Aerogel must be one of the strangest supermaterials to ever exist. Ghostly and shimmering in appearance, it's insanely light, incredibly strong, and an amazing thermal insulator. And its tricks look absolutely impossible when you see them up close.
Aerogel material—often referred to as frozen smoke, solid smoke, solid air or blue smoke—is actually, like all the best things, the result of a wager. It was first created by Samuel Stephens Kistler in 1931, after he bet Charles Learned that he could replace the liquid in a jelly with gas, without causing shrinkage. Turned out, he was right!
To produce an aerogel, you take a normal gel and then—very slowly and carefully—remove the liquid, leaving behind just the solid structure. That process varies depending on the gel in question, but invariably requires some complex chemistry to facilitate removal of the liquid by supercritical drying, which carefully avoids the liquid-gas transition by using pressure and temperature variation to go from liquid to solid to gas instead. Otherwise, the evaporation process can destroy the structure. The result is a substance that looks like the original gel but feels like expanded polystyrene to the touch.
And the material properties! Oh, the material properties. Just look at what it can do.
All the trapped air in aerogel makes it a remarkable insulator. In fact, silicon aerogel has a thermal conductivity of about 0.03 W/mK in atmospheric pressure down to 0.004 W/mK in modest vacuum—values similar to those exhibited by air itself. Hold it to a flame, and you won't notice much happen, either—silica aerogel doesn't melt until it reaches upwards of 2,000 °F. No, really. Look.
It's not all silica gel, though. Yes, graphene aerogel sounds like someone combined the two buzziest of materials buzzwords—but the results are amazing. In fact, this graphene aerogel snatched the title of the world's lightest material just a few of months ago—with a density lower than that of helium and just twice that of hydrogen at 0.16 mg/cm3. This stuff practically floats. (Incidentally, it's got air inside, which means that it doesn't.) The material was created using a new technique which involves freeze-drying solutions of carbon nanotubes and graphene to create a kind of carbon sponge. The resulting material is both strong and elastic, as well as incredibly light.
Silica aerogel is also oddly strong. Just look at this image: it shows a 2.2-kilo brick supported by a piece of aerogel which weighs just 1.9 grammes. That strength is a results of—brace yourself for this—its dendritic microstructure. All that means is that it's made up of roughly spherical, nanoscale particles which are fused together in clusters. Those clusters are strung together in 3D shapes which are almost fractal, providing an endlessly complex and strong structure. It can even support the weight of a car:
For all of these amazing properties, though, aerogel is still insanely expensive to manufacture, which is why there aren't any consumer products made using the stuff. Yet. In the meantime, let's just daydream of super-light, flame retardant, bullet-proof suits of armour. Not bad for a little bit of frozen smoke.
Images by NASA, NASA/Caltech and Zhejiang University