The legend of the aerogel is shrouded in mystery. What we do know is that in the late 1920s, American chemistry professor Samuel Kistler had a bet with colleague Charles Learned. Kistler believed what made an object a gel was not its liquid properties but its structure: specifically, its network of tiny, microscopic pores known as nanopores. Trying to prove this by simply evaporating the liquid led to the gel deflating like a soufflé. So, the object of the game was to be the first to replace the liquid in "jellies" with gas, but without causing damage to the structure [source: Steiner, Zero Gravity].
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After much trial and error, Kistler was the first to successfully replace the gel's liquid with a gas, creating a substance that was structurally a gel, but without liquid. By 1931 he published his findings in an article called "Coherent Expanded Aerogels and Jellies" in the scientific journal Nature [source: Ayers, Pioneer].
Aerogel begins as a gel, called alcogel. Alcogel is a silica gel with alcohol inside its pores. Simply evaporating the alcohol out of the silica structure would cause the structure to contract, much like a wet sponge will deform when left on a counter to dry. Instead of relying just on evaporation, the gel has to be supercritically dried. Here's what it takes:
- Pressurize and heat the gel past its critical point -- the point at which there's no difference between gas and liquid.
- Depressurize the gel while it still remains above its critical temperature. As the pressure decreases, molecules are released as a gas and the fluid grows less dense.
- Remove the gel from your heat source. After the structure cools, there's too little alcohol to recondense back into liquid, so it reverts to a gas.
- Check out your final product. What's left behind is a solid made of silica, but now filled with gas (air) where there was once liquid.
Supercritical drying is how the liquid "alco" part of the alcogel turns into a gas within the silica's nanopores without the structure collapsing. The alcogel with its alcohol removed is now called aerogel, as the alcohol has been replaced by air. With only 50 to 99 percent of the original material's volume, aerogel is a light, flexible and useful material [source: Steiner, Zero Gravity].
Continue to the next page to learn about the most common types of aerogels in use today.