Simulation of Magnetorheological Fluids: Microdevices and Self-Assembled Structures

abstractcts - 0326702 m。对超顺磁珠的可控操纵可以形成自组装结构，适合用作微光学滤光片，用于DNA分离，或用于探索微纳米制造的新概念，特别是在三维中。迄今为止的证据来自于磁流变（MR）流体的实验，这些实验中，带有微米级珠子的流体在外部磁场作用下形成了具有规则分布和间距的柱状链。该过程是完全可逆的，并且已经发现可以避免与电流变流体相关的困难。通过实验室演示，最近已经取得了进展，如何在微通道中操纵颗粒以进行细胞分选、细胞移除或制造新的泵、阀和混合器。虽然实验已经证明了MR流体的一些有趣的特性，但它们的全部潜力仍有待开发。在本提案中，我们的目标是模拟和研究MR流体的基本特性和产生的自组装结构，并研究原型胶体微器件的新设计和最佳性能。在更广泛的背景下，我们提出了不使用光刻技术制造微器件的新方法。我们将考虑两类不同的问题，第一类涉及数十个顺磁微球，而第二类涉及数千个。为此，我们将采用分层模拟方法，该方法在精度和计算复杂性方面在一定范围内的参数中表现最佳。它将包括新的随机技术来表示布朗噪声；几何粗糙度或与边界条件、颗粒大小和相互作用力有关的其他不确定性。具体来说，我们将采用基于高阶离散化和三种不同公式的直接数值模拟：(1)任意拉格朗日欧拉法（ALE），(2)分布式拉格朗日乘数法（DLM）和(3)力耦合法（FCM）。随机贡献将采用最近发展的广义多项式混沌方法进行频谱建模。该项目的第一个主要目标是开发和评估胶体微器件的拟议模拟方法。然后将其应用于设计微泵，微阀和其他微设备，如混合器和分选器，并优化其性能。我们还将研究制造三维微器件的新概念。第二个目标是研究从微米级和亚微米级顺磁珠悬浮液中形成自组装结构，如链或链阵列。布朗运动对较小的颗粒起着重要的作用，磁场的相对强度是一个潜在的参数，还有空隙率、通道几何形状和任何施加的流体流动。将考虑时变磁场或非均匀磁场的动态特性。这项工作的更广泛的研究影响是巨大的，因为它首次解决了许多不同构型的磁流变流体的模拟。利用外场以动态方式瞄准和精确控制微结构的电光和力学性能的可能性，将为微流体研究开辟新的视野，并为微纳米制造提供新的方案。自组装磁性矩阵可以在DNA和其他中等大小物体的分离中找到广泛的应用。胶体的自组装可以用于自底向上的方法来制造纳米系统和三维微系统。更广泛的教育影响也很大，因为拟议的工作将有助于对MR流体特性、自组装过程和新的纳米技术应用的基本理解。自组装过程发生在从分子（晶体）到行星（天气系统）的所有尺度上，这种普遍性将吸引年轻人的好奇心。我们希望吸引具有不同科学背景的本科生和研究生参与这个项目。我们还计划通过演示和可视化来告知更广泛的社区，我们将加强布朗大学的ARTEMIS项目，在纳米技术和计算科学问题上教育和激励年轻女性。之前的国家科学基金会的首席PI资助了两名非裔美国女博士生。

AbstractCTS-0326702M. Maxey, Brown UniversityControlled manipulation of super-paramagnetic beads can lead to formation of self-assembled structures suitable for use as micro-optical filters, in DNA separation, or for exploring new concepts in micro- and nanofabrication, especially in three-dimensions. The evidence to date has come from experiments where magneto-rheological (MR) fluids with micron-size beads subject to external magnetic fields form columnar chains that have a regular distribution and spacing. The process is fully reversible and has been found to avoid the difficulties associated with electro-rheological fluids. Recent advances have been made, through laboratory demonstrations, of how particles can be manipulated in microchannels for cell sorting, cell removal or to fabricate new pumps, valves and mixers.While the experiments have demonstrated some of the intriguing properties of MR fluids, their full potential remains to be developed. In this proposal, we aim to simulate and study the fundamental properties of MR fluids and resulted self-assembled structures, and to also investigate new designs and optimum performance of prototype colloidal microdevices. In a broader context we propose new ways of fabricating microdevices without the use of lithography. We will consider two different classes of problems, the first involving tens of paramagnetic microspheres whereas the second involving thousands.To this end, we will employ a hierarchical simulation methodology that performs best in a certain range of parameters in terms of both accuracy and computational complexity. It will include new stochastic techniques to represent Brownian noise; geometric roughness or other uncertainties associated with the boundary conditions, particle size and interaction forces. Specifically, we will employ direct numerical simulations based on high-order discretizations and three different formulations: (1) the arbitrary Lagrangian Eulerian (ALE), (2) the distributed Lagrangian multiplier method (DLM), and (3) the force-coupling method (FCM). The stochastic contributions will be modeled spectrally using the recently developed generalized polynomial chaos method. The first primary goal of the project is to develop and evaluate the proposed simulation methodology for colloidal microdevices. We will then apply it to design micropumps, microvalves and other microdevices such as mixers and sorters, and optimize their performance. We will also investigate new concepts in fabricating three-dimensional microdevices. The second goal is to study the formation of self-assembled structures such as chains, or arrays of chains, from a suspension of micron-scale and sub-micron paramagnetic beads. Brownian motion plays a significant role for smaller particles, and the relative strength of the magnetic field is an underlying parameter, together with void fraction, channel geometry and any imposed fluid flow. The dynamic characteristics for time-varying magnetic fields or a nonuniform patterning of the field will be considered.The broader research impact of this work is great as it addresses for the first time simulation of magneto-rheological fluids in many different configurations. The possibility to target and precisely control the electro-optical as well as the mechanical properties of microstructures in a dynamic way using external fields will open new horizons in microfluidics research and will suggest new protocols in micro- and nanofabrication. Self-assembled magnetic matrices can find a large range of applications for the separation of DNA and other intermediate-size objects. Self-assembly of colloids can be used in a bottom-up approach to the fabrication of nanosystems and three-dimensional microsystems.The broader education impact is also great in that the proposed work will contribute to fundamental understanding of properties of MR fluids, self-assembly processes, and new nanotechnology applications. Self-assembly processes occur at all scales from molecular (crystals) to the planetary scale (weather system), and this universality will attract the curiosity of young minds. We expect to attract undergraduate and graduate students with diverse scientific backgrounds to be involved in this project. We also plan to inform the broader community through demonstrations and visualizations, and we will enhance Brown's ARTEMIS program in educating and inspiring young women on issues of nanotechnology and computational science.The previous NSF grant of the lead PI supported two African-American female PhD students.