Collective dynamics in suspensions of swimming particles

项目描述：该项目将直接模拟低雷诺数下大量流体动力学相互作用的游动粒子，以获得对这些系统中实验观察到的集体动力学的理解。提出了一种能有效地用于大尺度模拟的游动粒子最小模型。在这个模型中，每个游泳者都有一个头杆哑铃，在那里施加液体阻力，还有一个虚幻的鞭毛，对液体和哑铃施加相等和相反的力。该模型的初步结果是有希望的；这些游泳者的悬浮液产生大规模的运动和反常扩散，这与实验观察一致。拟议的工作将从描述这个简单模型在散装溶液中的集体动力学开始，具体解决所产生的流体运动的相干性以及这些运动如何改变悬浮液中的运输。悬而未决的问题包括运动保持连贯的长度和时间尺度，以及大尺度运动对游泳者的形状和推进方式细节的依赖。其他将被探索的现象包括诸如施加剪切和将游泳者的溶液限制在狭缝或薄膜上的效应。除了拟议的研究外，还将为中学和高中教师和学生编写教材。这些材料的主题是游泳微生物的力学，将有两个主要组成部分：一个说明低雷诺数推进基础知识的简短视频和一个包含许多视频元素的工具包，将供科学和教师使用。该项目将被纳入西澳大学MRSEC的教育工作，该机构拥有优秀的资源，可以将这些材料分发给教师和学生。智力优势：最近的实验表明，在游动的微生物群体中，生物之间相互作用的主要模式仅仅是由每个微生物在游动时驱动的流体运动，这种情况下，微生物群体中有各种令人着迷的集体运动。集体游动导致了比生物本身大得多的尺度上的时空相干流体运动，同时也增强了微生物和随流运动的被动示踪粒子的运输。事实上，游泳微生物最近已经被探索作为一种手段，以加强微流体装置的运输。文献中没有对这一现象的模拟：先前关于低雷诺数游泳的工作主要集中在详细分析单个游泳者的运动或一对游泳者之间的相互作用。这项工作的最终结果将是对游动粒子群的低雷诺数动力学的基本理解。更广泛的影响：提议的工作将加强流体力学和生物学之间的知识联系，通过提供对生物体群体集体运动的见解。多体系统的集体动力学也是非平衡统计物理的一个重要方面，所提出的工作提供了一个模型系统的集体运动的“主体”群体，其中主体之间的多体相互作用是已知的第一性原理。这项工作还可能提供与医学和纳米技术的联系：例如，自主游泳的微型或纳米机器长期以来一直被设想为诊断甚至治疗疾病的潜在方法。拟议工作的教育影响也将有几个方面。研究生教育将部分提供给两名研究生，他们将学习和发展最先进的计算方法，以及生物推进的基本机制和一些结构生物学。这些学生与PI和一名本科生一起，还将开发令人兴奋的教育材料，分发给K-12学生和教师。

ABSTRACT - 0522386University of Wisconsin MadisonProject description: The proposed work will use direct simulations of large populations of hydrodynamically interacting swimming particles at low-Reynolds number to gain understanding of the collective dynamics that have been experimentally observed to arise in these systems. A minimal model of swimming particles, which can be efficiently used in large-scale simulations, is proposed. In this model each swimmer consists of a bead-rod dumbbell where the fluid drag is exerted, along with a phantom flagellum that exerts equal and opposite forces on the fluid and the dumbbell. Preliminary results with this model are promising; suspensions of these swimmers produce largescale motions and regimes of anomalous diffusion that are consistent with experimental observations. The proposed work will begin by characterizing the collective dynamics of this simple model in bulk solutions, specifically addressing the coherence of the fluid motions that are generated and how these motions alter transport in the suspension. Open questions include the length and time scales over which the motions remain coherent, and the dependence of the large scale motion on the details of the shape and mode of propulsion of the swimmers. Other phenomena that will be explored include effects such as imposed shear and confinement of the solution of swimmers to a slit or thin film.In addition to the proposed research, educational materials will be developed for secondary and high school teachers and students. The theme of these materials will be the mechanics of swimming microorganisms and will have two main components: a brief video that illustrates the basics of low Reynolds number propulsion and a kit that will contain many of the elements of the video and will be accessible to science and teachers. The project will be integrated into the educational efforts of the UW MRSEC, which has outstanding resources for distributing these materials to teachers and students.Intellectual merit: Recent experiments show a variety of fascinating collective motions in populations of swimming microorganisms, in situations where the primary mode of interaction between the organisms is simply the fluid motion driven by each of the microorganisms as it swims. The collective swimming leads to spatiotemporally coherent fluid motions on scales much larger than the organisms themselves, as well as to enhanced transport of the microorganisms and passive tracer particles moving with the flow. Indeed, swimming microorganisms have recently been explored as a means of enhancing transport in microfluidic devices. There are no previous simulations of this phenomenon in the literature: prior work on low Reynolds number swimming has focused on detailed analyses of the motion of individual swimmers of on the interaction between a pair of swimmers. The end result of the work will be a basic understanding of the low-Reynolds number dynamics of groups of swimming particles.Broader impacts: The proposed work will strengthen the intellectual connections between fluid mechanics and biology by providing insight into the collective motion of groups of organisms. Collective dynamics of many-body systems is also an important aspect of nonequilibrium statistical physics, and the proposed work provides a model system for collective motion of groups o f"agents" in which the many-body interactions between agents are known from first principles. The work also potentially provides links to medicine and nanotechnology: for example, autonomous swimming micro- or nanomachines have long been envisioned as potential ways to diagnose or even treat diseases. The educational impact of the proposed work will also have several facets. Graduate education will be provided in part for two graduate students, who will learn and develop state-of-the-art computational methods as well as the fundamental mechanisms of biopropulsion and some structural biology as well. These students, along with the PI and an undergraduate student, will also develop exciting educational materials that will be distributed to K-12 students and teachers.