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Water-Repelling Metals

New metals will keep engines and turbines dry and ice-free.
October 15, 2008

Researchers at GE have come up with a way to treat metals so that they repel water. The extreme water-repelling property, called superhydrophobicity, means that water forms drops on the surface instead of spreading and sticking to it.

Staying dry: A chemically treated plastic surface is rough on the nanoscale, forcing water droplets to form beads that can roll off. GE researchers have now done the same with metal.

The advance builds on previous work that came out of GE’s Global Research Center, in Niskayuna, NY. Two years ago, researchers showed that they could make Lexan–a widely employed plastic that’s used to create CDs, iPods, aircraft windscreens, and car headlamps–water-repellant. They did this by chemically treating the surface to make it rough. The researchers have now demonstrated the same effect on metal surfaces.

Many other superhydrophobic materials have been demonstrated, but most have used some kind of plastic. Superhydrophobic metals open up many new applications, says Jeffrey Youngblood, a professor of materials engineering at Purdue University. “Metallic structures are more robust and can survive in harsher environments, allowing for their use in applications where plastic is infeasible, [such as in] planes, trains, automobiles, heavy machinery, and engines,” Youngblood says.

GE has some ideas about how to use the materials. One possibility is in de-icing aircrafts. Ice buildup on engines due to condensation can be catastrophic. Right now, aircraft use heat to prevent ice, which requires power. De-icing on the ground, meanwhile, is done with de-icing fluids, which contain toxic chemicals; spraying aircraft with de-icing fluids on the ground also takes a lot of time. “It would be very desirable if we could … just be able to have a material on which ice didn’t stick,” says Margaret Blohm, advanced technology leader for the nanotechnology program at GE’s Global Research Center.

Another application for the metals could be in gas and steam turbines. The superhydrophobic metals could reduce the buildup of moisture and contaminants on the turbines, increasing their efficiency and requiring fewer shutdowns for maintenance.

GE researchers have not published their work, and they declined to divulge much about their research achievements. But they do say that their inspiration comes from lotus-plant leaves, which have a nanocrystalline wax structure. On the leaf’s surface are tiny wax crystals tens of nanometers wide, which hold water drops as almost perfectly spherical beads.

Blohm says that the team is toying with two different approaches to making the metals. One is to texture the metal surface and then put a water-repelling chemical coating on it. The other approach is to leave the metal surface untouched and texture the coating itself. The technique is very general and should work with metals currently used for engines and turbines, such as titanium alloys.

The material’s robustness will be key because of the high-performance applications that GE is targeting, says Gareth McKinley, a mechanical-engineering professor at MIT. He thinks that out of the two different approaches to making the superhydrophobic metal, altering the material surface itself would last longer. With a coating, he says, “there’s a possibility that it’s going to come off or flake off. So something intrinsic to the material will be more robust.”

Blohm says that both approaches–roughening the metal or coating it with a textured material–might have their advantages, depending on how the material is used. “Most of the environments we’re looking at with metals are rather harsh, whether it’s temperature, moisture, corrosion, or other contaminants,” she says. “So in some applications, you might choose textured metals that might be more robust, but in others, you might want to have the coating carry the performance with options to replace the coating.”

The researchers are testing many different models of superhydrophobic metals. They are tinkering with the texture of the metals and coatings to see what works best in certain harsh environments. The material would eventually have to be tailored to the application, Blohm says. “If we feel good about [a material]–one we know that might be more expensive and maybe not robust enough for the environment, but we see performance in those model textures–then it’s worth the investment,” she says. “Then we’ll work on making it manufacturable and robust in a specific environment.”

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