Cubatic structural transformation of granular cylinders packing

Packing structures of granular cylinders with aspect ratio close to one have been reconstructed with the help of magnetic resonance imaging techniques. By controlling the container boundary condition and preparation protocols, a structural transformation from a disordered liquid-like state to an orientationally-ordered state with cubatic symmetry at high packing fraction is observed. This ordering process is accompanied with the formation of more faceted contacts, which lower the elastic energy between jammed granular particles to drive the transformation. With the help of Edwards’ volume ensemble theory, this granular structural transformation is explained with a phenomenological thermodynamic model and a self-consistent mean-field statistical mechanical model. Both models predict a sharp but continuous change of order parameter when the effective granular temperature is lowered. The intrinsic difference and connection between this granular structural transformation and the entropy-driven phase transition of conventional thermal hard-particle systems are discussed.