Sandia researchers have
developed a process that can mix tiny volumes of liquid, even in complicated
spaces.
Kyle Solis, a graduate student intern in Nanomaterials Sciences, prepares a sample for mixing using a new approach called vortex field mixing. (Photo by Randy Montoya)
Researchers currently use all types of processes to try and create mixing,
with only "mixed" success. "In small devices," says
Sandia materials scientist Jim Martin "people have tried all kinds of
pillars and mixing cells to initiate mixing, but these approaches don’t
work well." Researchers need simpler and more reliable ways to mix in
tiny places such as micrometer-sized channels, Martin said.
"Mixing liquids in tiny volumes," Martin said, "is surprisingly
difficult." When fluid is pushed down a big pipe, eddies are generated
that create mixing. But if fluid is pushed down a small pipe no eddies are generated
and mixing does not occur unless you subject the fluid to tremendous pressure,
which isn’t usually easy or feasible, he said.
Martin’s discovery of how to mix tiny liquid volumes arose from LDRD-funded
research directed at improving the sensitivity of the chemical sensors developed
in his lab. That project, "Field-Structured Composite Studies,"
was a joint effort with Rod Williamson (now retired). While their LDRD project
did not lead to the expected results, Martin and Williamson were surprised by
the wide variety of physical effects they discovered along the way, including
magnetic mixing. These effects, Martin said, ended up being much more interesting
and important than the original goal.
Since the project began, Department of Energy’s Division of Material
Science and Engineering, Office of Basic Energy Sciences, has started a new
project whose goal is to better understand the fundamental science of field-structured
composites. So the program succeeded even as it failed, and eventually Martin
and graduate student intern Doug Read developed better ways to increase sensor
sensitivity.
In the new method of mixing, when one turns on a particular kind of magnetic
field, the magnetic particles suspended in the fluid form chains like strings
of pearls. The chains start swirling around and that’s what does the mixing.
The particles are then removed magnetically, leaving a nice mixed-up liquid.
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 increasing
the speed of the stir bar instead. It’s easy to feel this effect by simply
holding the beaker slightly above the stir plate. In vortex field mixing increasing
the speed of the wobbling doesn’t help, because the chains simply break
into smaller pieces and the mixing torque doesn’t change at all.
Vortex field mixing stirs just as effectively with magnetic nanoparticles as
with traditional micrometer-size powders. In fact, excellent mixing torques
have been obtained using 100 nanometer particles. This means even the tiniest
fluid volumes can be mixed, as well as the largest.
As strange as these effects are, they were initially predicted by Martin in
a theory paper published in the January 2009 issue of Physical Review. This
paper also explains why a simple rotating magnetic field doesn’t induce
mixing, and predicts the optimal wobbling angle of the magnetic field.
Vortex field mixing requires only the modest magnetic fields provided by simple
wire coils that can be scaled to the size of the fluid cavity. After mixing,
a researcher can trap the particles with a permanent magnet, decant the mixed
liquid and recycle the particles endlessly.
This work was supported by the Division of Materials Science and Engineering,
Office of Basic Energy Sciences, U.S. Department of Energy (DOE).