No longer just laboratory marvels, smart fluids have snapped up a wide range of commercial applications, from automotive to medical. With their burgeoning commercial appeal and the development of improved chemistries, these advanced materials are attracting renewed interest. They are already used as dampers in vehicles, as rotary brakes in aerobic exercise machines and as erasing mechanisms in Braille display tablets for the visually impaired.

Smart fluids are considered smart materials, a category that also includes shape memory alloys and piezoelectrics. The two principal types of smart fluids are magneto-rheological fluids (MRFs) and electro-rheological fluids (ERFs). While MRFs stiffen in the presence of a magnetic field, ERFs solidify when an electrical field is applied. Both fluids return to their liquid form when such forces are removed. They are very similar; both can take the place of some elaborate moving parts and are thus appealing to manufacturers on the lookout for inventive ways to cut costs.

While ERFs were discovered by accident in the 1940s, MRFs were invented in 1947 by Jacob Rabinow, who worked for the National Bureau of Standards. In the 1950s, MRFs found usage as magnetic power clutches in automotive transmissions, and in the 1960s, they appeared in the Apollo service modules. However, interest in smart fluids stagnated in the 1970s and 1980s and did not revive until the 1990s, with the commercial debut of stable MRFs and the development of the technical infrastructure—sophisticated computer algorithms, control circuits and sensors. In the late 1980s, ERFs received a commercial boost with the invention of so-called dry ERFs by Frank Filisco of the University of Michigan. These materials do not need water as part of their fluid in order to solidify in the presence of an electrical field. Thus, the currents used could be greatly reduced.

"That really invigorated the field, because now these materials could be practically applied," Henri Gavin, assistant professor of civil and environmental engineering at Duke University, tells The Industrial Physicist. "The power supply could be driven by a simple car battery." And as the smart fluids claimed commercial applications, they also raised research and development activity to its current high.

In general, these two materials don't directly compete against each other, as they are more suited for different applications because of their particular strengths. According to Constantinos Mavroidis, associate professor of mechanical and aerospace engineering at Rutgers, the State University of New Jersey, the main advantage of using MRFs is their ability to resist large forces while the key benefit of using ERFs is their small size. As a result, MRFs work well in vehicle-suspension systems and structural seismic-vibration damping because these applications call for large forces. Meanwhile, ERFs are well suited for newer applications such as haptic (tactile) interfaces and biomedical devices because these uses call for smaller forces and configurations.

The most profitable MRF application has proven to be in automotive suspensions. The leading supplier of MRF materials—Lord Corp.—provides the smart fluids for Michigan-based Delphi Corporation's new MagneRide automotive suspension system—which has appeared in such General Motors vehicles as the 2002 Cadillac Seville STS and the 2003 Chevrolet Corvette and is set to appear in two 2004 Cadillac models. Moreover, MRFs are showing promise in steer-by-wire systems, which do not rely on a mechanical connection between the steering wheel and the drive wheels and therefore allow manufacturers to cut vehicle weight.

ERFs, meanwhile, may have a future in automotive applications as well—possibly in haptic elements to remotely control various robotic devices. Additionally, England-based Smart Technology Ltd. is working on ERF-based suspension damping systems, which it plans to initially market to European car manufacturers.

Other current and emerging uses for MRFs include…

• In damping technology to help steady buildings during earthquakes.
• As clutches in vehicles to improve torque transfer control.
• As dampers in car steering columns and seatbelts to disperse more energy during collisions.
• As dampers in helicopter blades.
• As dampers in washing machines to minimize noise and vibration.
• As real-time controlled dampers in sophisticated prosthetic devices.
• In magneto-rheological optical finishing.
• And further into the future, in the bloodstream, where they could regulate the flow of blood to malignant tumors.

Other ERF applications under development include…

• As actuators in haptic devices, e.g. in erasable Braille display tablets for the blind.
• In prosthetic devices for rehabilitation.
• In a virtual-reality suit for astronauts to generate resistance so their muscles won't atrophy in zero gravity.
• In virtual surgery, with ERFs mimicking the resistance of human flesh.

In short, smart fluids are no longer just novelty materials, with their growing list of current and emerging applications. While numerous technical and engineering challenges remain—from control issues to settling problems to mass manufacturing concerns—their journey from laboratory marvel to commercial success is already well underway.

Source:

Smart Fluids Move into the Marketplace
Jennifer Ouellette
The Industrial Physicist, Dec. 2003/Jan. 2004
www.tipmagazine.com/tip/INPHFA/vol-9/iss-6/p14.html