The aerogel' or frozen gel is a colloidal material similar to the gel, in which the liquid component is changed by a gas, obtaining as a result a solid of very low density (3 mg/cm³ - milligrams per cubic centimeter - or 3 kg/m³ - kilograms per cubic meter -) and highly porous, with certain very surprising properties, and enormous thermal insulating capacity.

This material is generally composed of 90.50 to 99.98% air, and is one thousand times less dense than glass and about three times more dense than air. Familiarly, it is called icy smoke, solid smoke or blue smoke, due to its semi-transparent nature; However, it has a consistency similar to Styrofoam to the touch.

It has a refractive index of 1.0, very low for a solid. The speed of sound through it is very low, 100 m/s (meters per second).

This material was created by Steven Kistler in 1931,[1]​ as a result of a bet between him and Charles Learned over who could replace the liquid in a jam jar with gas without decreasing the volume of the jam.

Airgel can be made from very different materials; Kirstler's research consisted of aerogels based on silica (Silicon (IV) oxide), zirconium, alumina, chromium oxide, tin and carbon.

In 2013, Gao Chao and his research team at Zhejiang University in China developed graphene airgel,[2][3]​ whose density of 0.16 kg/m³ (kilograms per cubic meter) is the lowest ever achieved.[4]​.

Aerogels can be organic or inorganic, they can be organic-inorganic hybrids, or they can be carbon. Most are prepared using the Pekala method, in which resorcinol, formaldehyde and a catalyst are used, where a polymerization reaction is generated. Aerogels must go through stages of gelation and curing, supercritical drying, carbonization and activation.

The fundamental variables that control each of these stages determine the properties of the carbon gel obtained. Among these variables are: