Simulation of Magnetorheological Fluids:

Microdevices and Self-Assembled Structures

In this movie, one hundred swimmers, oriented in the same direction, are actuated using the method described by Dreyfus et al. with sperm number Sp=3 and magnetoelastic number Mn=3. The volume fraction is ~3%. The beating of the swimmers' tails are synchonized with the oscillation of the applied field. Zippering of the magnetic tails is also evident in the simulation. (Eric Keaveny)

Self-assembly processes occur at all scales from molecular (crystals) to the planetary scale (weather system), from bacteria colonies and swarms of ants to solar systems and galaxies, as described by Whitesides & Grzybowski (Science 295, 2418-2421,2002). It is one of the few practical strategies for making systems of nanostructures, where the individual components are hard to manipulate directly but where we can exploit the competing interaction forces between the components to form ordered structures. Self-assembly is seen in nature in the formation of fish schools or the swimming of bacteria. Dynamic self-assembly typically involves dissipation and a reversible process, whereby the structures break up once the forces are removed.

One context in which self-assembly can be used is the controlled manipulation of super-paramagnetic beads by magnetic fields. This 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. Once the field is removed, the paramagnetic beads lose their magnetic dipole moment and Brownian motion causes the chains to break apart. The use of paramagnetic beads and magnetic fields has been found to avoid the difficulties associated with electro-rheological fluids and the large electric potentials involved. 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.

Snapshot of super-paramagnetic beads in a suspension aggregating to form chains in a magnetic field that is rotating at low frequency, Mason number Ma = 0.2. The beads that have aggregated to form a chain are shown in blue. At this frequency, the chains rotate more or less in phase with the magnetic field. At higher frequency, the chains are distorted into S-shapes or form small clusters. Correctly estimating the hydrodynamic interactions of the beads is critical for determining the cluster size. Click on the image for an animation (5MB AVI) of the aggregation process. (Eric Keaveny)

Research

The focus of the present research is on:

  1. The dynamics of magnetic beads in suspensions and shear flows and their manipulation by magnetic fields in confined geometries to produce self-assembled structures and form colloidal micro-devices such as flow actuators, mixers or pumps. Simulations involve tens to thousands of magnetic beads.
  2. The development of mesoscopic simulation techniques that bridge molecular scale dynamics to nano- and micro-scale systems, important for colloidal systems and Brownian motion.
  3. The development of new micro-scale simulations for particle motion and suspensions.

This material is based upon work supported by the National Science Foundation under Grant No.0326702. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).