Sunlight streaming through a window can really heat up a room. In winter, when heating bills can soar, people tend to welcome that extra warmth. But in summer, that heat just boosts cooling costs. A homeowner could keep out some of that warming light by drawing the curtains or lowering the blinds. Or the window could change its transparency — blocking out some light, as needed — all by itself. That’s the idea behind new “smart” windows.

Some smart windows already exist. They work just like large versions of the LCDs (liquid crystal diodes) found in watches and other electronic devices. When an electric current flows through an LCD window, a coating on the panes of its glass darken. That blocks out some of the light. A homeowner can control the window’s light-blocking ability — or opacity — simply by flipping a switch. Or, a sensor connected to the window can automatically control the current, just like the thermostat used to control a furnace or air conditioner.

But the new smart window does not require such electronics. It depends only on the temperature outdoors, says Xuhong Guo. He’s a chemical engineer at the East China University of Science and Technology in Shanghai. His team designed a new liquid that it sandwiches between two panes of window glass. The researchers describe how this makes their window “smart” in the December 3 issue of Industrial & Engineering Chemistry Research.

The key: A heat sensitive gel

The material that Guo’s team designed is a colloid. That’s a substance in which tiny particles or droplets that don’t dissolve are spread throughout a larger volume of some other material. (Smoky air is one type of colloid. Milk is another.) The larger part of the new mix is a blend of water and alcohol. Floating inside are tiny globs of a gel.

Each glob is only between 200 and 700 nanometers across. That makes the diameter of the thinnest human hair about 24 to 85 times wider than each glob. The gel contains a heat-sensitive polymer (a chemical made from chain-shaped molecules). It also contains water and glycerol, a type of alcohol. The water and glycerol attach loosely to the polymer. This keeps the gel from dissolving into the larger volume of liquid. This also ensures that the gel globs don’t react with each other to form one big lump of goo.

In the gel recipe that Guo and his colleagues use, the polymer changes shape whenever the temperature rises above 32° Celsius (about 90° Fahrenheit). At lower temperatures, the polymer’s molecules remain long and straight. This allows them to dissolve throughout the gel. Now, lots of light can pass through the gel, making it appear clear. But once the gel’s temperature rises above 32 °C, the polymer molecules coil into small balls. These can’t dissolve into the gel. That makes the gel look cloudy. When dispersed throughout the liquid in between the window panes, these globs now block some light.

For their tests, the engineers built small boxes to simulate rooms in a house. In one box, they installed a smart window. A second box had the same sort of liquid-filled window, but its liquid didn’t contain any globs of the light-blocking polymer.

The new smart window blocked one-fourth, or about 25 percent, of the visible light and infrared energy (heat) emitted by a sun lamp. “That made a big difference in the temperature inside the box,” Guo toldScience News for Students. The plain window reduced the temperature inside the lamp-lit box by 10 °C (18 °F). That’s largely because the liquid between the panes of glass absorbed some of the light’s energy, he explains. But his team’s smart window reduced the temperature inside that box by 20 °C — fully twice as much. Here, too, the liquid in between the window panes absorbed some of the lamp’s energy. But as the polymer-filled globs turned cloudy, more energy was blocked.

The globs turn clear again as soon as their polymer molecules uncoil. This occurs when they cool below 32 °C.

How smart can a window get?

It’s possible to design globs that block even more light, says Guo. When his team added tiny particles of a mineral called vanadium oxide to the polymer, the new smart window blocked 40 percent of the sun lamp’s light.

It also might be possible to essentially choose the temperature at which the polymer changes its shape, Guo says. Experiments show that increasing the proportion of glycerol in the gel globs, for instance, lowers the temperature at which the polymer changes shape.

The new windows are “a great example of researchers finding a new behavior for a material and then taking advantage of it,” says Robert Prud’homme. He’s a chemical engineer at Princeton University in New Jersey.

But further study will be needed to see if the team’s “smart” window is really a smart idea after all, Prud’homme adds. While a cloudy window blocks radiation, that’s not the only way energy gets transferred. Conduction is another way. In that process, energy is transferred when atoms and molecules bump into one another. During such collisions, the slower, colder particles gain energy from the faster, warmer ones slamming into them.

So it’s possible, Prud’homme says, that the liquid-filled layer might actually increase the total amount of heat transferred through the window. Only more research can settle that question. “It’s up to scientists to find out what is possible,” he says. Then, he adds, engineers must work out “what is practical.”

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