Amazing Magnetic Fluids
A grey blob oozes down the side of a laboratory beaker. It's heading for the table, but before it gets there a low hum fills the air. Someone just switched on an electromagnet. The goop stiffens, quivers, then carries on oozing only after the hum subsides.
Is it alive?
No, just magnetized.
"We call them magnetorheological fluids--or 'MR fluids' for short," says Alice Gast, a professor of chemical engineering at MIT. "They're liquids that harden or change shape when they feel a magnetic field."
You can make some of this exotic stuff at home. Just mix some powdered iron filings with a thick liquid like corn oil, and presto: a simple MR fluid. Hold a magnet nearby and the bits of iron will line up end-to-end; they form a rigid lattice that stiffens the mixture. Take the magnet away and the fluid will relax again.
If you own a sports car or a Cadillac, you might have MR fluids in your shock absorbers. The stiffness of magnetic shocks can be electronically adjusted thousands of times per second, providing a remarkably smooth ride. Similar but more powerful devices have been installed at Japan's National Museum of Emerging Science and China's Dong Ting Lake Bridge. They're there to counteract vibrations caused by earthquakes and gusts of wind.
Above: The Dong Ting Lake Bridge in China is equipped with magnetorheological motion dampers to counteract gusts of wind. Image courtesy: Lord Corporation.
Motion damping is perhaps the most practical use for MR technology today, but much more is possible. Says Gast: "There are many potential applications that make these fluids very exciting." Some are quite far out: For example, MR fluids flowing in the veins of robots might one day animate hands and limbs that move as naturally as any humans. Book makers could publish rippling magnetic texts in Braille that blind readers could actually scroll and edit. It might even be possible to train student surgeons using synthetic patients with MR organs that flex and slice like the real thing.
Below: Applying a magnetic field (represented by the arrows) to magnetorheological fluids causes particles in the fluid to align into chains. Image credit: Lord Corporation.
That's the goal of an experiment called InSPACE now orbiting Earth onboard the International Space Station. Gast developed InSPACE, short for "Investigating the Structure of Paramagnetic Aggregates from Colloidal Emulsions," in collaboration with scientists and engineers at the Glenn Research Center. Gast is the principal investigator; Lekan is the project manager.
InSPACE will explore a curious phenomenon: When some low-density MR fluids are exposed to rapidly alternating magnetic fields, their internal particles clump together. Over time they settle into a pattern of shapes that look a bit likeviewed from the top of a tank. Such clumpy MR fluids don't stiffen as they should when magnetized.
The fishtank pattern is fragile and takes about an hour to fully develop. It doesn't occur in MR fluids that are constantly mixed and agitated, as in a car's suspension, but it could prove troublesome in other situations.
Above: The structure of particles in an MR fluid gradually changes when an alternating magnetic field is applied. The leftmost picture shows an MR fluid after 1 second of exposure to a fast-changing magnetic field. The suspended particles form a strong, fibrous network. The pictures to the right show the fluid after 3 minutes, 15 minutes and 1 hour of exposure. The particles have formed clumps that offer little structural support.
Even the modest pull of gravity can distort the pattern--a frustration to scientists trying to study the underlying physics. That's why Gast and colleagues are sending MR fluids into orbit. On the space station, astronauts can expose a free-falling fluid to magnetic pulses and record what happens.
"Astronauts are an integral part of our study," notes Lekan. They will reach into the Microgravity Science Glovebox, where the experiment is located, to align and focus cameras on a spot only 0.2 mm wide. If a fluid bubble gets in the way of the shot ... flick! they can remove it.
Right: a prototype MR washing machine. [more]
Meanwhile, some companies are already forging ahead with new magnetorheological devices. Lord Corporation of North Carolina, for example, is designing an MR washing machine. Magnetic dampers inside the machine will decrease noise and vibration--and save energy. They're also studying MR technology for seat belts and airbags in cars. Because MR fluids can generate large forces quickly and flexibly, they could be used by automakers to adjust the arresting force of a seatbelt to the size and weight of a passenger.
Saving lives and silencing washing machines--and that's just the beginning. Not bad for a bunch of grey oily goop.
NASA's Office of Biological and Physical Research -- supports studies of fundamental physics for the benefit of humans in space and on Earth
More far-out applications of MR fluids:
- magneto-liquid mirror telescopes that bend and deform to cancel the twinkling of starlight
- prosthetic limbs for humans (a prosthetic knee based on Lord Corporation MR fluid technology is already available from Biedermann Motech)
- active engine mounts that reduce vibration and quiet noise before it can get into a vehicle
- shock absorbers for payloads in the space shuttle
- active hand grips that conform to the shape of each individual hand or fingers
Alice Gast notes that such homemade fluids are not very stable. "We work with ferrofluid emulsions," she says. "These are made of droplets of an oil-based (octane or kerosene) ferrofluid suspended in water by soaps (sodium dodecyl sulfate). They are carefully fractionated to be all the same size, around 1/2 micron droplets. The ferrofluid is an oil with 100nanometer grains of iron oxide (rust) mostly Fe2O3 suspended in oil (octane or kerosene). The whole system is tailored to give us a large magnetic effect for the applied field."
Learn more about the extraordinary earthquake and wind dampers at Japan's National Museum of Emerging Science and China's Dong Ting Lake Bridge
InSPACE -- home page for the experiment aboard the International Space Station highlighted in this article
Lord Corporation, which funded much of Gast's early research, is one company working bring MR-fluid based products to market. CSA Engineering, Inc. is another.
Buck Rogers, Watch Out! -- Science@NASA article: NASA researchers are studying insects and birds, and using "smart" materials with uncanny properties to develop new and mindboggling aircraft designs.
Astronaut Fingers -- Science@NASA article about the new Microgravity Science Glovebox aboard the International Space Station
More information about smart structures: How Smart Structures Will Work (How Stuff Works); Calming Bad Vibes (Science News Online); MR Fluid Damping (Sensors Online)
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