But this means we need materials to put in a bracket. I have some ideas below. If we want to really this be materials and not just chemicals, we probably need to specify structure/processing in addition to composition. So I say Czochralski silicon instead of just “silicon” because that process makes the silicon used in modern electronics. However, I’m not sure if we need to split up related categories of materials more. On the other hand, I’m fine with not 100% specifying composition if changing the elements may be part of processing/design, which comes up a lot with transition metal dichalcogenices and perovskites. But do we need to specify those more into sub-groups that have more consistent crystal structures? (See this recent paper on what exactly IS a perovskite?)
Maybe we don’t need 64 for a first attempt, but 32 might nice. I would love to hear more ideas from people on WordPress and Twitter!
Transition metal dichalcogenides
Perovskites – does this need to be split out more?
Snowflakes (usually) have hexagonal symmetry because this reflects the crystal structure of water molecules in the solid. This old article from Scientific American goes a bit more into the details on the formation of snowflakes, but it’s also something we still don’t entirely understand
The New York times has already run the most whimsical materials science piece of the day and I won’t attempt to top the delightful Caity Weaver. So discover What is Glitter, or aluminum metalized polyethylene terephthalate.
I’ll slightly tweak my rules again, because you know what silver and gold look like, and I actually think it’s interesting to tell people that we have lots of practical uses for them. Gold and silver are transition metals, that special middle part of the periodic table which represents the addition of a new set of electron orbitals (the d-orbitals). Electrons in the d-orbitals are special because they tend to overlap in energy with those in the s- or p-orbitals, helping increase the number of electrons that are free to move. This is actually why gold and silver are shiny – they have electrons that are easily excited and interact with visible light and reflect it back. Gold gets its unique yellow color because its electrons move so fast they actually need to be described by relativity and it shows that their energies decrease (essentially because of their increased relativistic mass at high speed).
You’re probably less familiar with the nanoscale forms of gold and silver. Or you might be with silver because we now use the nanoparticles in lot of things for their antimicrobial properties. Our lab actually makes a lot of metal nanoparticles, and so I can show you a high-resolution image of these particles.
Gold nanoparticles (and many other metals) are neat because they can completely change the color of solutions they are in, often to red in the case of gold. This is different from the yellow we see for large gold pieces because at a very small size, the electrons in the particles have different energy levels than at the macro scale, and so they absorb light from different colors. Old stained glass actually gets its colors from tiny amounts of metallic nanoparticles being incorporated into the glass while it was being formed. We’re still not entirely sure how the nanoparticles were incorporated in stained glass. Theories range from it being caused by poor cleaning of gold residue from the surfaces glass was worked on or contamination in the source materials.
I’m going to (hopefully) permanently end the popular misconception that glass is a liquid with this post. Glass is not a really slow liquid – it is in fact a solid, based on its flow properties (yay rheology!). But glass is a solid without structure, or in fancy terms, an amorphous solid. Many solids you see are crystalline, not just the pretty stones pop cultures tends to reserve the name “crystal” for. A crystalline material is one where their atoms or molecules are arranged in a repeating 3D pattern. The metals in your car, the silicon in your computer, and the calcium phosphate mineral in your bones are also all crystals because we can see their atoms follow some crystal structure. While the atoms/molecules can differ, mathematicians have found out that there are only about 200 distinct ways to make a repeating pattern in 3D with no gaps or overlaps (and only 17 for 2D). This might seem low, but the point is that while tiny details may change, there’s only so many ways to combine the symmetries you can find, like reflections or rotations, and still fill up all of space (or your wall).
The image above is a high-resolution transmission electron micrograph literally showing you the atoms in silica (silicon oxide) – the material that makes up regular glass. On the left, it’s a crystal and the dots on the bottom show you in red and green where the different atoms are in a hexagon arrangement. Around 3/4 of the way to the right you see that the atoms are no longer always in hexagons and the shapes start to change. This side is amorphous.
Liquid crystals are everywhere now (if you’re looking at this on a computer, your display is probably LCD, and a decent chance any TV you looked at lately is an LCD too). They’re another one of those weird in-between states like we see in rheology. Liquid crystals are liquids based on their mechanical properties, but their molecules show some large-scale ordering that resembles solid crystals. This is because the molecules in liquid crystals tend to be relatively large, like a polymer chain, so you make distinct structures by lining them up, but it can also still be hard to pack them closely to make a true solid. The first liquid crystal was actually found studying cholesterol!
Most LCDs are based on liquid crystals in the nematic phase seen above. (Although evidently not LCD TVs) Because the molecules are asymmetric, they can be oriented by electric fields because their electrons will be pulled by the electric force. The power in an LCD is basically to turn on and off the voltage to twist the crystals. This twisting is done to change how light passes through. A bunch of nematic liquid crystals lined up next to each other essentially act as a filter called a polarizer and line up the waves of light passing through. At the top and bottom layer of your display are two permanently oriented filters that are perpendicular to each other, an arrangement which would not let any light pass through without the crystals being lined up in a way to help align the light somewhere in between. (This is also why you can sort of see the pixels of LCDs at off-angles and why the picture can look so off away from the center of LCD TVs – the light is really only lined up for someone looking straight through the display.)
Pearls are quite beautiful and also (kind of) rare, which makes us appreciate them even more. But it turns out they are made of very common components – a variant of chalk (calcium carbonate) bound in strings of the polymer that makes up insect, shrimp, and crab shells (called chitin, as well as a few other polymers). The calcium carbonate variant (or “polymorph“) is called aragonite, which technically has a different crystal structure from the form we actually use as chalk, which is called calcite. To make pearls even more interesting, aragonite isn’t actually the stable form of calcium carbonate at typical temperatures on the surface of Earth, but something about the way clams and other molluscs grow shells assembles that structure instead of calcite (and researchers still aren’t entirely sure why).
Despite essentially being a combination of chalk and insect shells, pearls and the related materials in mollusc shells are incredibly strong. We often attribute this to the strength of curves at our macro level, but it also turns out the way these parts are combined at the micro level is really important. The main solid part is essentially laid out slabs made of smaller bricks of aragonite, not unlike a brick house. And they are separated by small spaces due to rough bits on each brick, which is also where the entangling bits of polymers go. It’s really hard for cracks to get far in this arrangement – to travel through a slab it has to keep going through the alternating order of bricks and through the more flexible polymers separating them, and to go through multiple slabs a crack has to get through even more polymer and also change preferred directions along the crystals because the bricks change arrangement across slabs. As a result, pearls and clam shells are about 10 times stronger than individual hunks of aragonite or the polymers. This makes pearls an incredibly interesting composite that engineers want to mimic. In fact,a professor I work with at UVA has published multiple papers about the strength of mother-of-pearl (often called nacre in technical contexts) and tried to mimic this structure with graphene.
Industrially, humans get a lot of chitin from industrial processing of shellfish and use it as fertilizer as a food additive to help improve texture.