Scientists Develop Self-Cleaning Windows

If you don't like to "do windows," you're going to love Bharat Bhushan.

The Ohio State University professor of mechanical engineering is developing technology that could result in windows that actually clean themselves.

Now, clean windows aren't exactly at the top of Bhushan's worry list, but it turns out that the same technology that can reduce friction in the moving parts of tiny devices could also keep your windshield clean.

And we know that because awhile back Bhushan got all worked up over lotus leaves.

Stay with me here.

"The primary application is nano-technology," Bhushan said.

You might say he lives in a very small world, where new devices are being developed at a rapid rate on a scale that almost defies imagination. Motors so small they can power robots smaller than a dime. Sensors so tiny they can't be seen, but so sensitive they can detect even traces of toxic chemicals.

All those tiny gizmos face a common problem, friction. And since it isn't possible to climb inside with an oil can, some way has to be found to virtually eliminate friction between moving parts for the lifetime of the device.

But how do you do that?

Like Water Off a Leaf

For an answer, Bhushan turned to the fabled lotus leaf, because the surface of a lotus leaf seemingly defies the laws of friction. Even water just rolls right off the leaf.

"Nature is doing something right," Bhushan said during an interview while he was waiting to catch an airplane. "So I said, let's see if we can mimic it, and even improve upon nature. That's hard to do."

But as he studied the lotus leaf, he came to a conclusion that has been reached by others. The lotus leaf sheds water because it is what scientists call "hydrophobic." That's a Greek term meaning, literally, fearful of water.

That's important, Bhushan says, because in order to have low friction, a surface has to be hydrophobic. "In general, what's good for water-repellency is good for fighting friction," Bhushan said.

"You need the surface to be hydrophobic if it is to have low friction," he said, because fluids, and even tiny particles, have more difficulty holding on. It turns out that a surface that is a little rough works better than one that is perfectly smooth.

Helped by Tiny Bumps

The lotus leaf is able to send water on its way because it is covered with tiny bumps that prevent the water droplets from attaching themselves to the surface. The droplets are kind of suspended above the surface as they ride on the bumps, so they just slide right off.

Scientists have known that for some time, Bhushan says, but what they haven't known is exactly what size bumps work the best.

"What's the optimum size?" he asked. "That's what got us started."

So he and his colleagues at Ohio State developed a computer model that is helping answer that question. It varies, of course, depending on the materials used and the specific application, but in general the answer is very small. It seems that the best bumps are just a few millionths of a meter in size.

Some manufacturers have already recognized that, so there are commercial products that can be sprayed on a windshield, or a micro-gear, that use tiny particles to reduce friction. But that's not good enough for Bhushan.

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