Orientational order in random packings of ellipses

By means of Monte Carlo simulations, we examine the behavior of random packings of hard ellipses formed by pouring into a two-dimensional container. The particles pack so that their semimajor axes are preferentially aligned with the horizontal, and their orientational alignment increases with increasing aspect ratio. This self-organizational effect is accompanied by a corresponding reduction in the translational order of the centers of masses of the particles. For sufficiently elongated particles no translational order is present: The packings are amorphous, but still exhibit long-range orientational order. We call this state a ``nematic glass.'' We find that the orientational order also increases as the rate of deposition is reduced. The orientational order decreases if the packings are shaken. The behavior of the packings can be explained heuristically as being the result of competition between two local-potential-energy-minimization processes. The relevance of our results to materials science and petroleum engineering is discussed.