Directional Cluster Migration Driven by Escape-Rate Asymmetry in Multi-Compartment Granular Systems

TL;DR

This study uncovers how asymmetric interactions between small and large particles in vibrated granular systems cause directional cluster migration, explained by a flux model based on direct measurements.

Contribution

It introduces a minimal flux model derived from direct flux measurements that explains directional cluster migration in vibrated granular systems.

Findings

Small particle flux is enhanced by large particles.

Large particle flux is suppressed by small particles.

The flux model reproduces observed directional migration.

Abstract

Granular materials are inherently out-of-equilibrium systems due to energy dissipation through inelastic collisions and friction. When driven by mechanical agitation such as vibration, they exhibit rich collective behaviors including segregation, clustering, and spontaneous oscillations. Here, we report directional stepwise migration of particle clusters from one compartment to the next in a vertically vibrated granular system composed of small and large particles. To clarify the underlying mechanism, we directly measured how the flux of both particle species depends on the instantaneous particle populations. The measurements reveal an asymmetric interaction between particle species: the flux of small particles is enhanced by the presence of large particles, whereas that of large particles is suppressed by small particles. A minimal flux model incorporating these measured fluxes reproduces the observed directional dynamics and provides an experimentally grounded framework for collective transport in vibrated granular systems.