Paramagnetic beads, about 1μm in diameter, initially in a random dispersion will self-assemble to form chain-like structures when subject to a uniform magnetic field. Magnetorheological (MR) fluids are high volume fraction (φ > 0.2) suspensions of such particles whose rheological properties are tuned by adjusting the strength of the applied field. Lower volume fraction suspensions are used in a wide array of microfluidic applications ranging from DNA separation chips to micro-mixing devices.
|
|
|
The above movies (7MB avi) show the behavior of the suspension below (left) and above (right) the critical frequency of the rotating field. The particles that form an aggregate are shown in blue. In each of the simulations, there are N=1320 particles corresponding to a volume fraction of φ=0.003.
Using a newly developed model to efficiently handle the many body magnetic interactions, we have been investigating the behavior of suspensions of paramagnetic beads when subject to a rotating field. At low frequencies the chains will rotate, remaining aligned with the applied field. As the frequency increases, the average chain length decreases as the aggregates deform into S-shaped structures and rupture. Above a critical frequency, the beads assemble into planar clusters rather than chains. We are interested in ascertaining the effects of the unsteady field on the aggregation process, such as changes in growth rates and cluster shape.
Above the critical frequency, the force between two particles switches periodically from being attractive to being repulsive. This change in the force produces oscillations in the separation distance between the two beads.
|
|
|
Oscillations in the doublet separation distance as a function of time (center figure - 2MB avi). The left figure are the measured oscillations taken from Dominguez-Garcia et al., Colloids Surf A, 270, 270 (2005) while the right figure shows the corresponding simulation results.
The surfaces of the beads are treated with a polymer agent to produce a stabilized dispersion in the absence of the applied field. The adsorbed polymer changes the near-contact hydrodynamics and inter-bead surface forces affecting the doublet oscillations. We are incorporating these two effects into our existing models and comparing the amplitude and frequency of the oscillations with recent experiments. We are examining further the changes in suspension mechanics and self-assembled aggregates as a result of including an accurate description of the near-contact dynamics.