Aerogel: The Frozen Smoke That Stops Blowtorches and Catches Comets

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Aerogel can be up to 99.98 percent empty space, yet a sheet of it will shield a flower from a blowtorch and a two-gram piece will hold up a two-and-a-half kilogram brick. This episode starts in 1931, when Samuel Stephens Kistler accepted a bet from his colleague Charles Learned: remove all the liquid from a jelly and replace it with gas without letting the solid structure shrink. Kistler won with supercritical drying, pushing the liquid past its critical point so that surface tension vanished and the fragile framework survived.

From there the episode explains why the material behaves so strangely. Rayleigh scattering gives silica aerogel its smoky blue tint, fractal chains of particles two to five nanometers wide spread loads like billions of tiny arches, and pores under 100 nanometers trap air so tightly that the Knudsen effect makes the solid a better insulator than the gas inside it. The same architecture can be built from carbon, metal oxides, seaweed, cellulose, and graphene, each with its own uses.

Bare silica aerogel is so hygroscopic that handling it without gloves pulls water out of your skin and leaves dry, brittle patches.
Carbon aerogels, developed in the late 1980s, are pitch black, conduct electricity, and are used in supercapacitors.
NASA's Stardust mission used blocks of silica aerogel to stop hypervelocity comet dust intact, each grain resting at the end of a cone-shaped track.
Chalcogels made from sulfur and selenium bind mercury, lead, and cadmium, filtering heavy metals out of contaminated water.
An aerogel built from whey protein fibrils, a cheese industry byproduct, can pull gold ions out of dissolved electronic waste.

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