Peter Tsou looking at a large piece of aerogel at Jet Propulsion Laboratory, California Institute of Technology

Where is the border between air and solid?  To put it in other words how much material does it take to make a solid?  Aerogel is a very interesting material that is made of over 75% air by weight and over 99.9% by volume.  It holds 15 world records including being the best insulator and lowest density solid.  It is made of silica which is a main part of glass and quartz crystals, but what separates aerogel is the silica structure is in spherical particles between 2-5 nanometers in size.  These particles are then arranged in lose three dimensional structure that contains pockets of air.  Think of a milk crate, the crate is strong enough to stand on but it is mainly open space between plastic structure.  These nano particles of silica are not neatly arranged as the milk crates plastic but the same general effect occurs.

A flower resting on a thin aerogel block as a bunsen burner is heating the bottom of the aerogel.

Why is this useful?  Well this material is used in many cases that need a really good insulator from heat.  The empty structure means that it is hard to transfer heat across.  Think about a cup of hot chocolate, if you hold you hand a centimeter or so way you can barely feel the heat from the cup but if you touch it, you definitely feel it.  This is because air as a gas has a very low density and each of the molecules do not interact with one another.  Most solids such as you and the mug the atoms and molecules are tightly packed and easily interact and transfer energy.  But in aerogel there is mostly air and not much actual contact between spherical nano particles of silica.  This leads to slow transfer of heat from one side to the other of the solid.

Now you may be thinking why is not air or vacuum the perfect insulator?  This is because there is a second type of heat transfer called radiant heat.  All things emit electromagnetic radiation better known as light.  This emitted radiation from heat is called blackbody radiation.  At room tempature most of this light is in the inferred which is below the range of human eye sight.  But if you have seen a red hot object, this red color comes from the heat emitting light in the range that we can see.  Another nice property of silica is that it is good at absorbing inferred spectrum.  This means that even radiant heat is absorbed into the aerogel and does not pass through.  This helps make aerogel one of the best insulators.

The collector arm from the Stardust spacecraft, filled with aerogel blocks.

Aerogel is being used as heat insualtion and as a high surface area material in super compaciters.  Also the Stardust spacecraft brought back samples of comet dust in aerogel.  NASA needed a low density material to expose to comet dust that would not destroy the comet dust.

For more information:
Wiki: Aerogel
Blog: Aerogel
Stardust Spacecraft