Quick change. An electric field strings together tiny beads in a liquid solution (inset), forming a solid.

Liquid Takes a Hard Turn

In a split second, an electric field can turn a new material from a liquid as smooth as glycerin to a solid as hard as rubber, say researchers.

Liquids that solidify when electrified would allow faster, more precise control of valves, clutches, and shock absorbers. Magnetic versions of such fluids have already found their way into exercise machines and car suspensions, but they require bulky magnets to activate. And electric versions have always been too soft to live up to the hype. Typically they stiffen only to about the consistency of firm tofu.

Now a fluid full of nanosized particles has leapt past the threshold for practical use, say researchers at the Hong Kong University of Science and Technology. The group coated 50-nanometer-wide beads of barium and titanium with urea, a nitrogen-containing compound, and suspended them in silicon oil. Then they applied an electric field, which lined up the urea molecules and caused one "pole" of each bead to acquire a negative charge and the other a positive charge. As a result, the beads attract each other and form columns oriented parallel to the field, turning the liquid into a solid.

The material can be turned from liquid to solid (or vice versa) in 1/10th of a second, the group reports online 5 October in Nature Materials. The researchers measured the material's strength by electrifying it between two parallel plates, which they either pulled apart or slid past each other with increasing force. The new material increases in strength with the applied field, up to the strength of hard rubber, they found. Some electric smart fluids get nearly as hard, but only at high pressure. The urea coating is key to the enhanced effect, the researchers say.

"This does suggest a new route to [smart electric] fluids that may have useful properties," says John Ginder, a physicist in Research and Advanced Engineering at Ford Motor Co. But he adds that the material would have to withstand even higher sheer to survive in clutches or shock absorbers. Bill Bullough, an independent engineer and consultant at the University of Sheffield, U.K., says he has performed such tests on the fluid "and it does work well. It's a big improvement."

Related sites
Hong Kong University of Science and Technology electric smart fluids page
More info on electric smart fluids