Transport and Dynamics of Swimming Microorganisms in Time-Periodic Flows

摘要：本提案的主要目标是了解流动（即游泳）微生物的动力学和行为。许多微生物在有流体流动的环境中生存和发挥作用。例子包括低地河流和海洋中的藻类、肠道中的细菌、海洋中的浮游植物和人类生殖道中的精子细胞。在这里，PI对微生物在流动中的运输和混合感兴趣，以便深入了解许多鲜为人知的现象，其中一些如上所述。从技术的角度来看，运动和流动相互作用在包括疫苗和食品生产的发酵过程、污水处理厂和生物燃料生产等应用中非常有趣。这些过程将从更好地理解流动和运动之间的非平凡耦合中受益匪浅。在这里，PI提出了一个系统的实验研究(i)流动对游泳微生物的运输和混合特性的影响以及（ii）对施加的二维时间周期流动的主动应力的影响。研究团队由一名研究生组成，他正在接受流体动力学、生物物理学、实验与统计方法和非线性动力学方面的训练。研究团队还包括本科生，由PI和研究生指导。从这项研究中获得的基本知识可以用于发展新的模式，为输送和混合的活动物质和强迫活动流。技术摘要：本研究的主要目的是对复杂时空结构流中游动微生物的迁移、混合和动力学进行基本认识。这些过程在放置在磁体阵列上方的电磁驱动薄流体层中进行了实验研究。水平流过流体层的时间周期电流产生驱动流体（时间周期）流动的洛伦兹力。利用粒子跟踪方法得到空间分辨和时间分辨的速度场，并对速度场进行微分，得到流动拉伸场或拉格朗日结构。拉伸场与初始附近点的散度率密切相关，在混沌流中，散度率在时间(t)上平均为指数。这些拉伸场被用来表征混合动力学，预测混合速率，以及被动杂质和颗粒的输运，并被用于研究流动中的活性物质（即游动的微生物）。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，如霍乱弧菌和莱茵哈特藻。从拟议的工作中获得的知识可能对成功设计可控水下自主航行器（微型游泳机器人）、预防与饮用水有关的水传播疾病暴发以及开发海洋浮游物质扩散的精确模型有用。利用这些方法，PI希望解决许多悬而未决的问题，例如：(i)在时间周期流动中控制游泳微生物的运输和混合的主要流动参数是什么？（ii）水流如何影响游泳悬架的动力学？“细菌超流动性”会导致增强的转运吗？（iii）微生物是否与高拉伸和不稳定的流形区域对齐？（iv）微生物的游动作用是促进混合还是阻碍混合？推手或推手如何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?