Sweating the small stuff Home

'Nanograss' Turns Sticky to Slippery in an Instant

by Kenneth Chang

With possible applications in everything from microscopic plumbing to slick boat hulls to switches for optical networks, a new chameleonic material developed at Bell Labs sheds water droplets like a newly waxed sports car, but, at the flick of a switch, turns absorbent like a "quicker picker upper" paper towel.

Depending on the chemical structure of a solid, water and other liquids either cling to it -- making it wet -- or it repels them. Usually a surface is absorbent or repellent, but not both.

"What we're trying to do is make a surface which you can control on the fly," said Dr. Tom N. Krupenkin, a scientist at Lucent Technologies' Bell Labs who led the research. "If you can change that on the fly, it opens up applications everywhere."

Writing in an article that will appear May 11 in the chemistry journal Langmuir, Dr. Krupenkin and his collaborators at Bell Labs and the University of Pennsylvania describe carving a microscopic bed of nails out of a piece of silicon. Bell Labs, using a more bucolic metaphor, calls it "nanograss."

The nails, each about one three-hundredth the width of a human hair and about one four-thousandth of an inch tall, were coated with a polymer that repels water.

Next, the scientists placed droplets of liquid -- the scientists tried several including water and alcohol -- on the surface. The droplets, much larger than the space between the nails, lay intact on top, much like a person lying on a bed of nails. Because of the nonstick coatings on the nails and with just the tops of the nails touching the liquid, the droplets turned into little balls that rolled easily like tumbleweed.

An electrical voltage applied to the silicon generates an electric field that draws the liquid down into the spaces between the nails, wetting the surface. "So it becomes more like a sponge," Dr. Krupenkin said.

The new technology gives engineers the ability to control whether liquids stick to a solid surface or slide along it, said Dr. David Bishop, vice president of nanotechnology at Bell Labs in Murray Hill, N.J. "You can start to tune that interaction in novel sorts of ways."

Dr. Michael L. Steigerwald, a research chemist at Columbia University who was not involved in the research, said the Bell Labs work was "really very clever" and should allow the study of the fundamental physics of how a liquid wets a solid surface.

"It's just a lot of unexplored territory," he said.

Potential applications include tiny batteries that use the nails to hold apart the chemicals until precisely when electricity is needed. The surface could also help to cool future computer chips, where droplets sink in just in the spots that are hot. Installing a surface of the tiny nails on the exterior of boats or a torpedoes could allow them to slice through water more easily. It could also be used for filters and switches for optical networks by moving droplets in and out, turning a surface from clear to opaque.

Dr. Krupenkin said he also imagined that the technology could be used to control reactions of droplets that contain different chemicals. He said many such applications could be realized in a few years.