During the last decade on XXth century significant changes took place in the entire world from political to economical and technological point of view. The communism felt in Eastern Europe, it was a period of many economical, military and political unrest worldwide. It was also the time for many new things that happened in material science and technology in general. This was the late 1990s, a time when chinese espionage in the US national laboratories was a very sensitive issue. The US scientist Dr. Ho Lee had just been caught, indicted and put in solitary confinement on a charge of stealing nuclear secrets for China. He was eventually only charged with improper handling of secure data, to which he pleaded guilty, the judge eventually apologizing to him for the solitary confinement. After this incident there were a lot of strict measures on security issues and materials science labs all over the US were permanently under surveillance and the staff working there was regularly interviewed about the security issues related to their work. The american scientists working in such labs were under increasing pressure to report any conversation that were out of ordinary.
On 2 January 2004 , they announced on the TV news that the NASA mission to capture stardust had successfully engaged with the comet Wild 2. The news programme then showed a picture of a NEW MATERIAL. The material turned out to be a substance known as aerogel. It was the aerogel that was being used to collect the stardust. Aerogels were not of alien origin, they were invented in the 1930s by a man called Samuel Kistler, an American farmer turned chemist, who conjured them into existence solely to satisfy his curiosity about jelly. Jelly?
What was jelly, he asked. He knew that it wasn’t a liquid, but it wasn´t really a solid either: it was, he decided, a liquid trapped in a solid prison, but one in which the prison bars were like an invisibly thin mesh. In the case of edible jelly, the mesh is made from long molecules of gelatin, which is derived from the protein collagen, that makes up most connective tissues, such as tendons, skin and cartilage. When added to water, these gelatin molecules unravel and connect with one another to form a mesh that traps the liquid within it and prevents it from flowing. Thus jelly is basically like a water balloon, but instead of being an outer skin that holds the water within, it inhabits the water from the inside. The water is held inside the mesh by a force known as surface tension – the same force that makes water feel wet and form drops and causes it to stick to things. The surface tension forces inside the mesh are strong enough for the water to be unable to escape the jelly, but weak enough for it to slosh around – which is why jelly wobbles. It’s also why jelly feels so amazing in the mouth: it’s almost 100% water, and with a melting point
35°C the internal gelatin network promptly melts, freeing the water to burst in the mouth. The simple explanation – a liquid trapped by a solid internal mesh – was not enough for Samuel Kistler. He wanted to know whether the invisible gelatin mesh within a jelly was all of a piece. In other words, was it a coherent, Independent internal skeleton, such that if you could find a way to remove all of the liquid from it, the mesh could stand on its own?
To answer the question he conducted a series of experiments, the results of which he published in a letter to the scientific journal Nature in 1931 (No. 3211, Vol. 127, p. 741). The letter is entitled “Coherent Expanded Aerogels and Jellies”, and here is how he introduced the report:
“The continuity of the liquid permeating jellies is demonstrated by diffusion, syneresis, and ultra-filtration, and the fact that the liquid may be replaced by other liquids of very diverse character indicates clearly that the gel structure may be independent of the liquid in which it is bathed.”
What he is saying in this opening paragraph is that various experiments have shown that the liquid in a jelly is connected throughout, rather than being compartmentalized, and can be replaced by other liquids. This demonstrates, in his opinion, that the solid internal skeleton may indeed be independent of the liquid in the jelly. And in using the word ‘gel’, as a more general word for jelly, he is saying that this is true of a whole range of jelly-like materials that span the gap between being truly solid and truly liquid, from hair gel, to solid chicken stock to setting cement (where the internal mesh is formed by calcium silicate fibrils). He goes on to point out that no one had yet managed to separate the liquid of a jelly from its internal skeleton:
“Hitherto the attempt to remove the liquid by evaporation has resulted in shrinkage so great that the effect upon the structure may be profound.”
In other words, those in the past who have tried to remove the liquid by evaporation have found that the internal skeleton simply collapses. He then goes on to say triumphantly that he and his collaborators have found a way to do it:
“Mr. Charles learned and I, with the kindly assistance and advice of Prof. J. W. McBain, undertook to test the hypothesis that the liquid in a jelly can be replaced by a gas with little or no shrinkage. Our efforts have met with complete success.”
Their cunning idea was to replace the liquid with a gas while it was inside the jelly, and so use the pressure of the gas to keep skeleton from collapsing. First, though, they found a way to replace the water in the jelly with a liquid solvent (they used alcohol), which would be easier to manipulate. The danger of using a liquid solvent was that it too would evaporate, but they found a way to stop it:
“Mere evaporation would inevitably cause shrinkage. However the jelly is placed in a closed autoclave with an excess of liquid and the temperature is raised above the critical temperature of the liquid, while the pressure is maintained at all times at or above the vapour pressure, so that no evaporation of liquid can occur and consequently no contraction of the gel can be brought about by capillary forces at its surface.”
