Surface-energy-driven clustering and flotation instability in orbitally shaken granular beds

A series of experiments and simulations were conducted on a bed of air-immersed glass beads subjected to horizontal orbital vibration. When the peak acceleration relative to gravity exceeds a critical threshold Γc, the bed develops Faraday-type tilting and air-driven convection, which is analogous to the behavior observed under vertical vibrations. However, at substantially higher accelerations, a qualitatively different regime emerges: the beads flow downward along the container walls and recirculate upward through the interior, while solid particles in the core region migrate away from the strongly fluctuating boundary layers and form denser clusters. These phase-separated aggregates are interpreted as macroscopic manifestations of granular contact-line pinning and surface-energy minimization in regions of low granular temperature. Three-dimensional numerical simulations based on non-linear contact mechanics and coupling to the interstitial air reproduced the observed non-Faraday tilts and resolved the internal flow, bubbling, and particle clustering dynamics. Together, the experiments and simulations demonstrate that surface-energy-driven contact-line pinning can promote cluster formation not only in weakly agitated beds but also in highly excited, orbitally shaken granular systems, providing a mechanics-based explanation for phase-separated clustering in dense gas–granular flows.