Transport and collective dynamics in suspensions of swimming particles

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

CBET-0754573, Graham 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. Prior results with this model are promising; suspensions of these swimmers produce large scale 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 that will be addressed include the length and time scales over which the motions remain coherent, the physical mechanisms (e.g. instabilities of ordered states) leading to large scale motions, and the dependence of the motions 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. Interactions between swimming and chemotaxis will also be studied. Preliminary experiments will complement the proposed simulations and theory. 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 students 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. This enhanced diffusivity also has important consequences for how cells reach nutrients or flee from toxins, as it indicates that the very act of swimming alters the distribution of chemical species in a fluid. The proposed work provides a conceptual basis for understanding and exploiting this important and widespread class of problems. 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 of ?agents? in which the manybody 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 for a graduate student, who will learn and develop state-of-the-art computational methods as well as the fundamental mechanisms of biopropulsion and some microbiology as well. The student, along with the PI and undergraduate students, will also develop exciting educational materials that will be distributed to K-12 students and teachers.