Emergence of a crystalline phase in sheared granular matter

One-half century ago a classic experiment by G.D. Scott (Nature 188, 908, 1960) showed that pouring steel balls into a rigid container filled the volume to an upper limit volume fraction of 0.64, which is well below the volume fraction 0.74 filled by spheres in a hexagonal close-packed (HCP) or face-centered cubic (FCC) lattice. Subsequent experiments confirmed a ``random closed-packed" (RCP) volume fraction of about 0.64. However, the physics of the RCP limit has remained a mystery. We have conducted an experiment on a cubical box filled with 49400 precision glass spheres under weak shear imposed by a small slow angular oscillation of two opposite sidewalls. A phase transition occurred at a volume fraction of 0.645, from a disordered to an ordered state, consisting of crystallites of mixed FCC and HCP symmetry that coexist with the amorphous bulk. The transition is initiated by homogeneous nucleation. In the shearing process small crystallites with about ten or fewer spheres dissolve, while larger crystallites grow (doi.org/10.1103/PhysRevLett.120.055701, JSP doi.org/10.1007/s10955-018-2144-4).