The current understanding of aggregation of such trapped matter involves collisions (owing to Brownian motion, shear, and differential settling) and adhesion 8, 9. Numerous mechanisms have been described by which particles may aggregate in such environments. These fluid systems all have stable density stratification, which is known to trap particulates 1, 4, 5, 6, 7 through upper lightweight fluid coating the sinking particles, thus providing transient buoyancy. Some examples include: sedimenting “marine snow” particles in lakes and oceans (central to carbon sequestration) 1, dense microplastics in the oceans (which impact ocean ecology and the food chain 2), and even “iron snow” on Mercury 3 (conjectured as its magnetic field source). Particle sedimentation and aggregation in stratified fluids is observed throughout natural systems. Numerical force calculations with two spheres are used to build many-body simulations which capture observed features of self-assembly. Control experiments isolate the individual dynamics, which are quantitatively predicted by simulations. 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. We show that these flows yield attractive horizontal forces between particles at the same heights. This phenomenon arises through a complex interplay involving solute diffusion, impermeable boundaries, and aggregate geometry, which produces toroidal flows. 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. An extremely broad and important class of phenomena in nature involves the settling and aggregation of matter under gravitation in fluid systems.
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