Anisotropic energy distribution in three-dimensional vibrofluidized granular systems

Reticulate collisional structure in boundary-driven granular gases

We report a peculiar head-on collision network between two vibrating boundaries in experiments performed during a parabolic flight and in a laboratory using horizontal vibration. This structure is a new ordering, which is due to an orientation correlation between the relative position and velocity of any particle pair. It weakens the collision frequency and produces a long-range boundary effect. Moreover, we find the molecular chaos assumption is violated in a larger portion of the phase space. Using an anisotropic distribution model, we modify angular integration results and compare them to the results of the kinetic theory.

Boltzmann statistics in a three-dimensional vibrofluidized granular bed: idealizing the experimental system

The importance of the method of energy injection into a vertically vibrated granular bed was investigated using positron emission particle tracking. A comparison was made between two experimental systems. The first was driven by a flat base, the second by a base comprising a monolayer of spheres, each free to move in three dimensions but constrained to the bottom of the system, effectively randomizing energy transfer into the bed. The latter system exhibited more Gaussian velocity distributions, a more isotropic temperature, and a better approximation of molecular chaos than the former, behavior more akin to the idealized situation of white noise forcing than is generally observed in an experimental system.

Shear-induced crystallization in jammed systems

Simulations are used to address the effects of oscillating shear strain on jammed systems composed of spherical particles. The simulations show that shear oscillations with amplitudes of more than a few percent lead to substantial crystallization of the system. To ensure that the conclusions are independent of the simulation methodology, a range of simulations are carried out that use both molecular dynamics and athermal dynamics methods, soft and hard potentials, potentials with and without attractive forces, and systems with and without surrounding walls. The extent of crystallization is monitored primarily by the Q(6) order parameter, but also in some simulations by the potential energy and the radial distribution function, and by direct visual inspection. A mechanism is proposed for shear-induced crystallization of jammed systems, based on fold catastrophes of the free-energy landscape.