ER fluids: the plot thickens.

ER fluids: The plot thickens

Imagine turning a liquid into a solid at the flip of switch. That's what a class of materials called electrorheological (ER) fluids can do. When these fluids -- made are put in an electric in oil or other nonconducting liquid -- are put in an electric field, the particles stick together and the flowing liquid becomes increasingly viscous, sometimes to the point of solidifying. Research on ER fluids, discovered in the 1940s, has been jealously guarded by companies that see an estimated $20 billion a year market for improved valves, clutches, brakes and other hydraulic devices based on ER fluids. Such devices promise lower costs, better performance and more precise control if the electric field is directed by a computer.

But the problem with most ER fluids developed so far is that they contain adsorbed water, says Frank E. Filisko at the University of Michigan in Ann Arbor. This means that at the high temperatures often found in engines, the water boils off, and if the ER effect depends on the presence of adsorbed water, the fluid will not operate. Moreover, the escaping water can corrode the device and lead to "thermal runaway," in which the fluid gets increasingly hotter while more current is needed to maintain the electric field.

Last month, Filisko received a patent for a class of ER fluids he says can operate without adsorbed water, and in fact contain less than 1 part per million of water. Made with aluminosilicate ceramic particles, the fluids operate above water's boiling point and beyond at least 120[deg.]C. Success, he says, involved finding materials whose structure and chemistry do roughly what he suspects the adsorbed water does in other ER fluids.

Researchers at the Cranfield Institute of Technology in Cranfield, England, already have announced making an ER fluid containing less than 5 percent water. According to team member Jeff Kelly, their fluids, which consist of semiconducting particles or polymers immersed in oil, operate at temperatures between --30[deg.]C and 200[deg.]C. Filisko, however, contends the Cranfield group has not proved the fluids can operate without water.

In general, scientists are uncertain how ER fluids work. The Cranfield group believes that the electric field impedes the particles in their fluid from rotating--something the particles normally do when the fluid is flowing -- and that this in turn makes the fluid more viscous. Another theory holds that water molecules form strong bridges between the particles. Filisko believes that neither of these ideas explains his fluid, but without more work he is reluctant to discuss his own model.

Whatever their disagreements over mechanisms, most scientists agree the possible applications of ER fluids are enormous, and not only for the auto and machine industries. "We have the potential of directly connecting tiny hydraulic flow devices to a computer brain [for making agile robotic fingers and other robotic machines]," says Filisko. "Ultimately a whole new generation of devices is going to develop out of this."