Text for FP and LN. SS04
Going With the Flow
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THESE days everything seems to be getting smarter. So-called “smart materials” include shapememory alloys, which change shape according to the temperature, and piezoelectric materials that
mechanically deform when an electric field is applied. These materials are said to be smart because
they can be used to sense and respond to the environment. But smart materials are not limited to
solids; liquids, it turns out, can be smart too. Such “smart fluids” are now finding their way into all sorts
of devices, from cars to bridges to digital cameras.
The term “smart fluid” is generally applied to fluids whose properties can be changed by the
application of an electrical or magnetic field. In particular, there are electro-rheological (ER) and
magneto-rheological (MR) fluids that can change, in an instant, from free-flowing liquids to become
more viscous, or even solid. ER fluids consist of tiny dielectric particles dispersed in an insulating fluid
such as silicon oil, while MR fluids use magnetisable particles suspended in a non-magnetisable
carrier liquid. Normally, the particles are randomly aligned. But when a field is applied, they line up
with it, forming long chains that make the liquid more viscous. The process is reversible: once the field
is removed, the fluid flows freely again. And by varying the field, the fluid's viscosity can be carefully
controlled.
For more than half a century, smart fluids have been little more than a laboratory curiosity. But gradual
improvements in their properties—the development, for example, of additives that prevent suspended
particles from clumping or settling—and recent advances in the systems used to control them have led
to the first commercial applications.
Magnetic attraction
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MR fluids are the most widely used kind of smart fluid. They can resist large forces more efficiently
than ER fluids, since they require less energy to change their properties. The world leader in the
commercial development of MR-fluid technology is Lord Corporation, of Cary, North Carolina, a private
firm with an interest in vibration and motion control. Its first commercial success came in 1997, using
MR fluids to dampen vibration for drivers of 18-wheel trucks. Then Lord, along with Delphi
Corporation, of Troy, Michigan, a former subsidiary of General Motors and a big supplier of car parts,
applied MR fluids to car suspension.
The result, called MagneRide, first appeared on Cadillac's Seville models and is now standard
equipment on its XLR convertible roadster, and an option on some other models. Sensors monitor the
profile of the road surface and provide a permanent stream of information as to what damping is
necessary. An electromagnetic coil inside the piston of the damper creates a magnetic field that
adjusts the viscosity of the fluid up to provide continuously variable damping. Unlike traditional
suspension, the system has no electromechanical valves or small moving parts that wear out, and
provides particularly good control at low frequencies. The industry also claims it offers a smoother ride
and improves road-holding. The only problem is its high price. About 100,000 cars with MagneRide
suspension have been sold since production started in 2002, says David Hoptry of Delphi. So far only
General Motors is using the technology, and only in luxury, high-end sports and specialist vehicles.
But as the price of the technology falls it will, he predicts, spread into cheaper cars too.
Ford, another carmaker, is researching other applications of MR fluids. It thinks they will be useful one
day in automotive suspension, but that they are still too expensive. Instead, Ford is concentrating on
their use in automatic clutches. Prototype designs being studied at Ford's Scientific Research
Laboratories are extremely quiet compared with mechanical clutches, and can engage gradually by
slowly increasing the strength of the magnetic field. This reduces noise and means the car pulls away
smoothly. Ford is also examining the use of smart fluids to make quieter air-conditioning compressors.
Steady improvements in the technologies used to control smart fluids, such as microprocessors and
sensors, are opening up a diverse range of new markets. Lord, for example, has figured out how to
use MR fluids to reduce vibration in washing machines. Its system works well; again, the barrier to
adoption is cost. MR fluids are also appearing in much larger structures. Japan's National Museum of
Emerging Science and Innovation, in Tokyo, has installed seismic-scale MR-fluid dampers developed
by Lord. They are integrated with the building's structure and are designed to act as huge shock
absorbers in the event of an earthquake, soaking up energy and protecting the building from damage.
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Large MR-fluid dampers are also being put into bridges, such as the Dong Ting Lake Bridge in China's
Hunan province, to steady it in high winds.
Meanwhile, CSA Engineering of Albuquerque, New Mexico, has developed a damping system based
on MR fluids to suppress vibration during rocket launches, which can damage satellites. MR fluids
have also been suggested as a way to reduce vibration in surveillance satellites. Another military use
is being explored at the Massachusetts Institute of Technology: the idea is to create a fluid-filled
uniform that is flexible and comfortable to wear in normal conditions, but that can instantly be made
much more resistant—and ultimately bullet-proof.
The development of ER fluids has been much slower, since they require large amounts of energy to
keep them in a viscous or solid state. But there have been some recent and significant scientific
developments in this area, so it may only be a matter of time before ER fluids begin appearing in
commercial products too. For if the energy-consumption problem can be overcome, they have
advantages over MR fluids. They do not need bulky magnets to be activated and can, instead, rely on
only a couple of electrodes. In particular, they might find applications in “haptic” (touch-based)
systems. One idea is to use ER fluids to develop Braille readers and writers. Another is to use them as
dampers in prosthetic devices.
Existing haptic devices rely on MR fluids. More than a dozen prototype artificial knees, based on MR
fluids, are already in use. MR fluids are also used in one type of drive-by-wire forklift truck to impart a
sense of rigidity and stiffness to the steering. And carmakers are looking into the use of MR fluids to
impart the sensation of road resistance to drive-by-wire vehicles, where the steering wheel has no
mechanical connection to the wheels of the car. (The physical sensation of the road is an important
component of driving, but is missing in drive-by-wire systems.) Smart fluids could thus be used in
haptic devices ranging from joysticks for gaming to instruments for remote surgery.
The benefits of smart fluids are also becoming apparent to the makers of digital cameras. Although the
technology is different, the benefit is similar: using a fluid in place of mechanical parts enhances
control and reduces the number of bits that can break. Camera designers are finding that, as cameras
become ever smaller, and are integrated into mobile phones and portable computers, the friction
between the moving parts of the lens is increasingly problematic. So lenses based on smart fluids
could have widespread appeal. They would also be more resilient than traditional lenses, which could
be particularly useful in portable devices.
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Look smart
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Varioptic, a firm based in Lyon, France, and Philips Electronics, a consumer-electronics giant based in
the Netherlands, have both developed tiny lenses based on smart fluids. (The two firms are currently
involved in a patent dispute over the technology.) Both firms' designs rely on an effect called
“electrowetting” where the application of an electrical field alters the surface tension of a liquid. As with
other kinds of smart fluids, the effect is not new, but was seen as a curiosity until recently.
Two non-mixing fluids are placed in a short tube with transparent end caps. One is an aqueous (waterbased) fluid, the other a transparent oil. The internal surfaces of the tube wall, and one of its end caps,
are coated with a water-repellent coating that causes the water-based fluid to form itself into a
hemispherical mass at the opposite end of the tube, where it acts as a spherically curved lens.
The shape of the lens is adjusted by applying an electric field across the hydrophobic coating,
reducing the surface tension and making the coating less hydrophobic. The water-based fluid is then
less repelled by the walls of the tube, which makes the lens change shape. By increasing the applied
electric field, the normally convex surface of the lens can be made completely flat or even concave.
This allows the lens to adjust its focus. Using two smart-fluid lenses in combination, it is possible to
make a zoom lens.
Varioptic recently announced that it has licensed its technology to Samsung of South Korea, for use in
camera-phones, handheld computers and other devices. With tens of millions of camera-phones sold
each year, that could carry smart fluids into the consumer mainstream. But however they end up being
used—whether in cars, buildings or mobile phones—it appears that smart fluids have a solid future
ahead of them.