Magnetic Mixing Offers Strong Attraction

The method subjects particles to a rotating magnetic field with a precession-like motion.

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A new approach to using magnetism for mixing provides a simple and robust way to achieve instantaneous and homogeneous mixing at any scale, say its developers at Sandia National Laboratories, Albuquerque, N.M. The technique avoids agitators or other mechanical parts contacting fluid. Instead, it relies on magnetizable particles suspended in the fluid forming chains and then swirling to create effective mixing throughout the fluid. When mixing is done, a magnet can quickly and completely remove the particles, which can be reused, notes Jim Martin, a materials scientist at Sandia.

The method, called vortex field mixing, subjects particles to a rotating magnetic field with a precession-like motion. This causes the particles to assemble into a multitude of microscopic chains that follow the field motion and stir the entire volume of fluid regardless of the complexity its shape (Figure 1). (A YouTube video showing the mixing can be downloaded via http://www.youtube.com/watch?v=_2izq-XOvlg.)

"Because the effect is independent of particle size, there is really no lower limit to the volumes we can mix… It would also be interesting to implement this technology to large mixing volumes that might be encountered in industrial processes," says Martin. "Pretty much any nonmagnetic material is suitable for the mixing chamber construction."

High viscosity fluids can cause chain fragmentation and thus less effective mixing, but this can be countered by adjusting field strength or frequency, he notes.

"With our process, if we make the magnetic field twice as strong, the stirring becomes four times as strong because the stronger field makes the particle chains longer," he explains.

Use of anisometric particles such as nanorods and platelets is being investigated. "These particles give greater mixing torque, but at the expense of greater sedimentation volume, and thus a greater volume of trapped fluid," says Martin.

The researchers hope to interest a company in joint development of commercial applications of the technology.

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