Aerogel (fragments, stubby jar)

This is a rare sample of silica aerogel: a glass foam whose nano-structure contains up to 99.8% air, making it the world's lightest solid. Aerogels are a class of extremely porous solid materials that exhibit an array of superlative properties: they are barely denser than air, are incredibly good heat insulators, and, like a sponge with billions of minuscule holes, have a huge surface area. 

This ghost-like material seems to trick our sense of vision. With its ultra-low density, aerogel contains so little solid material that its edges look fuzzy, as if it wasn’t really there. It manages the confusing feat of being somewhat more ethereal yet less transparent than normal glass. This eerie material has an opalescent, cloudy quality to it, to the point where it is sometimes referred to as solid smoke. Against a dark background, it even appears slightly blue; placed in front of a light source, it looks slightly orange. And yet, no pigmentation involved: it is made of pure, clear glass! This is the same reason why the air around us is transparent, but the sky is blue and a sunset is orange. It is due to the way that light interacts with a material structured at the nanoscale. When light waves enter the maze of incredibly small pores of this nano-sponge, they echo and reverberate through the structure in a disorderly way. Instead of passing straight through, light is scattered wildly in all directions, causing the material to lose transparency. Blue light is strongly scattered and comes out of the material from every direction at once, while red/orange light is weakly scattered, passing almost straight through, in line with the original light source.

Aerogels were first produced in the 1930s as the result of an investigation on the nature of… jelly. To us, jelly is a deliciously sloshy, wobbly food; but to a scientist, it is a material confused as to whether it wants to be a liquid or a solid. The best hypothesis at the time was that jelly was made almost entirely of water, but that this liquid was trapped in a solid prison: an invisible internal mesh of gelatine. Scientists at the time wanted to know whether the gelatine skeleton would stand on its own if you could make the water disappear. It was easier said than done: when a gel dries through normal evaporation, the liquid pulls on the delicate interior walls as it recedes, squeezing the tiny, microscopic spaces shut, and causing a collapse of the fragile internal network. 

The introduction of a process called supercritical drying solved this issue in one stroke of brilliance. Supercritical fluids are an extreme state of matter which exists when liquids and gases get so compressed and so hot that these two, usually distinct states become impossible to tell apart. If the liquid in the jelly is pressurized and heated up into a supercritical fluid, and in turn, the supercritical fluid is cooled down and expanded into a gas, then the whole liquid has become a gas without ever having been through the destructive process of evaporation. And left behind is the delicate, highly porous skeleton, all in one piece… and full of hot air!

If gelatin was the OG aerogel backbone, this high-tech recipe doesn’t really care about what the skeleton is made of, so all sorts of different gel-like materials have since been turned into aerogels. They include other organic polymers like egg white (making a delicious aero-meringue!), metal oxides like alumina or zirconia, and of course glasses like silica, probably the most famous of the gang. Silica aerogels retain some properties of glass: they are fragile and hydrophilic. Add to that their sponge-like porosity, and one can understand why they are highly hygroscopic materials, meaning that they suck up and trap water like a very thirsty towel. Over time, water-logged aerogels become extra brittle and crumble away. Surprisingly, silica aerogels also emit a characteristic metallic ring when dropped. 

Aerogel could have had a bright life as some of the best thermal insulators in the world. A one-centimetre slice of aerogel is enough to dissipate the heat of a flame and maintain a temperature which is safe to touch. They are great at blocking convective heat (as hot air is trapped in the tiny pores and can’t rise) and conductive heat (as silica does not conduct heat well), but they’re almost useless against radiant heat (contrary to metals, infrared radiation passes right through them). But their complexity and cost made them hard to democratise. Instead, their superlative abilities were put to good use in thrilling scientific endeavours, first in some of CERN’s particle detectors, then in a NASA space mission to capture cometary dust. 

These small aerogel granules in a jar are bits and pieces coming from the manufacturing or the breaking apart of larger aerogels chunks. They can be touched, but know that the moisture on your fingers is likely to accelerate the breakdown of these fragments into even smaller pieces.