Noun
aerogel (countable and uncountable, plural aerogels)
A porous, ultralight solid-state substance, similar to gel, in which the liquid component is replaced with gas.
Almost all solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Source: Internet
Virtually any fluid can be used as an extender including water ( hydrogels ), oil, and air ( aerogel ). Source: Internet