Silicon Aerogel
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Guitars used to just be called guitars, but then electric guitars were invented and you had to start calling regular guitars "acoustic guitars" to distinguish them from electric guitars. Same thing with gels. A gel, as in gelatin, used to mean a sparse three-dimensional web of solid material supported by a liquid solvent. Now you have to call that a "solgel", to distinguish it from an "aerogel", in which the liquid solvent is replaced by air.
Aerogels are very, very light, a tenth of a gram per cubic centimeter or less (for comparison water is one gram per cc). They are often referred to as frozen smoke, an apt description if you've ever held one.
But the often-made claim that they are the least dense solid material strikes me as suspect. They are also said to have a very large internal surface area, and it seems to me that if something has internal surface area, then it's not solid.
While aerogels have a very modern NASA air about them, they are actually quite old: In the 1950's a model of refrigerator was available that used aerogel insulation!
Source: eBay seller oboyoberta
Contributor: Theodore Gray
Acquired: 5 January, 2006
Text Updated: 5 February, 2006
Price: $37.50
Size: 1"
Composition: SiO2