Transport and Dynamics of Swimming Microorganisms in Time-Periodic Flows

非技术摘要：本提案的主要目标是了解流动中能动（即游泳）微生物的动力学和行为。许多微生物在存在流体流动的环境中生活和发挥作用。例如，低地河流和海洋中的藻类、肠道中的细菌、海洋中的浮游植物以及人类生殖道中的精子细胞。在这里，PI感兴趣的是微生物在流动中的运输和混合，以深入了解许多知之甚少的现象，其中一些在上面提到。从技术的角度来看，运动性和流动相互作用在包括用于疫苗食品生产的发酵过程、污水处理厂和生物燃料生产的应用中非常感兴趣。这些过程将大大受益于更好地理解流动和运动之间的非平凡耦合。在这里，PI提出了一个系统的实验研究的影响（i）流动的游泳微生物的运输混合性能和（ii）施加的二维时间周期流的主动应力。研究小组由一名正在接受流体动力学、生物物理学、实验统计方法和非线性动力学培训的研究生组成。研究团队还包括本科生，他们由PI和研究生监督。从这项调查中获得的基本知识可以是有用的，在新的模型的开发的运输和混合的活性物质和强制活性流。 技术摘要：这项建议的主要目标是发展的运输，混合和流动的游泳微生物的动力学与复杂的时空结构的基本理解。这些过程的实验研究在良好的控制流在电磁驱动的薄流体层放置在一个阵列的磁铁。水平穿过流体层的时间周期性电流导致洛伦兹力，该洛伦兹力驱动流体中的（时间周期性）流动。空间和时间分辨的速度场得到使用粒子跟踪方法和微分，以获得流拉伸场或拉格朗日结构。拉伸场与初始附近点的发散率密切相关，在混沌流中，发散率平均随时间（t）呈指数变化。 这些拉伸领域已被用来表征混合动力学，预测混合速率，和被动杂质和颗粒的运输，并适用于研究流动下的活性物质（即游泳微生物）。实验计算的拉伸场有助于理解拉格朗日动力学，运输和自推进微生物的混合，如细菌V. cholerea和pH2C。莱因哈德氏菌从拟议的工作中获得的知识可能是有用的成功设计的可控水下自主车辆（微型游泳机器人），预防与饮用水有关的水传播疾病的爆发，并制定准确的模型，在海洋中的扩散物质。使用这样的方法，PI希望解决许多悬而未决的问题，如：（一）什么是主要的流动参数管理的运输和混合的游泳微生物的时间周期性流？(ii)游泳悬挂的动力学如何受到水流的影响？“细菌超流性”会导致运输增强吗？(iii)微生物是否与高拉伸和不稳定歧管区域对齐？(iv)微生物的游动行为是促进还是阻碍了混合？如何拉或推影响流动的有限时间李雅普诺夫指数？

Nontechnical Abstract: The main goal of this proposal is to understand the dynamics and behavior of motile (i.e. swimming) microorganisms in flows. Many microorganisms live and function in environments in which fluid flow is present. Examples include algae in lowland rivers and ocean, bacteria in the gut and intestines, phytoplankton in oceans, and sperm cell in human reproductive tracts. Here, the PI is interested in the transport and mixing of microorganisms in flows in order to gain insight into many poorly understood phenomena, some of which are mentioned above. From a technological point of view, motility and flow interactions are of much interest in applications that include fermentation processes for vaccine & food production, sewage treatment plants, and production of biofuels. These processes stand to greatly benefit from a better understanding of the nontrivial coupling between flow and motility. Here, the PI proposes a systematic experimental investigation on the effects of (i) flow on the transport & mixing properties of swimming microorganisms and (ii) of active stresses on the imposed 2D time-periodic flows. The research team is composed of a graduate student who is receiving training in fluid dynamics, biophysics, experimental & statistical methods, and nonlinear dynamics. The research team also includes undergraduate students, who are supervised by the PI and the graduate student. The fundamental knowledge obtained from this investigation can be useful in the development of new models for the transport and mixing of active matter and of forced active flows. Technical Abstract: The main goal of this proposal is to develop fundamental understanding on the transport, mixing, and dynamics of swimming microorganisms in flows with complex spatiotemporal structures. These processes are experimentally investigated in well-controlled flows in an electromagnetically driven thin fluid layer placed atop an array of magnets. A time-periodic current that travels horizontally through the fluid layer results in Lorenz forces that drive a (time-periodic) flow in the fluid. Spatially- and time-resolved velocity fields are obtained using particle tracking methods and differentiated to obtain the flow stretching fields or Lagrangian structures. Stretching fields are intimately related to the rate of divergence of initially nearby-points, which in chaotic flows is exponential in time (t) on the average. These stretching fields have been used to characterize the mixing dynamics, predict mixing rates, and the transport of passive impurities and particles, and are applied to study active matter (i.e. swimming microorganisms) under flow. Experimentally computed stretching fields are instrumental in understanding the Lagrangian dynamics, transport, and mixing of self-propelled microorganisms such as the bacterium V. cholerea and the alga C. reinhardtii. The knowledge obtained from the proposed work can be potentially useful for the successful design of controllable underwater autonomous vehicles (micro-swimming robots), the prevention of waterborne disease outbreaks associated with drinking water, and development of accurate models for the dispersion of planktonic matter in oceans. Using such methods, the PI hopes to address many outstanding questions such as: (i) What are the main flow parameters governing the transport and mixing of swimming microorganisms in time-periodic flows? (ii) How are the dynamics of the swimming suspension affected by flow? Does 'bacterial superfluidity' leads to enhanced transport? (iii) Do microorganisms align with regions of high stretching and unstable manifolds? (iv) Is mixing enhanced or hindered by the microorganisms' swimming action? How pullers or pushers affect the flows finite time Lyapunov exponent?