The Solid That Is Almost Entirely Air

Step 1 of 5

It is a solid the way a cloud is a solid.

A typical silica aerogel is 99.8 per cent air by volume. The remaining fraction is a continuous skeleton of glass strands, each a few millionths of a millimetre thick, holding the whole shape up. You can rest a block on a flower and the flower will not bend far.

Step 2 of 5

The making is the whole story, and it is a drying problem.

Start with a wet gel — a jelly, essentially, with a solid mesh holding liquid in its pores. The mesh you want already exists at that point. The entire difficulty is getting the liquid out and leaving the mesh standing.

Step 3 of 5

Let it dry normally and it destroys itself.

As a liquid evaporates from a pore, its surface curves and pulls inward. Those surface-tension forces are tiny, but the strands are tinier, and the gel crushes itself into a dense shrunken lump. The fix is to heat the liquid under pressure until the boundary between liquid and gas simply stops existing — supercritical drying. With no surface, there is no surface tension, and nothing pulls.

Steven Kistler settled this in 1931 by betting that the liquid, not the solid, was the problem — and won the bet.

Step 4 of 5

All that trapped air makes it the best insulator we have.

Heat crosses a material three ways, and aerogel obstructs all three. The pores are too small for air to circulate, the strands are too thin and tangled to conduct well, and the material can be made to scatter infrared. That is why it has flown to Mars, insulated the pipes on the seabed, and caught comet dust in space.

Step 5 of 5

So why isn't everything made of it?

Because it is brittle, dusty and expensive. Silica aerogel crumbles under a fingernail and the supercritical drying step needs a pressure vessel. The engineering answer has been to give up on pure blocks. Modern aerogel insulation is a blanket of fibres with aerogel grown through it, which bends, holds together, and costs rather less.