A FLUID that becomes as stiff as plastic in an electric field could lead to a new generation of brakes and clutches for cars. The material is already being tested in a prototype clutch.
Twenty years ago, car makers had high hopes for such 鈥渆lectro-rheological鈥� (ER) materials. They have the potential to dramatically simplify vehicles鈥� design by allowing the same fluid that lubricates moving parts to be used to lock them in place and then release them at will.
But enthusiasm has waned because the materials could never be made stiffer than bean curd, says Ping Sheng of the Hong Kong University of Science and Technology. He says a new material developed by his team performs much better: 鈥淥ur material can go from water-like to as hard as hard plastic.鈥�
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ER materials rely on the ability of certain materials to become highly polarised in an electric field. When particles of these materials are suspended in an insulating liquid, the mixture behaves like a fluid. But apply an electric field and the particles become polarised, lining up in rows, positive end to negative end, which stiffens the material.
Sheng and his team coated particles of barium titanyl oxalate measuring several tens of nanometres across with a layer of the simple chemical urea just a few nanometres thick. The size of the particles and the thinness of the urea layer make all the difference by allowing the coated particles to become highly polarised and stick together more tightly. This is crucial, because the electric forces involved are far stronger on these nanoscales than at longer distances. Sheng鈥檚 material produces up to 20 times the stiffening achievable with previous ER materials (Nature Materials, DOI: 10.1038/nmat993).
Engineers tended to prefer magneto-rheological materials over ER materials because they produce a much wider range of stiffness. But the electromagnets needed to activate them tend to be power hungry. Weihua Li of the University of Wollongong in New South Wales, Australia, says Sheng鈥檚 new material may now have the advantage. A square metre of the new material requires less power than a light bulb to maintain its stiff state.
