Materials Advent 2018 Part 17 – Snow

Hexagonal plate with dendritic extensions. (precipitating snow).
Snow in an electron microscope.

The US Department of Agriculture has a seriously fascinating page on microscopy of snow flakes because it helps advance understanding of weather and crystal formation.

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

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Materials Advent 2018 Part 16 – Glitter

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.

PET polymer chain
Model of polyethylene terephthalate. Oxygen atoms are red, hydrogen atoms are white/gray, and carbon atoms are black.

Materials Advent 2018 Part 12-15 – Silver and Gold, large and small

Uses of silver in the U.S.
uses of gold in the USA

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).

The energy levels of silver hydride (AgH) and gold hydrige (AuH) before (n.r.) and after accounting for relativity. From this Reddit conversation on relativity in chemistry.

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.

Silver nanoparticles on amorphous carbon. The lines in the particles are actually the organized rows of atoms. From this paper by my research lab.

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.

The Lycurgus Cup is a particularly special kind of stained glass that changes color depending on how it is lit.

Materials Advent 2018 Parts 9, 10, and 11 – Weird Glasses

A teardropped shaped piece of glass seems to have an iridescent sheen
A photo of a Prince Rupert’s drop taken with a special polarizer that helps reveals stresses in the material

Following up on yesterday, I thought it would be fun to look at some weirder glasses.

Prince Rupert’s drops are teardrop (or tadpole) shaped pieces of glass made by dropping molten glass into cold water. They are famous for their bizarre strength. You can pound on the head all you want, and it will almost never break, but nick the tail a little bit and it spectacularly explodes.

Poof

This turns out to come from the way the drop forms. That initial bit that hits the water cools so fast it actually gets compressed by the cooling, making it stronger, but the tail is basically a path to the weak core. The trippy oil-puddle-esque image above is taken with a special kind of set-up that looks at light that ends up being polarized by stresses within the glass. Prince Rupert’s drops turn out to be technologically important, because efforts to understand them since the 1600s have inspired research into ways to make other kinds of glass stronger, leading to the Gorilla Glass and other toughened glass that now lines our smartphones and many other displays.

Metallic glasses are just what they sound like. Just like how I mentioned yesterday that metals are usually crystals, it turns out we can also try making them into glasses by cooling them so quickly their atoms can’t form an ordered structure. This requires either incredibly fast cooling (on the scale of at least 1000 degrees a second for some compositions) or an interesting work around using a lot of different metals together. It turns out that mixing a bunch of atoms of different sizes makes it harder for them to pack into a neat pattern.

The mix of atoms in metallic glasses helps them stay disordered.
A microscopy image showing the real atoms in a glassy alloy of unspecified composition. From Physical Metallurgy (Fifth Edition)

You might wonder why we want to make glass out of metals. It turns out to provide a special property – bounciness. And we literally demonstrate that with “atomic trampolines”. It’s really easy to deform a crystalline metal because that orderly crystal structure makes it easy to slide rows of atoms past each other when you hit them hard enough, just like it’s easy to push a row of desks lined up in a classroom. The glass can’t deform – there’s no preferred direction to push the atoms, so instead the energy just goes back to whatever it hits. This has a cost though – if you hit it too hard, just like regular glass, a metallic glass just shatters instead of accepting a dent. There was initially a lot of hope for them as new materials for the shells of devices like smartphones since they don’t transmit that energy to the components inside, but that’s proved harder to make than hoped. However, you can buy a golf club that takes advantage of the bounciness to essentially transmit all the energy from your swing into the ball. (Going farther back, they evidently also form the basis of most of those theft prevention tags that ring alarms.)

Finally, I’m breaking my rule a bit with this last one by not having an image, but did you know that toffee is also a glass? (Sorry, no one has put toffee under a high-resolution microscope or run it under an X-ray source for weird images for me yet) Or at least good toffee is. That crisp crunch you get from well-made toffee is because of glass shattering. When toffee feels gritty, it is because it has actually started to crystallize and typically has hundreds of little mini-crystals that want to deform. This is why some recipes suggest adding corn syrup. The bigger sugar molecules in corn syrup mixed up with the sucrose in regular table sugar mix up in a way like the metallic glasses above and make it harder for them to set into their crystal structure. Similarly, an early kind of stunt glass for special effects was literally made by boiling sugar into a clear candy.

Materials Advent 2018 Part 8 – Glass

Silica goes from crystalline to amorphous (glassy) from right to left

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.

Materials Advent 2018 part 7 – Liquid Crystals

An image shows bright regions of color on a black background. The left shows features reminiscent of oil spills. The right shows multiple butterfly-like features of four same-colored spots around a central point.
A polarized light image shows the patterns caused by two different arrangements of liquid crystals within the same sample. From Wikipedia: Liquid crystal

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!

Rods pointing in the same direction but randomly spaced out.
A “nematic” liquid crystal – the molecules are lined up in the same direction, but there is not clear pattern in how they are spaced. 

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.)

Do These Units Make My Mass Look Big?

Humanity did something really cool in November –  as a species, we redefined all our main measuring units to be based on universal physical constants. This represents the confluence of the two favorite subjects of many people – large international bureaucracies and science, more specifically metrology, or the science of measurement, so I’m sure everyone already knows about this. Just kidding. But it actually is interesting. The International Bureau of Weights and Measures (BIPM) had its 26th General Conference and voted to redefine the kilogram, ampere, degree kelvin and mole (that famous unit you learned about in chemistry class). The vote was unanimous, which is kind of a big deal when you consider that BIPM members include countries with historically tense relations like Israel and Saudi Arabia or the US with Iran and Venezuela. So what does it mean to change these definitions and why did we do this? 

