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Destabilizing a Buoyant Multilayer Granular Raft by Heavy Grains: The Role of Inertia
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This physics study investigates how floating layers of particles on a liquid surface — a phenomenon relevant to microplastic "rafts" on the ocean — become destabilized when heavier objects fall into them. Through lab experiments and mathematical modeling, the researchers identified how the size and speed of impacting particles determine when the floating layer collapses, with fast-moving jets destabilizing the raft much more easily than slow loading. While primarily a fluid dynamics study, the findings contribute to understanding how microplastics accumulate and are redistributed on the ocean surface.
Buoyant multilayer granular rafts are seen in oceanography across many scales, from microplastic pollutants on the ocean surface to ice mélange in the arctic region, and are capable of supporting loads that would otherwise sink. This study investigates the load-carrying capacity of a buoyant multilayer granular raft subjected to dynamic deformation by heavy grains. Experiments are performed to quantify the critical number of heavy particles Nc required to destabilize the raft, as it depends upon the particle and liquid properties and the inertia of the heavy particles. Two experimental loading protocols are utilized, (1) quasi-static and (2) inertial jet, which are distinguished according to the inertia or lack thereof and give rise to distinct destabilization morphologies. Destabilization occurs when the width of the particle cluster reaches a maximum, and this coincides with the turning point of a preferred bifurcation diagram. This critical width can be more than two times larger for the quasi-static case than for the inertial jet case. A mathematical model is developed to predict Nc for a buoyant multilayer raft, which compares well with our experimental observations and those from prior literature over 7 orders of magnitude.
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