Aerogel

A solid that is 99% air — yet it holds a car's weight, insulates a Mars rover from −100 °C nights, and caught comet dust moving at 6 km/s without destroying it.

Aerogel insulates against heat flow better than any other solid. A slab a few centimetres thick can separate a surface at 650 °C from one at room temperature with a temperature drop of hundreds of degrees. It also absorbs sound, filters particles, and — because its pores are smaller than the mean free path of air molecules — suppresses gas-phase heat conduction below that of still air, which is already an excellent insulator.

Aerogel is made by forming a gel — typically from silicon alkoxide precursors in a sol-gel reaction — then removing the liquid without collapsing the porous skeleton. Ordinary drying collapses the pores by surface tension; supercritical drying (Kistler's method, 1931) brings the liquid above its critical point and vaporises it, leaving the skeleton intact. The result is a silica network with pore sizes of 20–40 nm and porosity up to 99.8%. At those pore sizes, gas molecules can barely travel one pore diameter before hitting a wall — the Knudsen effect — so gas-phase conduction drops dramatically below its free-space value. The remaining contributions are solid conduction through the wispy silica skeleton and infrared radiation, both minimised by the low density and opaque dopants in insulation products.

In plain terms

Imagine a sponge made not of foam but of a web of glass strands each a thousand times thinner than a human hair, with the tiny holes filled with air instead of water. The whole thing feels almost weightless, yet the glass skeleton is so tortuous that heat can barely find a path through it — like trying to run through a maze where every turn leads to a dead end.

Also called: silica aerogel, frozen smoke, solid smoke, blue smoke.

Why it matters

Where it lives

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Surprising facts

A short history

Samuel Stephens Kistler, working at the College of the Pacific and later the University of Illinois, published 'Coherent Expanded Aerogels and Jellies' in Nature (127, 741) on 16 May 1931. He had accepted a bet with colleague Charles Learned that it was possible to replace the liquid inside a gel without shrinkage. After failed attempts with supercritical water — which dissolved the silica — he washed the gel with alcohol and then took the alcohol past its critical point, allowing it to be removed as vapour and leaving the gel skeleton intact. Early aerogels were milky and fragile. Commercial production of silica aerogel under the Santocel brand by Monsanto began in the early 1940s. The modern era of aerogel research restarted in the 1970s–80s when Stanislas Teichner's group at the University of Lyon used a one-pot sol-gel alkoxide route that compressed the synthesis from weeks to hours. NASA's Jet Propulsion Laboratory developed low-density aerogels specifically for the Stardust particle-capture mission, launched in 1999, which returned cometary samples in 2006.

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