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Dr. Rich McLaughlin, UNC – Self-assembly and particle aggregation in stratified fluids
September 28 @ 4:00 pm - 5:00 pm
Title: Self-assembly and particle aggregation in stratified fluids
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 mechanism by which particles suspended within stratification self-assemble and form large scale aggregates without adhesion. We will review how this behavior was discovered in our fluids lab, and revisit particle settling through stratified water and the known mathematical results for settling in stratification. The aggregation 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 on the order of hundreds of piconewtons. 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, Stokesian simulations which capture observed features of the experimental self-assembly. Mathematically exact solutions are presented in the low Reynolds and low Peclet limits for a host of different boundary conditions. We lastly discuss mixing induced by such diffusion induced flows.
This is joint work with Roberto Camassa, Lingyung Ding, Dan Harris, Robert Hunt, and Zeliha Kilic.