Aerogel

An aerogel is a low-density solid material derived from a gel by replacing the liquid component with a gas (air). The result is an extremely low density solid, with remarkable heat insulating properties. Aerogels feel like extruded polystyrene to the touch. They are nicknamed frozen smoke, solid smoke or blue smoke due to their semi-transparent appearance and the way light scatters in the material. [Longer wavelength red light is preferentially scattered, leaving a blue tint. A very similar effect colors the daytime sky blue.]

Aerogels were first made by Steven Kistler in 1931, following a bet with Charles Learned, over who could replace the liquid inside a jam (jelly) jar with gas, without causing shrinkage. The first results were silica gels. Kistler went on to experiment with many different materials, making aerogels based on silica, alumina, chromia, and tin oxide. Carbon aerogels were not developed until the early 1990's.

The most widely used aerogels are made from silica (SiO₂). The lightest solid known is desribed on Wikipedia as a silica based aerogel, with a density of 10_-3 g/cc. Since the gel is 99% air, the editor assumes that the density is around 2x10_-2 g/cc and the buoyancy effect in air accounts for the lower figure, which would properly be described as weight (?).

A 2.5 kg brick resting on Aerogel weighing only 2.38 grams.