Disordered spherical bead packs are anisotropic

¹  Theoretische Physik, Fried.-Alex.-Universität Erlangen-Nürnberg - Staudtstr. 7, 91058 Erlangen, Germany, EU
²  Center for Nonlinear Dynamics and Department of Physics, University of Texas - Austin, TX 78712, USA
³  CSIRO Mathematics, Informatics and Statistics - Private Bag 33, Clayton South, Victoria 3168, Australia
⁴  Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University Canberra ACT 0200, Australia
⁵  Foams and Complex Systems, School of Physics, Trinity College Dublin - Dublin, Ireland, EU
⁶  School of Physical Sciences, University of Kent, Canterbury - Kent, CT2 7NH, UK, EU

Corresponding author: Gerd.Schroeder-Turk@physik.uni-erlangen.de

Abstract

Investigating how tightly objects pack space is a long-standing problem, with relevance for many disciplines from discrete mathematics to the theory of glasses. Here we report on the fundamental yet so far overlooked geometric property that disordered mono-disperse spherical bead packs have significant local structural anisotropy manifest in the shape of the free space associated with each bead. Jammed disordered packings from several types of experiments and simulations reveal very similar values of the cell anisotropy, showing a linear decrease with packing fraction. Strong deviations from this trend are observed for unjammed configurations and for partially crystalline packings above 64%. These findings suggest an inherent geometrical reason why, in disordered packings, anisotropic shapes can fill space more efficiently than spheres, and have implications for packing effects in non-spherical liquid crystals, foams and structural glasses.