Abstract
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.
Highlights
An extremely broad and important class of phenomena in nature involves the settling and aggregation of matter under gravitation in fluid systems
We identify and isolate an unexplored mechanism, which induces particle attraction and self-assembly in stratified fluids in the absence of adhesion arising as a first-principle fluid dynamics phenomenon
We present a series of control experiments and calculations to explain the underlying mechanism responsible for the attraction
Summary
An extremely broad and important class of phenomena in nature involves the settling and aggregation of matter under gravitation in fluid systems. 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 present a series of control experiments and calculations to explain the underlying mechanism responsible for the attraction This involves using Lagrangian tracer particle dynamics to observe flows created by single bodies of different aspect ratios. To explore many-body effects, we first simulate the flow induced by two fixed, same size spheres and evaluate numerically the induced force at an array of separation distances This resulting force law is used to develop a modified Stokesian dynamics simulation (capable of simulating hundreds of spheres), which is in turn validated with the experiments involving two same size, moving spheres. Modified Stokesian dynamics results are presented for hundreds of spheres exhibiting selfassembly features which strongly resemble those observed in our many-body experiment
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