An autoclave is simply a pressure tank that can be heated. By increasing the pressure in the autoclave, the liquid inside the jelly is prevented from evaporating, even when the temperature is increased beyond its boiling point. The capillary forces he talks about, meanwhile, are caused by the surface tension of the liquid. Kistler speculates that, when the liquid is gradually removed through evaporation, it is these same forces, which hold the jelly together, that are responsible for tearing it apart. But when he raises the temperature of the whole jelly above so-called “critical temperature” – the point at which there is no difference between a gas or liquid because both have the same density and structure – the whole liquid becomes a gas without going through the destructive process of evaporation. He says:
“When the critical temperature is passed, the liquid has been converted directly into a permanent gas without discontinuity. The jelly has had no way of “knowing” that the liquid within its meshes has become a gas.”
This is a stroke of genius: under the pressure from the autoclave, the newly created gas cannot escape from the jelly and so the internal skeleton stays intact. “All that remains is to allow the gas to escape, and there is left
behind a coherent aerogel of unchanged volume.” Only now does he let the gas escape slowly, leaving the internal skeleton of the jelly completely intact and mechanically sound, thus proving his hypothesis. It must have been a very satisfying moment.
But he didn’t stop there. These internal skeletons of jelly were incredibly light, fragile things, comprised mostly of air. They were, in fact, foams. Perhaps he could make them stronger, he thought, by making a jelly not out of gelatin but out of something more rigid. So it was that he engineered a jelly in which the internal skeleton was made of the mineral silicon dioxide SiO2: the main constituent of glass. Using exactly the same process described above, he then created from this jelly a “silica aerogel”: the lightest solid in the world. This was the material that was kept secret in later 1990s by the US labs which later started to be widely used by NASA for its space exploration projects.
Not content with this achievement, Kistler went on to make other aerogels, and he lists them in the paper: “So far, we have prepared silica, alumina, nickel tartarate, stannic oxide, tungstic oxide, gelatine, agar, nitrocellulose, celulose and egg albumin aerogels and see no reason why this list may not be extended indefinitely”.
Note that despite his triumph with silica aerogel he couldn’t resist making an aerogel from egg albumin – that’s egg white, so while the rest of the world were using egg whites to cook light fluffy omelettes and bake cakes, Kistler did a different type of cooking using an autoclave to create egg aerogel: the lightest meringue in the world.
Silica aerogel looks extremely odd. Placed against a dark background, as in the photograph above, it appears to be blue, but put it against a light background and it disappears almost entirely. In this sense, it is harder to see – more invisible, even – than normal glass despite being less transparent. When light passes through glass, its path is distorted slightly – it is refracted – and the degree of distortion is known as glass’s refractive index. In the case of aerogel, because there is simply less of the stuff, light’s path is hardly distorted at all. For this same reason there is no hint of reflection on its surfaces, and because of its ultra-low density it appears to have no distinct edges, to not be fully solid at all. Which of course it isn’t. The internal skeleton of a jelly has a structure not unlike that of bubble bath foam with one main difference, which is that all of the holes link up. Silica aerogel is so full of holes that it is typically 99.8% air and has a density only 3 times greater than air, which means that it has practically no weight at all.
At the same time, when placed against a dark background silica aerogel is undoubtedly blue. And yet, since it is made from clear glass, it ought to have no colour at all. For many years, scientists wondered why this might be? The answer when it came, was rather satisfyingly odd. When light from the sun enters the Earth’s atmosphere, it hits all sorts of molecules (mostly nitrogen and oxygen molecules) on its way to Earth and bounces off them like a pinball. This is called scattering; which means that on a clear day, if you look at any part of the sky, the light you see has been bouncing around the atmosphere before coming into your eye. If all light was scattered equally, the sky would look white. But it doesn´t. The reason is that the shorter wavelengths of light are more likely to be scattered than ‘the longer ones, which means that blues get bounced around the sky more than reds and yellows. So instead of seeing a white sky when we look up, we see a blue one.
This Raleigh scattering, as it is called, is very slight indeed , so you need an enormous volume of gas molecules to see it: the sky works but a room full of air doesn’t. Put another way, any one bit of the sky doesn’t look blue but the whole atmosphere does. But if a small amount of air happens to be encapsulated in a transparent material that happens to contain billions and billions of tiny internal surfaces, then there will be sufficient Raleigh scattering off these surfaces to change the colour of any light that passes through it, silica aerogel has exactly this structure and this is where its blue hue comes from. So when you hold a piece of aerogel in your hand, it is, in a very real way, like holding a piece of sky.
