RUI: Lagrangian studies of active mixing -- barriers, enhanced transport and collective behavior

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Non-Technical DescriptionThis project addresses the question of how fluid mixing affects processes going on in the flow. During the past decade, the principle investigator's research group has conducted experiments on the behavior of reaction fronts moving in fluid flows. Those experiments have verified a theory that predicts the existence of invisible barriers that block the motion of reaction fronts in a flow. The new experiments apply very similar theories to a seemingly very different process: the swimming of self-propelled ("active") tracers such as microscopic bacteria in a flow. The experiments investigate whether there are invisible barriers that block the motion of swimming bacteria and, if so, how these barriers affect both the individual and group behavior of the bacteria. Ultimately, these experiments may lead to a better understanding of blooms of harmful algae in the oceans that can damage marine ecosystems; "self-assembly" of novel materials; the role of microscopic flows in developing embryos; and the effects of moving populations on the spreading of a disease. All of this research is done with undergraduate students who play an active role in all aspects of the studies, including experimental design, building and testing of the apparatus, data collection and analysis, and publication and presentation of the results. These are the first real research experiences for most of these students, and play an important role in their development as scientists.Technical DescriptionMany fluid processes are heavily influenced by the manner in which tracers are mixed by flows. This project is composed of a series of experiments that study the mixing and transport of active (self-propelled) tracers in a flow. Two strains of bacteria (bacillus subtilis) are used in the experiments as the active tracers: (a) wild type bacteria, which follow a random run and tumble trajectory in the absence of a flow and chemical gradients; and (b) smooth swimming bacteria, which have been genetically mutated to eliminate their ability to change swimming directions. The flows are either laboratory scale, vortex-dominated flows or microfluidic flows on a small enough scale to track individual bacteria. The experimental results are analyzed with a theory proposed by Kevin Mitchell (University of California at Merced) that is based on "swimming invariant manifolds" that act as one-way barriers to the motion of the swimmers. This theory is similar to a theory of "burning invariant manifolds" for reaction fronts that has been verified experimentally by the principle investigator and his students to predict and explain one-way barriers that block reaction front propagation. The research project addresses the following issues: (a) the existence and topology of one-way barriers that restrict the motion of active tracers in a flow; (b) the effects of nutrient gradients (chemotaxis) in the flow on these barriers; (c) enhanced long-range transport of active tracers due to a combination of swimming and fluid advection; and (d) the effects of mixing and transport on collective behavior of the organisms.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.