Collective Hydrodynamics of Swimming Bacteria: A Living Fluid

美国国家科学基金会化学与运输系统分部？微粒和多相过程计划（1415）提案编号：0730579主要研究者：Koch， Donald隶属关系：康奈尔大学提案标题：游泳细菌的集体流体动力学；游动微生物（如大肠杆菌）的悬浮液构成了一种独特的非牛顿流体，它可以表现出负粘度不稳定性，负第一正应力差导致的二次流增强混合，浓度梯度诱导的应力导致的破裂，以及有利于新型分离方法的迁移现象。虽然单个细菌游动的物理机制——借助像螺丝钉一样转动的鞭毛束推动自己穿过液体——已经被很好地理解了，但控制细菌悬浮的运动方程以前还没有推导出来。在本研究中，我们将从对细菌-流体相互作用的基本描述出发，推导出这些方程，求解几个代表性流动的方程，并通过实验观察这些流动。智力优势：细菌细胞对液体施加阻力，而它的鞭毛施加相等且相反的力，导致力偶极子，平均在细胞平均方向上产生压力。这种情况可以与拉伸聚合物形成对比，拉伸聚合物在其取向方向上施加张力。在弱剪切流中，细菌随流动的伸展轴定向并加强伸展运动。因此，在临界细胞浓度以上，悬浮液粘度为负，静止悬浮液将不稳定，形成自发的流体运动。我们相信，这种不稳定性解释了先前在游动细菌系统中对涡旋运动的实验观察。我们计划使用细菌和被动胶体粒子的粒子跟踪来探测这种不稳定性。在强剪切流中，细菌沿着流线排列将产生负的第一法向应力差（或流线压力），而聚合物溶液的第一法向应力差（或流线张力）为正。这两种非牛顿流体都可以增强混合，因为在弯曲的微流体通道中流线曲率引起的二次流动，但是，正如我们将确认的那样，漩涡将集中在通道弯曲的内部，而在外部对于聚合物溶液。更广泛的影响：我们的研究应用包括一种基于细菌趋化行为分离细菌的新方法，一种新的活性？用于微流体混合的流体，其活性可以通过生化输入来调节，并深入了解细胞在响应生化线索时分散或聚集成簇的方式。人们对生物的集体行为有一种天然的好奇心。我们的研究将这种集体行为与动量和质量传递原理以及悬浮的动力学理论描述联系起来，这将提供一种吸引和激励学生思考生物学和工程学之间联系的方法。我们将在我们的本科和研究生课程以及纳米生物技术中心的高中教师拓展计划中利用这些机会。

National Science Foundation - Division of Chemical &Transport Systems ? Particulate & Multiphase Processes Program (1415)Proposal Number: 0730579 Principal Investigators: Koch, Donald Affiliation: Cornell University Proposal Title: Collective Hydrodynamics of Swimming Bacteria: A Living Fluid Suspensions of swimming micro-organisms such as the bacterium E. coli constitute a unique type of non-Newtonian fluid that can exhibit a negative-viscosity instability, enhanced mixing by secondary flows resulting from a negative first normal stress difference, break up due to concentration-gradient-induced stresses, and migration phenomena that facilitate novel separation methods. While the physical mechanism by which a single bacterium swims, pushing itself through the fluid with a flagella bundle that turns like a screw, is well understood, the equations of motion governing a suspension of bacteria have not been derived previously. In the proposed study, we will derive these equations starting from a fundamental description of bacteria-fluid interactions, solve the equations for several representative flows, and observe these flows experimentally. Intellectual Merit: A bacteria cell exerts a drag force on the fluid while its flagella exert an equal and opposite force, leading to a force dipole which on average creates a pressure in the direction of mean cell orientation. This situation may be contrasted with a stretched polymer which exerts a tension in the direction of its orientation. In a weak shear flow, a bacterium orients with the extensional axis of the flow and reinforces the extensional motion. Thus, above a critical cell concentration, the suspension has a negative viscosity and a quiescent suspension will be unstable to the formation of spontaneous fluid motion. We believe that this instability explains previous experimental observations of vortical motions in systems of swimming bacteria. We plan to use particle tracking of both bacteria and passive colloidal particles to probe this instability. The alignment of bacteria along streamlines in a strong shear flow will create a negative first normal stress difference (or streamline pressure) in contrast to the positive first normal stress difference (or streamline tension) for polymer solutions. Both non-Newtonian fluids can enhance mixing due to secondary flows caused by streamline curvature in a curved microfluidic channel, but, as we shall confirm, the vortices will be centered on the inside of a channel bend for bacteria and on the outside for a polymer solution. Broader Impacts: The applications of our studies include a novel method to separate bacteria based on their chemotactic behavior, a new ?active? fluid for micro-fluidic mixing whose activity can be modulated by biochemical inputs, and insights into the manner in which cells disperse or collect themselves into clusters as they respond to biochemical cues. People have a natural curiosity about the collective behavior of living things. Our studies which link such collective behaviors to the principles of momentum and mass transport and kinetic theory descriptions of suspensions will provide a means to engage and inspire students to think about connections between biology and engineering. We will exploit these opportunities in our undergraduate and graduate curricula and in the Nanobiotechnology Center's outreach program for high school teachers.