A polymer containing a color-changing molecule called a mechanophore turns red seconds before it snaps. The technology may one day allow damage to materials or structures to be easily spotted.

The color red often means danger — and by paying attention, accidents can be prevented. At railroad crossings, flashing red lights warn cars to stay back. A red light at a traffic intersection tells cars to stop, so they don’t run into other cars. And when a driver steps on the brakes, bright red taillights warn cars behind to slow down.

In the future, the color red also may help prevent danger at construction sites. Thanks to new work by engineers, bridge supports — or other kinds of materials — could one day contain a new kind of material that turns red before a structure collapses or falls apart.

The secret behind the color-changing material is a particular type of molecule. A molecule is a group of atoms held together by chemical bonds. Molecules come in all shapes and sizes, and make up everything you can see, touch or feel.
How a molecule behaves depends on what kinds of atoms it contains, and how they’re held together.

To get a rough picture of one way atoms are held together in a molecule, imagine you and your friends standing in a large circle, holding hands. Each person represents one atom, your clasped hands represent the bonds, and the entire circle represents a molecule.

The molecule being used to turn the material it’s in red is called a mechanophore. When one bond in the mechanophore molecule breaks, the rest of the molecule turns red. (Imagine your circle of friends again, and try to imagine that if two people let go, everyone turns bright red.)

When humans are injured, we bruise and our skin changes color. When a polymer containing a color-changing molecule called a mechanophore is about to break, it also produces a color. The process is similar. An injury breaks blood vessels under the surface

Kemter/iStockphoto

“It’s a really simple detection method,” says Nancy Sottos, one of the scientists who worked on the project. “We’re opening up this one bond, and it changes color.” Sottos works on the science of different kinds of materials at the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign.

Sottos and her team tested the color-changing molecule in two different kinds of polymers, long chains of similar atoms or molecules linked together. First, the team put the mechanophore into a stretchy, soft polymer — not so unlike a rubber band. When the researchers stretched the material, it turned bright red a few seconds before it snapped into two pieces. When they repeatedly stretched and relaxed the polymer, without breaking it, it started to turn red.

They also tested the molecule in beads of a brittle, glasslike polymer. When the beads were squeezed (but not hard enough to shatter), they turned red.

There is a way to get rid of the red color: light. When the scientists shone a bright light on the mechanophore, the broken bond was fixed — and the red danger sign disappeared. This “self-healing” may be a problem for engineers who want to use the color-changer in big construction projects that will be outside, in sunlight. And if bright light keeps the red hue from appearing, then the mechanophore’s warning system will be useless.

Sottos and her fellow scientists still have a lot of work to do before the color-changing molecules can be used outside the lab. If mechanophores can be used in the real world, she suggests employing them in a new kind of paint or even rollerblade wheels.

Going Deeper:

Goggles that fog up while you’re skiing can be a nuisance—maybe even dangerous.

The problems of fogging and glare don’t seem to have much in common. But materials scientists are finding that they can solve both problems at the same time. The secret is to put a coating with the right combination of chemicals and texture on the surface of the glass or plastic.

Clearing up fog

Fog forms on a mirror or window when water vapor in warm, moist air condenses to create tiny water droplets on the smooth, cool surface. Instead of letting light through, the droplets tend to scatter light in different directions. This makes it hard to see through the glass. What you do see looks blurry.

To deal with this problem, Michael Rubner considered the way droplets form on surfaces with different textures. He’s a materials scientist at the Massachusetts Institute of Technology.

Like many researchers who work on materials, Rubner was inspired by nature. In this case, he looked at the leaf of the Japanese lotus flower, which causes water to bead into rounded droplets (see “Inspired by Nature” and “Butterfly Wings and Waterproof Coats“).

A lotus flower and leaves with water droplets.

U.S. Fish and Wildlife Service

The surface of a lotus leaf is waxy and filled with tiny holes, so it looks a bit like a sponge. The waxy substances on the leaf’s surface repel water. At the same time, air trapped in the holes keeps water from sticking.

To create a coating that prevents fogging, Rubner’s idea was to replace the waxy substances with chemicals that attract water.

Making a nanosponge

To make the new anti-fog coating, Rubner and his coworkers use a material called silica. Silica consists of the elements silicon and oxygen. It’s found in most kinds of rock, and it’s the main chemical compound in sand and glass. Silica also tends to attract water molecules.

The researchers work with tiny silica particles, each one just a few nanometers wide. A nanometer is one-billionth of a meter. In comparison, a human hair is about 80,000 nanometers wide. A nanometer-sized particle is much smaller than a living cell and can be seen only by the most powerful microscopes available today.

The scientists form the coating layer by layer by dipping a glass surface into a mixture of water and silica (or glass) nanoparticles, then into a mixture containing a type of plastic, or polymer. The plastic substance acts like glue, holding the glass particles together. The final coating has as many as 20 thin, alternating layers of polymer and glass particles.

The result is a coating with lots of air pockets—like a thin “nanosponge” made of glass, Rubner says. Yet, the particles and holes are so small that the coated glass still looks and feels smooth.

When exposed to warm, moist air, the coated glass surface attracts water. Instead of beading into rounded droplets, however, the water gets sucked into the holes all over the surface. This spreads out the water, and the resulting water film doesn’t scatter light in the same way that droplets do. You can still see through the glass.

A coated glass slide (left) shows the lotus flower clearly, while an untreated slide (right) fogs the view.

Rubner

A glaring improvement

The new coating also changes the way in which light interacts with glass. Light moves very quickly, but it moves more slowly through glass than through air. Because of this mismatch, when light hits glass, some of it reflects off the surface, leading to glare.

Because of the air pockets within the spongy coating, the coated glass surface acts like a mixture of glass and air when light hits it. This combination means that almost all of the light goes through the glass. Very little light is reflected. There’s no more glare.

In addition to improving Game Boy screens, this type of coating could lead to better windows for greenhouses, Rubner says. In a greenhouse with coated glass that reduces glare and resists fogging, more light gets in, so plants have more light for growth.

Stronger coatings

A coating isn’t practical if it scratches or rubs off easily, however. To make the materials more durable, Rubner and his team heat the coatings to a high temperature, about 500 degrees C.

Heating works fine for glass, but heated plastics will melt or even burn up. So, at this point, only materials that can stand high temperatures can be coated with the new anti-fog and anti-glare film.

Although it hasn’t been done yet, constructing a greenhouse using improved glass that cuts glare and resists fogging would allow more light into the chamber.

Peggy Greb, Agricultural Research Service, U.S. Department of Agriculture

In Australia, Paul Meredith and Michael Harvey of the University of Queensland have taken a similar approach. They also use porous silica—a thin layer of glass filled with tiny holes—as a coating. But their procedure for creating the coating is somewhat different.

Meredith and Harvey have even solved the problem that Rubner and his group are still working on. Their porous silica coatings are durable on both plastic and glass. The two researchers have formed a company, called XeroCoat, to make such coatings for solar cells.

In 2 to 5 years, you might be shopping for improved eyeglasses or ski goggles or riding around in a car with a windshield that resists fogging and reduces glare. It’d be a new window on the world, thanks to materials research.

Going Deeper:

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