Simulation of Magnetorheological Fluids: Microdevices and Self-Assembled Structures

摘要CTS-0326702M。控制超顺磁珠的操作可以形成适合用作微型光学滤光器、DNA分离或探索微米和纳米制造的新概念的自组装结构，特别是在三维制造中。到目前为止，证据来自实验，磁流变(MR)流体具有微米大小的珠子，受到外部磁场的影响，形成具有规则分布和间距的柱状链。该过程是完全可逆的，并已被发现避免了与电流变液相关的困难。最近，通过实验室演示，颗粒如何在微通道中被操纵以用于细胞分离、细胞去除或制造新的泵、阀和混合器方面取得了最新进展。尽管实验已经展示了磁流变液的一些有趣的特性，但它们的全部潜力仍有待开发。在这个方案中，我们的目标是模拟和研究磁流变液的基本性质和由此产生的自组装结构，并探索新的设计和优化的胶体微器件的性能。在更广泛的背景下，我们提出了不使用光刻来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，第二类涉及数千个。为此，我们将采用分层模拟方法，在一定的参数范围内，在精度和计算复杂性方面表现最好。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相关的其他不确定性。具体地说，我们将使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(ALE)，(2)分布式拉格朗日乘子法(DLM)，(3)力耦合方法(FCM)。随机贡献将使用最新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所提出的胶体微器件模拟方法。然后，我们将应用它来设计微型泵、微型阀和其他微型设备，如混合器和分选器，并优化它们的性能。我们还将研究制造三维微器件的新概念。第二个目标是研究由微米级和亚微米级顺磁珠悬浮形成的自组装结构，如链或链阵列。布朗运动对于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，与空泡率、通道几何形状和任何施加的流体流动一起。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液的模拟。利用外场动态地定向和精确控制微结构的电光和机械性能的可能性将为微流体研究开辟新的视野，并将在微米和纳米加工方面提出新的方案。自组装磁性矩阵在分离DNA和其他中等大小的物体方面有着广泛的应用。胶体的自组装可以用于自下而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是巨大的，因为拟议的工作将有助于从根本上理解磁流变液的性质、自组装过程和新的纳米技术应用。自组装过程发生在所有尺度上，从分子(晶体)到行星尺度(天气系统)，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向更广泛的社区提供信息，我们将加强布朗的Artemis计划，在纳米技术和计算科学问题上教育和激励年轻女性。之前NSF资助的首席PI资助了两名非裔美国女性博士生。

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.