Energy non-equipartition in vibrofluidized particles

The aim of the present work is to investigate the influence of the realistic model parameters for particle interactions, specifically the spring stiffness coefficient for the tangential force between particles on the energy equipartition in a vibrofluidized system. To achieve this, a three-dimensional vertically vibrated granular system consisting of spherical particles is simulated using the discrete element method (DEM) implemented in the open-source software LAMMPS. Interparticle and wall-particle interactions are determined using the linear-spring dashpot model. Simulations are performed for particles ranging from nearly perfectly smooth to nearly perfectly rough. Two different values for the ratio of the tangential to normal spring stiffness coefficient $\kappa$ ($2/7$ and $3/4$) are chosen for most of the simulations. The ratio of the translational to the rotational kinetic energy ($K$) monotonically decreases with an increase in the friction coefficient, $\mu$ for $\kappa=2/7$; however, for $\kappa = 3/4$, after an initial reduction with $\mu$, $K$ increases and plateaus at $\approx 5$, indicating the absence of equipartition of energy between the translational and rotational modes. Further simulations performed for $0.67 \le \kappa < 1$ confirm non-equipartition of energy for particles with a very high friction coefficient.