Magnetically stimulated flow patterns offer strategy for heat transfer problems

March 7, 2014

( —Sandia National Laboratories researchers Jim Martin and Kyle Solis have what Martin calls "a devil of a problem."


But they're not sure how and why the occur, although clearly it's a complex scientific behavior stemming from fundamental phenomena.

"Just because an effect is easy to generate doesn't mean that it's going to be easy to understand," Martin said.

It's also a tough problem to simulate because of the huge scale of the flow patterns compared to the tiny particle size, he said. He and Solis, a Student Intern Program doctoral researcher, have been generating flow patterns in magnetic platelet suspensions for about three years. They published a paper in 2010 in the American Institute of Physics' Applied Physics Letters and another paper in 2012 in the Royal Society of Chemistry's journal Soft Matter, outlining how they created flow patterns. Martin led a topical review, called "Driving self-assembly and emergent dynamics in colloidal suspensions by time-dependent magnetic fields," for the international journal Reports on Progress in Physics in November.

The pair's research, funded by the Department of Energy's Office of Science, is concentrating on extending fundamental understanding of novel heat transport in liquids, evaluating the effectiveness of various flows and exploring what happens when researchers modify experimental parameters. Martin and Solis found the patterns occur only for magnetic particles shaped like plates, essentially magnetic confetti. Spherical and rod-like particles don't produce the effects.

Making fluid flow like convection

The goal is making fluid flow on its own, as in thermal convection. Convection is familiar to everyone who boils water or marvels at birds or gliders riding on rising columns of warmer air known as thermals. However, it doesn't work in outer space where there's no gravity or in a liquid that's beneath rather than above a hot object. The modern world forces convection by using pumps and fans with associated seals and valves in contact with the fluid, but sooner or later those moving parts corrode and break down.

Martin and Solis make fluids move by adding a small amount of magnetic platelets to a liquid and applying modest, uniform alternating current (AC) magnetic fields. The phenomenon, which they've termed isothermal magnetic advection, has shown very good results for noncontact heat transfer, and would be useful for cooling microsystems, cooling in microgravity or for transferring heat in circumstances that prevent convection, they said.

"We don't have a lot of understanding of why these things occur, but we can determine what the effects are and how well it works," Martin said. The ongoing experiments, coupled with modeling, are advancing the understanding of the phenomena.


The researchers discovered various flow patterns they call advection lattices. "The patterns are pretty remarkable because it's not easy to understand why the fluid should flow in the first place because a uniform magnetic field does not exert a force on a particle, just a torque," Martin said.

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He compared the flow lattices to the patterns, or murmurations, of flying, wheeling flocks of birds, with "every bird obeying some simple rules like avoiding crashing into neighboring birds. There's no leader. These patterns just spontaneously emerge from these simple rules. That's more or less the same thing here. Each particle is obeying simple rules but collectively there's this emergent behavior that's quite surprising."

Specially built magnet generates uniform field





Martin points out not all the "rivers" in the lattice flow in the same direction: Cutting through the fluid would reveal a checkerboard pattern of flow columns, some going one way and adjacent columns flowing the opposite.

"It's an enigmatic phenomenon," he said as he used a tiny light to illuminate the 3-centimeter (cm) or 1.2-inch square glass container of fluid sitting in the middle of the magnets.

Patterns evolve as magnetic field changes


"The sample will go through all these transformations," Martin said. "In any one moment it's trying to become what the field is directing it to become, but now the field is continuously changing, causing the pattern to evolve. In other words, there are lots of patterns that are possible and we can select these by carefully adjusting the phase angle between field components."


"This is the same kind of thing but it's all a fluid," he said.


Researchers demonstrate heat valve



Thus it acts like a valve because it can control the transfer of heat over a 1-cm gap by a hundredfold, he said. "Think of a water valve that can control water flow by a factor of 100—perhaps a little leaky, but still better than no valve." There's still room for improvement, he added.

"Heat transfer can be controlled over any size volume, and the relative efficiency of heat transfer actually increases with scale," Martin said. "It's easy to create over a large volume because the coils that produce magnetic fields are equally efficient at any size."

Isothermal magnetic advection could help efficiently manage overheating in computers. The chips in modern supercomputers run ever hotter and use more power, and drawing heat away from them is a technical challenge that's limiting development, Martin said. And it's not just large systems. "One of the limitations for cooling right now on personal electronics like laptops is just how fast people can run the fans inside of them before the noise becomes too obnoxious," Solis said.

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not rated yet Mar 07, 2014
Whoa...could this be used in more effective heat transfer for fission cores?


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