More technically, the new mixing method, which Jim calls vortex field mixing, subjects a suspension of microscopic, magnetizable particles to a magnetic field whose direction is constantly spinning in a motion similar to a spinning top as it is about to collapse on its side, but much faster. In this "vortex field" the particles assemble into countless microscopic chains that follow the field motion, stirring every nook and cranny of the fluid. The vortex field stirs the liquid vigorously, and surprising fluid effects are possible, such as a kind of washing machine agitation where the spinning direction alternates periodically.
Currently, Martin, Lauren Rohwer, and graduate intern Kyle Solis work with the vortex field mixing, among other projects. Their experimental report, recently appearing in the July issue of Physical Review, has generated interest, including a Physical Review Focus article and a Research Highlight in the September MRS Bulletin.
This type of magnetic mixing with particles that assemble into micro-stir bars isn't like the magnetic mixing done in high school chemistry class.
"In your high school chemistry class," Martin says "when you mixed a beaker of water on a stir plate, underneath the plate was a permanent magnet spinning around to make the stir bar spin. If that hidden magnet suddenly became twice as strong, the magnetic field would double but you wouldn't see any increase in the stirring at all.
"With our process," Martin said "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."
With conventional stir-bar mixing you can increase the mixing torque by increasin
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DOE/Sandia National Laboratories