The most diffuse solids are the aerogels, created by subjecting a conventional gel to supercritical drying. Supercritical drying allows the liquid to be extracted from the gel without collapsing the gel's matrix due to capillary action. Supercritical drying requires a high temperature (705 °F, 374 °C) and pressure (219 atmospheres). It forces the liquid to enter a state indistinguishable from a gas or liquid (a supercritical liquid), then heats it up and causes it to transform into a gas. The capillary action is prevented by avoiding a direct liquid-to-gas transition, as in evaporation. Aerogel was invented by Steven Kistler in 1931, allegedly as part of a bet with his friend to see who could extract the liquid from jam while still preserving its volume.
Aerogels is a remarkable material. Nicknamed "frozen smoke" for its appearance, aerogel is extremely diffuse, made of 99.8% air. Its density is only 3 milligrams per cubic centimeter (3 kg per cubic meter), 0.3% the density of water. Aerogel is 3780 times more diffuse than lead. Silicon aerogel is featured in an incredible 15 entries in the Guinness Book of Records, including most diffuse solid and best thermal insulator. Aerogel was recently used by NASA on a mission by the probe Stardust to capture small pieces of the comet Wild 2.
Despite its low density, aerogel can easily be handled. Riddled with 100 nanometer holes, it has a huge surface area, making it ideal for supercapacitors. Aerogel is such a fantastic thermal insulator that a thin layer of it can protect matches from being ignited by a blowtorch. Its high insulation capabilities have caused it to be proposed for use in space suits and for military armor.
Aerogel can be made using a variety of starting materials, including silicon, alumina, chromia, tin oxide, and more recently, carbon. Using carbon nanotubes, materials scientists have created an aerogel called carbon nanofoam, which has only 1% the density of previously produced aerogels, with only a few times the density of air. Carbon nanofoam is truly the most diffuse solid.
