Aerogels, Jelly and Shield Charms

Chapter 5 of Mark Miodownik’s Stuff Matters is indeed Marvellous! The substance of interest – aerogels. What is an aerogel? Basically, it is a porous substance containing lots of air. The most common aerogel, made of silicon dioxide, has been around for a while. Also known as “silica aerogel”, it’s amazing that the same elements found in sand and glass can also be restructured to form the lightest solid in the world, made up of 99.8% air.

Silica aerogel is a good thermal insulator. The Wikipedia page has a neat pictures including one that shows a flower being protected from the heat of a Bunsen burner. Another picture shows the aerogel supporting a brick. In the same picture you can also see the translucent quality of the aerogel – it seems to almost fade into the background. It looks like a lightweight “nothingness”, yet is structurally stable, functions as a capable barrier, and acts as a superb insulator. Applications range from the mundane (window replacements or additions to glass) to the exotic (trapping cometary stardust at high speeds in a NASA project).

Miodownik describes the history and the process of making aerogels. I was surprised to learn that the basic idea was formulated and successfully carried out back in the 1930s by Samuel Kistler, a farmer turned chemist, “who conjured them into existence solely to satisfy his curiosity about jelly.” This made me think a little more about the strangeness of jelly as a substance. Apparently what makes it delicious: When you put it in your mouth, and it reaches its “melting point of 35^oC the internal gelatin network promptly melts, freeing the water to burst in your mouth.”

Silica aerogel is a bit more difficult to make than jelly. While sol-gel processes are familiar today, back in Kistler’s time constructing the aerogel was more complicated. The trick was to “replace the liquid with a gas while it was still inside the jelly, and so use the pressure of the gas to keep the skeleton from collapsing.” This was done by using an autoclave – basically a pressure cooker. It also takes advantage of the blurring of boundaries between a liquid and gas above its critical temperature. (To my P-Chem students this semester: If you’re reading this, here’s another reason why critical points are important!)

Having recently watched the 2015 Fantastic Four movie (it’s not that good; I waited to borrow the DVD from the library), I was reminded about energy shields, or force fields (in the old parlance), as generated by Sue Storm. As a computational chemist, “force field” means something very specific in my field (no pun intended). Visually it looks like a shimmering transparent bubble that surrounds the person or object to be protected. This is in generally how energy shields are visually represented in science-fiction and fantasy TV shows or movies. The Protego shield charm of Harry Potter’s magical world works similarly. I particularly liked the blue tinge in Fantastic Four, because there is a nice correspondence to silica aerogel which looks blue in color against a dark background (due to Rayleigh scattering). As Miodownik puts it, “when you hold a piece of aerogel in your hand, it is, in a very real way, like holding a piece of the sky.”

This made me start to ponder how personal protective energy shields work and how you might generate them. I suppose it depends on what you’re protecting yourself from. Also, if you’re going to move around with them, they should not be too heavy. This puts a mass limitation on the shield. Thus, it would be difficult to deflect a massive object moving at reasonable speeds – a speeding car, perhaps, or maybe a piece of spacecraft debris thrown at you by a Sith Lord wielding the force. But a shield of ions, or plasma, or some sort of electromagnetic radiation, might protect you against the Imperial emperor’s crackling lightning bolts. Sue Storm’s shield protects her from Dr. Doom’s energy attack, which visually resembles the manipulation of ionizing or electromagnetic radiation. Perhaps this is similar to how Protego works, a magical electromagnetic shield that protects against magical electromagnetic spells. (See here for a discussion connecting electromagnetic radiation and spellcasting magic.)

But what if you could add some lightweight, almost transparent matter to your electromagnetic shield? That should increase your ability to stop macroscopic solid objects. Sand, rocks, and other silicon-oxygen-containing substances are abundant. The casting of Protego could involve the gathering together of small amounts of this material, chemically bonded to provide the skeletal framework of an aerogel. It’s transparent so you can still see past your protective field (presuming your electromagnetic shield is also non-opaque). The aerogel might even cause it to shimmer blue! This is why spellcasters should learn chemistry – the understanding of matter at a molecular level (I speculate) will lead to magic both more powerful and precise.

What if you’re not a magican, mutant or midichlorian-enhanced? Can you create your own personal shield to be deployed when needed? The image of an umbrella comes to mind. It can be carried in a folded compact form, and mechanically deployed with the press of a button. With the creation of x-aerogels, flexible in nature, perhaps foldable, one could imagine an aerogel umbrella. In combination with other polymeric materials that allow the expansion of balloon-like objects, one could imagine various deployment methods.

I’m looking forward to new gizmos inspired by and that make use of aerogels. As Miodownik concludes, “if ever there was a blue-sky material – it is aerogel.”