RUI: Experimental Studies of Active Mixing in Extended Fluid Systems

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Non-Technical AbstractExperiments in this project explore the effects of fluid flows on “active” agents such as swimming organisms and moving reaction fronts. Fluid flows are everywhere in nature and in technology, including in living organisms; in rivers, lakes and oceans; in the atmosphere and in stars; in numerous industrial processes; and even in “flows” of people in society. These experiments test predictions that fluid mixing can synchronize the growth of microbial colonies, similar to the rapid “blooming” of algae in the oceans; that mixing patterns in the flow can affect the formation of biofilms on surfaces; that the combination of swimming and the fluid flow can result in “chaotic” trajectories; and that flows can enhance – and sometimes inhibit -- the spreading of groups of swimming microbes and of reaction fronts. The results from these experiments could help explain how algae blooms form and how diseases spread in moving populations; an understanding of active mixing could also help in the development of “self-assembled” microstructures. All of the research is done with undergraduate students, giving them experiences that play an important role in their development as scientists.Technical AbstractThe experiments in this project study the effects of laminar flows on active impurities that consume energy, resulting in motion relative to the fluid. The active impurities are (a) self-propelled microbes, such as swimming bacteria and algae; and (b) propagating chemical reaction fronts. The flows include vortex chain and vortex array flows, hyperbolic flows in a cross-channel, time-dependent channel flows, and spatially-disordered flows. Previous experiments by the PI’s group have demonstrated that “burning invariant manifolds” and “swimming invariant manifolds” block the motion of active impurities in laminar flows. We will extend these studies to explore transport and pattern formation of active impurities over distances larger than typical flow scales. The experiments will investigate: (1) the effects of laminar mixing on the growth of microbial colonies, both with and without chemo- and phototaxis; (2) the effects of chaotic fluid mixing on biofilm formation; (3) large-scale propagation of reaction fronts and the leading edges of microbial ensembles; (4) chaotic motion of active impurities in flows where passive mixing is non-chaotic; (5) long-range transport of active impurities in laminar flows; and (6) how selection over multiple generations enhances certain swimming protocols in response to the fluid mixing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.