Pitt scientists think they might have found a way to combat freezing rain, making it safer to… Pitt scientists think they might have found a way to combat freezing rain, making it safer to travel during the winter.

Pitt researcher Di Gao and doctoral student Liangliang Cao think they’ve developed a solution that can keep freezing rain from forming ice when it hits a surface.

“Freezing rain” does not mean just very cold rain, though it certainly is very cold. The term refers to rain that falls while still at a temperature below water’s freezing point. Once this rain finds something to stick to — like roads or your windshield — it turns into ice.

Some methods already exist to combat freezing rain. Vehicles and handcarts full of chloride salts scatter their contents over Pitt’s sidewalks and streets every winter. Airlines spray polymers over their plane’s wings to keep ice from forming on them. Many of these compounds work by lowering the freezing point of water, meaning it must get colder than usual for ice to form.

Gao’s compound is different because it actually prevents the freezing rain from forming ice when it hits a surface.

Freezing rain is water that exists in what Cao described as a “meta-stable state.” You can cool water to minus 20 degrees Celsius until it forms ice on its own. Until that point, Gao said, it needs a “nucleation site” to stick to something.

“If some kind of freezing rain touches the solid surface, that meta-stable state will be destroyed and will instantly form ice,” Cao said. “So our idea is that some kind of superhydrophopic [really water resistant] surface can actually prevent the contact between the solid and the water.”

Inspired in part by the tiny ridges that naturally occur on water-resistant lotus leaves, Gao’s silicone-resin solution prevents this contact on a nano-metric level.

Gao’s solution is spray-on, and the silicone particles, or silica, form the solid that the spray becomes. These nanoparticles are anywhere from 10 nanometers to 1 micrometer in size. With the help of the rubber-like resin, they clump together and resist the water.

“You can imagine [the rain] suspended on these hills and valleys,” Gao said. “Only the top of the hill is in contact with water. The rest of the water is actually in contact with air.”

When Gao and Cao spray the solution on a surface, water can touch only about 10 percent of it. A video at www.pitt.edu/news2009/ice.html shows super-cooled water forming ice on two aluminum plates, one of which is covered in the solution. Ice piles up on the plate without the solution, and it can’t be shaken off afterward. Water mostly slides right off the plate with the solution, and the little ice that does form is easily shaken off.

Gao and Cao still need to tweak their solution a bit before people can begin using it.

Cao said researchers need to improve the solution’s “robustness.” Though hard to feel from stepping on or touching it, you could take a knife to the stuff and scrape it off, a state for which constant foot or vehicular traffic would not bode well if it were sprayed on roads.

Another possibility is to make it more transparent so it can be used on windows or to test its effects on an object’s aerodynamics and consider using it on airplanes.

Gao said his and Cao’s solution can stick to something for months, even a year. Ethylene glycol and other compounds that are sprayed on airplanes and elsewhere need constant reapplication and can sometimes be hazardous to plant and animal life.