Role of friction in pattern formation in oscillated granular layers

TL;DR

This study demonstrates that friction is crucial for accurately modeling pattern formation in oscillated granular layers, affecting pattern types, onset thresholds, and shock wave behavior in simulations.

Contribution

It shows that including friction in simulations is essential to replicate experimental patterns and dynamics, unlike frictionless models.

Findings

Friction raises the pattern formation threshold by about 30%.

Only stripe patterns form in frictionless simulations, unlike experimental square and hexagonal patterns.

Friction influences shock wave characteristics and hydrodynamic field behavior.

Abstract

Particles in granular flows are often modeled as frictionless (smooth) inelastic spheres; however, there exist no frictionless grains, just as there are no elastic grains. Our molecular dynamics simulations reveal that friction is essential for realistic modeling of vertically oscillated granular layers: simulations of frictionless particles yield patterns with an onset at a container acceleration about 30% smaller than that observed in experiments and simulations with friction. More importantly, even though square and hexagonal patterns form for a wide range of the oscillation parameters in experiments and in our simulations of frictional inelastic particles, only stripe patterns form in the simulations without friction, even if the inelasticity is increased to obtain as much dissipation as in frictional particles. We also consider the effect of particle friction on the shock wave that forms each time the granular layer strikes the container. While a shock wave still forms for frictionless particles, the height and time dependence of the hydrodynamic fields differ for the cases with and without friction.