Load-carrying capacity of buoyant multilayer granular rafts*

Granular rafts formed at liquid-gas interfaces can support loads that would otherwise sink. This combined experimental and theoretical study investigates the load-carrying capacity of a buoyant multilayer granular raft that is deformed by heavy grains through 1) quasi-static pouring or 2) impact of a jet of particles. The inertia of the heavy particles plays a significant role in both the destabilization dynamics and load carrying properties, as quantified in experiment through the number of heavy grains (N_c) required to destabilize the raft.  Balancing the destabilizing forces with the restoring ones at the onset of collapse, a mathematical model is derived to predict N_c, as it depends upon the geometric and material properties of this multiphase system. Limiting cases recover prior literature in simplified forms. Theoretical predictions agree well with experiment over a range of conditions, thus extending our understanding of granular rafts from monolayers to multilayers.