Collaborative Research: The Analysis and Simulation of Biologically Active Suspensions

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。生物活性悬浮液是流体系统，其微观结构是活的和可运动的，游动的细菌浴就是一个典型的例子。当系统的“活跃粒子”推动自己穿过周围的流体时，它们会产生扰动流，将它们的运动传递给其他游泳者，从而改变他们的游泳方向和速度。这种相互作用可以导致相关的、大规模的、复杂的流体流动，其移动的长度和时间尺度远远大于任何单个游泳者。这些由游泳者驱动的水流对微生物群落的进化和生存具有重要意义，因为它们通过颗粒运输和流体混合影响营养物质的输送，并且还可能在群体感应和生物膜形成等其他重要现象中发挥作用。它们也是非平衡模式形成系统的基本例子。在这个项目中，我们建议使用分析模型和数值模拟相结合的方法进一步加深我们对生物活性悬浮液的理解。研究将集中在这些系统中出现的相干结构的建模和分析以及它们与流体混合的关系。具体地说，边界和边界条件、约束和系统规模的影响将被检查。新的多尺度方法允许数十万相互作用的游泳者被模拟，从而接近生物现实主义也将被开发。通过这项研究，我们对活性悬浮液的理论认识将得到提高，并将揭示运动微生物菌落中营养物质运输和混合的核心生物物理机制。它还可能揭示运动策略的进化，特别是那些在生物膜中共同生存和繁衍的微生物。这项研究的广泛影响在于主动悬浮液对几个关键科学领域的重要性，包括生物学、人类健康和医学、软凝聚态物理和工程学。了解主动悬浮液以及驱动（或阻止）它们大规模混合的原因可能会带来控制感染的新方法。它为物理学提供了一个在生物学中产生的非平衡模式形成的典型例子，在工程上，这种理解可能会导致利用生物材料进行混合和泵送等任务的新设备。该项目的影响还在于开发了应用和计算数学的新的重要研究领域，并增加了应用数学家和理论工程师可以解决生物和复杂流体动力学问题的理论和计算工具包。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Biologically active suspensions, of which a bath of swimming bacteria is a paradigmatic example, are fluid systems whose microstructure is alive and motile. As the system's "active particles" propel themselves through the surrounding fluid, they produce disturbance flows that communicate their motions to other swimmers, thereby altering their swimming direction and speed. This reciprocal interaction can result in correlated, large-scale, and complex fluid flows that move on length- and time-scales much larger than those of any single swimmer. These swimmer-driven flows have important implications for the evolution and survival of micro-organismal colonies, as they impact nutrient delivery through both particle transport and fluid mixing, and may also play a role in other important phenomena such as quorum sensing and biofilm formation. They are also fundamental examples of non-equilibrium pattern-forming systems. In this project, we propose to further deepen our understanding of biologically active suspensions using a combination of analytical models and numerical simulations. The research will focus on the modeling and analysis of the coherent structures that arise in these systems and on their relation to fluid mixing. Specifically the effect of boundaries and boundary conditions, confinement, and system scale will be examined. New multiscale approaches allowing hundreds of thousands of interacting swimmers to be simulated will also be developed, thus approaching biological realism.As a result of this study, an improved theoretical understanding of active suspensions will be achieved and will reveal the core biophysical mechanisms underlying nutrient transport and mixing in colonies of motile microorganisms. It may also shed light on the evolution of locomotory strategies, particularly for microorganisms that live and thrive cooperatively, as in biofilms. The broader impacts of this research lie in the importance of active suspensions to several key areas of science, including biology, human health and medicine, soft-condensed matter physics, and engineering. An understanding of active suspensions and what drives (or stops) their large-scale mixing could lead to new ways of controlling infection. It provides to physics a well-characterized example of nonequilibrium pattern formation arising in biology, and in engineering this understanding could lead to new devices that exploit biological materials for tasks such as mixing and pumping. This project's impact also lies in the development of new and important areas of inquiry for applied and computational mathematics, and in its adding to the theoretical and computational tool-kit that applied mathematicians and theoretical engineers can bring to problems in biological and complex fluid dynamics.