inaccessible in experiments as well as in numerical simulations. Here, we study using event-driven Brownian dynamics simulations systems of self-propelled motorized hard spheres, and we find that with increasing activity the relaxation dynamics in hard-sphere systems can be sped up by orders of magnitude. Particles that are trapped in a cage formed by their neighbours can escape due to the activated dynamics of the self-propulsions. We find that the glass transition shifts to higher packing fractions upon increasing the activity, which allows the study of sphere packings at packing fractions close to RCP. Our study provides new insights in dense packings of hard spheres, and opens new possibilities to investigate RCP as well as the glass transition in systems of arbitrarily shaped particles.
View original: http://arxiv.org/abs/1306.3605