First, it’s worth going over some history. You can sort of view this as the ultimate culmination of the hopes of the original proponents of the metric system. Fitting the spirit of the times, scientists in revolutionary France proposed new units of measurement that could be used to describe all the universe, not just a single kingdom or even just a fiefdom. Their initial system only proposed two of the units we now use: the meter and the gram. The meter was defined as one ten millionth of the distance from the North Pole to the Equator, as measured on the line of longitude going through Paris (because, France), which they estimated with pretty solid accuracy even back then. The gram was defined as the mass of a cubic centimeter of water at freezing point. To point out a recurring problem, the French defined the gram to lay out the system, but decided that making a standard reference of something that small was too hard and so they commissioned a kilogram standard to be made as the reference instead. Thomas Jefferson heard about the new French units when he was Secretary of State and actually asked for a copy of the kilogram standard, but it never made it to  America because the ship (and scientist) carrying it were hijacked by pirates ofthe Caribbean, so America ended up without a reference kilogram for decades. 

(As an aside, temperature wasn’t defined in the revolutionary system at all. The Fahrenheit and Celsius scales were both developed earlier in the 18th century. For some weird reason, Celsius was also originally reversed, so that the numbers actually decreased with rising temperature – 100 degrees C was the freezing point of water and 0 was the boiling point. Also, my hot take is that Fahrenheit actually has a better zero point for reference than Celsius. As you know, Celsius is zero for the freezing/melting point of water, but it’s really easy to overshoot that depending on your cooling/heating rate as any video of supercooled water can show you. Fahrenheit used a mixture of water, ice, and the flavoring in salty licorice that always stabilizes to a specific temperature.)

On the left is a shiny grey cylinder. On the right is a rod with an X-shaped cross-section
The prototypes of the kilogram and meter 

In 1875, 17 countries (including the US) signed the Metre Convention, which established the definitions of the kilogram and meter based on new physical artifacts – a 1 kg cylinder of platinum-iridium alloy (the International Prototype Kilogram, affectionately called “le Grand K”) and a platinum-iridium bar with markings spaced 1 meter apart – and establishes the BIPM and associate organizations. The BIPM makes copies of these prototypes for national metrology organizations, and the US finally gets a kilogram, and a few years later, the US actually adopts the metric system. You might think that’s a joke, but Americans’ continued usage of imperial units is really just a surface level thing. Since 1893, the US has always defined the pound and yard in relation to their metric counterparts instead of a separate standard. We just can’t convince people to stick with the metric units in everyday life. 

The problem with these artifact-based definitions is that it makes it hard to pinpoint where uncertainty develops. Every time a physical object is being handled, you risk scratching some microscopic sliver off or leaving some tiny amount of residue changing the mass of the kilogram standard or perhaps creating some strain that changes the length of the meter standard. For instance, nearly all the national kilogram standards have gained mass relative to le grand K, but people aren’t sure if this is a sign of them gaining mass, le grand K losing mass, or le grand K gaining mass slower than the other standards. Defining in terms of a physical constant offers more stability because we do have good evidence that they are, well, constant. This is also only reliable if you can measure the constant to high precision.

The relative changes in mass of the national kilogram standards compared to le grand K since 1900.

The meter has already been redefined this way a few times. First in the 1960s, it was defined as being a certain multiple of the wavelength of light emitted by a certain atomic transition of krypton. As optics advanced, it was discovered that light spectrum had some irregularities that could result in different values depending on experiments, so it was later replaced by a definition relating the meter explicitly to the speed of light. This means the meter is now based on the second, which is very precisely defined by an atomic transition of cesium.

Until recently measurements of the relevant constants for mass weren’t precise enough to replace le grand K. However, it’s also been awkward because the process of “tracing” the mass from le grand K to smaller masses becomes more imprecise as you go down. The balances only compare masses, so the only way to standardize something smaller than a kilogram is to see if it along with a bunch of other masses add up to a kilogram, and then just keep working down to smaller and smaller values. This quickly adds up difficulty and accumulated error, so the smallest NIST standard you can obtain goes to half a milligram. Unfortunately, industrial processes like developing active ingredients in pharmaceuticals or manufacturing nanoscale parts for electronics routinely deal with masses a lot smaller than that. 

The traceability sequence of making NIST standards of less than a kilogram with the old definition. From NIST’s own research journal.

The new definition defines the kilogram in terms of the Planck constant, the unit that relates the frequency of light to the energy it carries. Or technically, it precisely defines the Planck constant as 6.62607015×10−3 joule-seconds or square meter-kilograms/second, and since we have physical values for seconds and meters, better measurements of the Planck constant now give the value of the kilogram. This is done with a watt balance, which uses the force of an electric current in a magnetic field to balance out the weight of a mass by varying the current until the forces are equal. A benefit of this definition (and the one for the meter) is that it is easier to define larger or smaller values for standards. Instead of needing to go through the full sequence of steps for scaling down mass standards, once a watt balance has found the current needed for a kilogram, you can easily scale that current down by 10 or 100 to make a new 100 or 10 g standard.

AND Materials Advent 2018 part 6 – Platinum-Iridium Alloys

So why make standard artifacts out of a mixture of platinum and iridium? It turns out this is really durable in a lot of ways. Pure platinum is already desired for how resistant it is to rusting. Iridium is an even rarer element – so rare that probably it’s biggest claim to fame is that a worldwide layer of it deep in the soil was one of the earliest proofs of the asteroid impact we think killed the dinosaurs. Adding a bit of iridium to platinum not only improves the rust resistance, but makes it stronger and harder, so handling the standards shouldn’t degrade them as much. It’s also a mixture that doesn’t change size much when heated or cooled, helping minimize another potential headache in measuring length.