Aerogel foams have other interesting properties, the most remarkable of which is their thermal insulation – their ability to act as a barrier against heat. They are so good at this that you can put the flame of a Bunsen burner on one side of a piece of aerogel and a flower on the other and still have a flower to sniff a few minutes later. Double glazing works by providing a gap between two glass panes which makes it hard for heat to conduct between them. Imagine that the atoms in glass are arranged like the audience in a rock concert, all packed together and dancing around. As the music gets louder and the audience dances more energetically people knock into each other more. The same happens in the glass: as the material heats up, the atoms jiggle about more. The definition of the temperature of a material is, in fact, the degree to which the atoms in it are jiggling around. In the case of double glazing, though, there is a gap between the two glass planes, which means that the jiggling glass atoms in one pane find it hard to pass on their energy to those in the other. Of course, this works both ways: the same double glazing can be used to keep heat inside a building in the Arctic and to keep it outside a building in Dubai.
Double-glazed windows work well enough, but they still leak a lot of heat – as anyone who lives in a hot or cold country knows by looking at their energy bill. Could we do better? Well, there is, of course, triple glazing and quadruple glazing which work by introducing a new layer of glass and so a new barrier to the heat transfer. But glass is dense, so these windows get heavier and bulkier and less transparent the more layers there are. Enter aerogel. Because it is a foam, it has within it the equivalent of a billion billion layers of glass and air between one side of the material and the other. This is what makes it such a superb thermal insulator. Having discovered this and other remarkable properties, Kistler reported them in the final sentence of his paper as folIows:
“Apart from the scientific significance of these observations, the new physical properties developed in the materials are of unusual interest.”
Unusual interest, indeed. He had discovered the best insulator in the world. The scientific community applauded briefly, but then promptly forgot all about aerogels. It was the 1930´s and they had other fish to fry; it was hard to know what would shape the future and what would be forgotten. In 1931, the year Kistler reported his invention of aerogels, the physicist Ernst Ruska created the first electron microscope. In the same edition of nature in which Kistler published his findings, the Nobel Prizewinning materials scientist William Bragg reported his findings on the electron diffraction within crystals.
These scientists paved the way for a new understanding of the inner structure of materials developing the tools with which to see and visualize them. It was the first time that a new microscope had been invented since the optical microscope of the 16th century and whole new microscopic world was opening up. Soon, materials scientists were peering into metals, plastics, ceramics and biological cells,and starting to understand how they worked from an atomic and molecular perspective. It was an exciting time, world of materials was exploding and materials scientist would soon deliver nylon, aluminium alloys, silicon chips, fiberglass and many other revolutionary materials. Somehow in all the excitement aerogels got lost and everyone forgot about them. Everyone except one man, Kistler himself. He decided that the beauty and thermal insulation properties of these jelly skeletons were so extraordinary that they should and must have a future. Although silica aerogel is as fragile and brittle as glass, for its weight (which is minuscule) it has good strength, certainly enough to make it industrially useful. So he patented it and sold the license to manufacture it to a chemical company called Monsanto Corporation. By 1948 it was making a product called Santogel, which´was a powdered form of silica aerogel.
Santogel seemed to have a bright future as the best thermal insulator in the world, but alas the time was not right for it. Energy was getting cheaper and cheaper, not more expensive, and there was no awareness of the problem of global warming an expensive thermal insulator like aerogel just didn´t make economic sense. Having failed to find a market in thermal insulators, Monsanto rather bizarrely found applications for it in various inks and paints, its role being to flatten them optically by scattering light, creating a matt finish. Aerogel finally ended up ignominiously being used as a thickening agent in screw-worm salves for sheep and in the jelly used to create napalm for bombs.
In the 1960s´and 1970s, cheaper alternatives usurped aerogel even from this rather limited repertoire of applications, and finally Monsanto gave up making it altogether. Kistler died in 1975 having never seen his most wonderful material find a place in the world. The revival of aerogels came not as a result of any commercial application but because their unique properties attracted the attention of some particle physicists at CERN studying something called Cherenkov radiation. This is the radiation given off by a subatomic particle when it travels through a material faster than light can travel through it. Detecting and analyzing this radiation gives clues to the nature of the particle and so provides a very exotic means of identifying which of the many invisible particles the scientists are dealing with. Aerogel is extremely useful for this purpose – providing a material through which the particle can travel – as it is, effectively, a solid version of a gas and it continues to be used for this today, helping physicist unravel the mysteries of the subatomic world. Once aerogels found their way into physicists’ labs, with their sophisticated equipment esoteric aims and big budgets, the material’s reputation started to grow again.