Aerogel – Weird Stuff Indeed

Using the Right Bait to Catch a Comet describes Aerogel—super light weight, least dense material, hard to see straight, yet a great insulator and hard to crush. It sounds cool. I want some.

At around 0.003 grams per cubic centimeter of material — only about three times as dense as air — aerogel is pure silicon dioxide. Not only is it the least dense solid in existence, it is also such a remarkable insulator that a layer of it surrounds the most vital electronics on the Mars rover Spirit. But the most striking feature, at least to the naked eye, is that up close, the cube looks like a blurry hologram.

“When you look at this,” says Dr. Tsou, holding the aerogel up, “you don't know where to focus. That's why some people call it solid smoke.”

Made of 99.6 percent empty space, the little cube is indeed barely there, with a density one-hundredth that of the hand that holds it.

To make this strange material, scientists start with a liquid alcohol like ethanol and mix it with silicon dioxide to form a gel. Then, through a process called supercritical drying, the alcohol is forced out of the gel, typically with high-pressure carbon dioxide. With this drying process, the gel does not collapse or lose its volume. It appears holographic because the silicon dioxide scatters shorter wavelengths of light much like air in the daytime sky.

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“It has 14 Guinness Book of World Records-type properties,” Dr. Tsou said. “It's the lowest density of any solid, and it has the highest thermoinsulation properties. Though it would be very expensive, you could take a two- or three-bedroom house, insulate it with aerogel, and you could heat the house with a candle. But eventually the house would become too hot.”

Additionally, aerogel slows soundwaves to about 10 percent of their speed in air, and because it has such a vast surface area for its volume, its use as a filtration agent could increase the capacity of desalination plants a thousandfold.

Because aerogel is transparent and releases light when struck by certain high-energy radiation, it provides an excellent means of counting atomic particles. It also has incredible compressive strength. “It can take 2,000 times its body weight without damage,” Dr. Tsou said. NASA's Web site shows a 2-gram cube of aerogel (less than 0.1 ounce) supporting a 2.5-kilogram brick (about 5.5 pounds).

What a shame it's so expensive.

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