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

非技术描述本项目解决了流体混合如何影响流动中进行的过程的问题。在过去的十年中，首席研究员的研究小组对流体流动中反应前锋的行为进行了实验。这些实验验证了一个理论，该理论预测存在无形的障碍，阻止流动中反应前锋的运动。新的实验将非常相似的理论应用于一个看似非常不同的过程：流动中的微生物等自行式(“活性”)示踪剂的游动。这些实验调查是否存在阻止游泳细菌运动的无形屏障，如果是的话，这些屏障如何影响细菌的个人和群体行为。最终，这些实验可能有助于更好地理解海洋中可能破坏海洋生态系统的有害藻类的大量繁殖；新材料的“自组装”；微观流动在胚胎发育中的作用；以及种群流动对疾病传播的影响。所有这项研究都是与本科生一起完成的，他们在研究的各个方面都发挥了积极的作用，包括实验设计、仪器的建造和测试、数据收集和分析以及结果的发布和展示。对于大多数学生来说，这些是第一次真正的研究经历，对他们作为科学家的发展起到了重要作用。技术说明许多流体过程受到示踪剂与流动混合的方式的严重影响。该项目由一系列实验组成，研究流动中活性(自推进)示踪剂的混合和传输。实验中使用了两种细菌(枯草芽孢杆菌)作为活性示踪剂：(A)野生型细菌，它们在没有流动和化学梯度的情况下遵循随机奔跑和翻滚的轨迹；(B)平滑游泳的细菌，它们已经经过基因突变，丧失了改变游泳方向的能力。这些流动要么是实验室规模的，要么是涡流主导的流动，要么是足够小的微流体流，以跟踪单个细菌。实验结果由加州大学默塞德分校的凯文·米切尔(Kevin Mitchell)提出的一种理论进行了分析，该理论基于对游泳者运动起到单向障碍作用的“游泳不变流形”。这一理论类似于反应前锋的“燃烧不变流形”理论，该理论已被主要研究人员和他的学生实验验证，以预测和解释阻碍反应前锋传播的单向势垒。该研究项目涉及以下问题：(A)限制活性示踪剂在水流中运动的单向屏障的存在和拓扑结构；(B)水流中营养梯度(趋化性)对这些屏障的影响；(C)由于游泳和流体平流的结合，增强了活性示踪剂的长距离运输；以及(D)混合和运输对生物集体行为的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，认为值得支持。

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.