The Solid Material That's Mostly Air

Silica aerogel looks almost like solid smoke: a pale, translucent block light enough to rest on top of a flower without crushing it, yet rigid enough to hold its shape. Despite that appearance, it is made of the same basic material as ordinary glass or sand. What makes it different is structure rather than substance. Instead of forming a dense, continuous solid, the silica is arranged into a delicate network of interconnected nanoparticles, leaving pores just a few nanometers across throughout the material. As a result, more than ninety-nine percent of an aerogel's volume can be empty space, making it one of the lightest solid materials known. That structure is also what makes aerogel such an effective insulator, since heat can travel through a material in three different ways, and aerogel interferes with all three at once. Its pores are smaller than the average distance an air molecule travels before colliding with something, so air trapped inside cannot circulate and carry heat by convection. Because so little of the material is actually solid silica, there is barely any continuous path for heat to conduct through directly. And its structure scatters and absorbs the infrared radiation that would otherwise carry heat straight through. A material built almost entirely from empty space turns out to block heat more effectively than most solid ones.