It is an enduring puzzle why in a boxful of ball bearings, even when compacted by many taps, the particles never pack denser than ~ 64% by volume. This is much less dense than Kepler's stacking of triangular layers, known to every grocer as the optimal packing of oranges (density ~74%). Yet despite their sub-optimal density and lack of periodic order, jammed packings are rigid and resist shear. Replacing spheres with less symmetric objects, such as rods or ellipsoids, introduces new degrees of freedom that alter the packing structure and create new modes of response. In this paper we describe the striking effects displayed by the jammed packing of extended, flexibly-connected granular chains. Although the density and coordination number decrease dramatically with increasing chain length, the packings remain rigid.

Using X-ray tomography to examine the chain conformations, we find long floppy chains effectively partition into nearly-rigid segments that then jam into a rigid structure. Just as the packing of spheres is a basic structural model for simple liquids and glasses, we find the packing of chains parallels the glass transition of molecular polymers in its response to varying chain length, topology, and stiffness.

Read more about this in a 2010 Argonne APS Science Highlight (link).

1. •Ling-Nan Zou, Xiang Cheng, Mark L. Rivers, Heinrich M. Jaeger, and Sidney R. Nagel, “The Packing of Granular Polymer Chains,” Science 326, 408 (2009). link to article