Simulation of Magnetorheological Fluids: Microdevices and Self-Assembled Structures

Brown University, Providence RI

Investigators

Abstract

Abstract CTS-0326702 M. Maxey, Brown University Controlled 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.