Understanding of collective hydrodynamic phenomena in microconfined suspension flows is crucial in diverse research fields that range from dynamics of bacterial colonies to microfluidics. Such phenomena, however, are far from being understood. To elucidate collective hydrodynamics of particulate systems we propose to investigate the evolution of strongly confined ordered suspensions in parallel wall channels. We will study two kinds of systems: particle arrays interacting only via hydrodynamic forces, and flow driven colloidal crystals where particle ordering stems from potential and hydrodynamic forces. Our preliminary studies of flow driven ordered particle arrays reveal that these systems show wave propagation, sudden rearrangements of particle lattice, order disorder transitions, and fingering instabilities. Using numerical, experimental and theoretical methods, we will investigate hydrodynamic mechanisms that produce this rich dynamical behavior. Insights from our studies will shed new light on basic questions of particulate flows, will be applicable to other ordered dissipative systems, and will also suggest new strategies for practical applications (e.g., multidrop microfluidic devices and particulate coating flows). The proposed numerical simulations will be performed using our novel Stokesian dynamics algorithm that is accurate and highly efficient. The experimental part will consist in assembling regular 2D particle arrays using holographic optical tweezers, and observing changes in suspension microstructure using confocal microscopy. Our theoretical investigations will involve Fourier analysis of displacement waves in regular particle arrays and an effective medium approach to describe the evolution and instabilities of the arrays.
Intellectual Merit: The intellectual value of our proposed research is both fundamental and practical. We will study an entirely new class of hydrodynamic phenomena in creeping particulate flows. Our results will be relevant to nonlinear physics (including dynamics of complex fluids, structural evolution in dusty plasma, and collective motion of flux vortices in superconductors). Our results will also have significant impact on engineering applications, especially in microfluidics. Emerging multidrop microfluidic applications include tunable optical devices, high throughput lab-on-chip assays, and manufacturing microstructured materials. Understanding hydrodynamic mechanisms governing suspension structure under strong confinement conditions is key in such applications, and our proposed research will uncover such mechanisms.
Broader Impact: This project will provide educational and research opportunities for graduate, undergraduate and high school students. Graduate and undergraduate students will present their work at New England Complex Fluids Workshops and at national scientific meetings. Students from under represented groups will be recruited through the STARS (Science, Technology and Research Scholars) program at Yale. The PIs will support K-12 education in Greater New Haven by serving as judges in New Haven science fairs. Interdisciplinary significance of our project consists in obtaining results that will be used in physics of pattern formation, physical chemistry, and microfluidic technology. Insights from our research will also be applicable to biological sciences in investigations involving collective hydrodynamic effects, e.g., in studies of bacterial motion. Software for particle tracking and some of the Stokesian dynamics codes will be disseminated through free websites. This research also involves international collaboration.
The purpose of this project was to determine how fluid flow can arrange suspended microparticles into disordered or ordered microstructures in tightly confined spaces (e.g., micropores and microfluidic channels). Understanding the organizing role of the flow is important for development of emerging technologies, such as fabrication of novel particle-based materials with functional microstructure, which, for example, allows to obtain microfiltration membranes. This knowledge is also indispensable to advance microscale particle sorting methods (e.g., for separation of different cells for biological assays). Investigations supported by this NSF award have identified key hydrodynamic mechanisms through which particles in confined spaces mutually affect their positions. This, in turn, led to the discovery of new flow-arranged microstructures and resulted in the development of innovative theoretical and numerical methods for describing these collective phenomena. An interesting and important mechanism identified by this research is the swapping-trajectory effect, where particles on collisional trajectories switch streamlines due to the lift produced by wall-scattered flow. As a result, the particles reverse their direction and avoid collision. This mechanism stabilizes a near-wall layered structure in flowing suspensions and ensures stable ordered motion in microfluidic channels. Maintaining ordered and well-controlled particle traffic in microfluidic devices is important for lab-on-chip devices. The project has investigated the consequences of the swapping-trajectory effect for the stability of flowing particle arrays (hydrodynamic crystals). The discovery of the swapping-trajectory effect has provided an impetus to research performed by other investigators (which led to a patent application describing microfluidic particle focusing). The swapping-trajectory effect can also have a stabilizing-destabilizing role, causing buckling instabilities in lattices of hydrodynamic crystals. The results of the project and the proposed theoretical models impact a vast spectrum of applications, ranging from hydrodynamic particle fractionation methods to analyses of cell and nutrient distribution in blood microcirculation. Moreover, computer simulation techniques originally developed to analyze the motion of passive suspended particles have also been used to harness hydrodynamic phenomena to study neuromuscular control of millimeter-size swimming organisms, which helps advance biological studies. This translational research demonstrates the broad applicability of the NSF-funded fundamental projects. The grant has provided partial support for three Ph.D. and two postdoctoral students (three of these coming from groups underrepresented in science), contributing to the formation of a new generation of well-educated professionals in engineering sciences. The research supported by the grant has also been presented to high-school students at a science fair organized by Texas Tech University, exposing young students to modern science and technology, and encouraging them to consider studies and future careers in STEM fields.
