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A first-principle mechanism for particulate aggregation and self-assembly in stratified fluids
Summary
Researchers discovered and mathematically modeled a surprising phenomenon where particles suspended in layered (stratified) fluids spontaneously attract each other and self-organize into disc-like aggregates without any sticky coating — a fundamental finding that helps explain how particles like microplastics and marine debris may clump together in stratified ocean layers.
An extremely broad and important class of phenomena in nature involves the settling and aggregation of matter under gravitation in fluid systems. Here, we observe and model mathematically an unexpected fundamental mechanism by which particles suspended within stratification may self-assemble and form large aggregates without adhesion. This phenomenon arises through a complex interplay involving solute diffusion, impermeable boundaries, and aggregate geometry, which produces toroidal flows. We show that these flows yield attractive horizontal forces between particles at the same heights. We observe that many particles demonstrate a collective motion revealing a system which appears to solve jigsaw-like puzzles on its way to organizing into a large-scale disc-like shape, with the effective force increasing as the collective disc radius grows. Control experiments isolate the individual dynamics, which are quantitatively predicted by simulations. Numerical force calculations with two spheres are used to build many-body simulations which capture observed features of self-assembly.
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