Orientational Order in Amorphous Packings of Ellipsoids

We perform Monte Carlo simulations of the pouring of hard ellipsoids into a three-dimensional container. Both oblate and prolate ellipsoids of revolution are examined. Oblate ellipsoids pack so that their symmetry (semi-minor) axes are preferentially aligned with the direction of gravity, and their orientational alignment increases with increasing aspect ratio. Conversely, for prolate ellipsoids, the symmetry (semi-major) axes tend to lie within the plane perpendicular to gravity. The ellipsoids form novel packings that we call "oriented glasses". No translational order is present: the packings are amorphous, but still exhibit long-range orientational order. In addition, the density of the packings is not a monotone function of the aspect ratio, and exhibits a global maximum for slightly elongated particles.