Homogeneous Crystallization in Cyclically Sheared Frictionless Grains.

Many experiments over the past half century have shown that, for a range of protocols, granular materials compact under pressure and repeated small disturbances. A recent experiment on cyclically sheared spherical grains showed significant compaction via homogeneous crystallization (Rietz et al., 2018).

Nucleation in Sheared Granular Matter.

We present an experiment on crystallization of packings of macroscopic granular spheres. This system is often considered to be a model for thermally driven atomic or colloidal systems. Cyclically shearing a packing of frictional spheres, we observe a first order phase transition from a disordered to an ordered state.

Sound speed in water-saturated glass beads as a function of frequency and porosity.

Sound propagation in water-saturated granular sediments is known to depend on the sediment porosity, but few data in the literature address both the frequency and porosity dependency. To begin to address this deficiency, a fluidized bed technique was used to control the porosity of an artificial sediment composed of glass spheres of 265 μm diameter. Time-of-flight measurements and the Fourier phase technique were utilized to determine the sound speed for frequencies from 300 to 800 kHz and porosities from 0.

Fluid-solid transition in a hard-core system.

We prove that a system of particles in the plane, interacting only with a certain hard-core constraint, undergoes a fluid-solid phase transition.

Most stable structure for hard spheres.

The hard sphere model is known to show a liquid-solid phase transition, with the solid expected to be either face centered cubic or hexagonal close packed. The differences in free energy of the two structures are very small and various attempts have been made to determine which structure is the more stable. We contrast the different approaches and extend one.

Structure of the hard sphere solid.

We show that near densest packing the perturbations of the hexagonal close packed (hcp) structure yield higher entropy than perturbations of any other densest packing. The difference between the various structures shows up in the correlations between motions of nearest neighbors. In the hcp structure random motion of each sphere impinges slightly less on the motion of its nearest neighbors than in the other structures.