March Materials Madness candidates

Inspired by March Mammal Madness and the Periodic Playoff, I have now mused to my fellow materials science colleagues:

https://platform.twitter.com/widgets.js

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!

  1. Gold nanoparticles
  2. Transition metal dichalcogenides
  3. Metallic glass
  4. Nacre
  5. Silver nanoparticles
  6. Nitinol
  7. Mesoporous metals
  8. Clays
  9. Collagen
  10. Silk
  11. Graphene
  12. Carbon nanotubes
  13. Diamond
  14. Perovskites – does this need to be split out more?
  15. Czochralski silicon
  16. PDMS
  17. hexagonal boron nitride
  18. Antimicrobial copper-alloy touch surfaces
  19. Cadmium telluride
  20. Stainless steel
  21. Neodymium magnets (Nd2Fe14B)
  22. Cellulose
  23. Chitin

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

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 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.)

Materials Advent 2018 3 (and 4 and 5): Pearl and its components

A black pearl rests on an iridescent shell in a black background
A black pearl on the shell of a black-lipped pearl oyster. From Wikipedia: https://en.wikipedia.org/wiki/Pearl

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). 

Image result for crystal structure of aragonite

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

original
An electron microscope image of flakes of aragonite in a mother-of-pearl sample

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. 

Image result for chitin

Materials Advent Calendar 1 and 2: graphite and graphene

I’m late, but an interesting thing I thought I would attempt this year is to do a materials science Advent calendar (or Christmas countdown, we’ll see if I I want to do 23 or 25) and write brief blurbs about neat materials. For belated start, I figured I would start with what I work with: graphite! Yes, your pencil lead is actually a super interesting material. Graphite is an allotrope of carbon, and actually the more stable one in everyday life – your diamonds will eventually decay if they stay on Earth’s surface, but that reaction takes at least thousand if not millions of years because of how slow the atoms can move in diamond.

A microscope image of graphite, showing it’s flake structure. 

Graphite makes for a great writing material because of its layered structure. It’s really easy to slide layers past each other because the carbon atoms don’t interact between layers, so you can easily leave flakes of graphite on paper with just a light pencil press. It also turns out to make graphite a good solid lubricant – you can buy graphite powder and it can be stable for a wide range of conditions. We would use graphite powder to help lubricate the nail axles in Pinewood Derby. Weirdly though, it turns out that even though the layers don’t interact, graphite seems to require something in air to slide easily because it doesn’t lubricate in vacuum (which means you can’t use it as a lubricant for parts exposed to space). 

Three layers of atoms in graphite – the orange layer in between is stacked slightly out of sync with the blue layers

And to try to catch up and get a second material, let me talk about graphene. If you can isolate a single layer of atoms from graphite, you have graphene. (And it turns out you can do this with Scotch tape if you’re patient enough.) And graphene turns out to be the strongest and most conductive material humanity has discovered. If you want to be more technical (and some more rigorous solid-state people do), lots of thing people call “graphene” are actually a few layers, but it turns out even up to 10 layers it still behaves differently than your pencil lead. But we’re good at making few-layer graphene, and it could be an additive we put in almost anything. Seriously. People have proposed putting it in things from water filters to flexible electronics (bendable smartphones anyone?). We’re currently still figuring out how to best scale that up to compete with other established materials though. But it’s exciting to think where this could go in another decade or two. 

What is rheology?

Inspired by NaBloPoMo and THE CENTRIFUGE NOT WORKING RIGHT THE FIRST TIME SO I HAVE TO STAY IN LAB FOR THREE MORE HOURS THAN I PLANNED (this was more relevant when I tried writing this a few weeks ago), I’ll be trying to post more often this month. Though heaven knows I’m not even going to pretend I’ll get a post a day when I have a conference (!) to prepare for.

I figure my first post could be a better attempt at better describing a major part of my research now – rheology and rheometers. The somewhat uncommon, unless you’re a doctor or med student who sees it pop up all the time in words like gonorrhea and diarrhea, Greek root “rheo” means “flow”, and so the simplest definition is that rheology is the study of flow. (And I just learned the Greek Titan Rhea’s name may also come from that root, so oh my God, rheology actually does relate to Rhea Perlman.) But what does that really mean? And if you’ve tripped out on fluid mechanics videos or photos before, maybe you’re wondering “what makes rheology different?”

RheaPerlmanAug2011.jpg

Oh my God, she is relevant to my field of study.

For our purposes, flow can mean any kind of material deformation, and we’re generally working with solids and liquids (or colloid mixtures involving those states, like foams and gels). Or if you want to get really fancy, you can say we’re working with (soft) condensed matter. Why not gas? We’ll get to that later. So what kind of flow behavior is there? There’s viscosity, which is what we commonly consider the “thickness” of a flowing liquid. Viscosity is how a fluid resists motion between component parts to some shearing force, but it doesn’t try to return the fluid back to its original state. You can see this in cases where viscosity dominates over the inertia of something moving in the fluid, such as at 1:00 and 2:15 in this video; the shape of the dye drops is essentially pinned at each point by how much the inner cylinder moves, but you don’t see the fluid move back until the narrator manually reverses the cylinder.

The other part of flow is elasticity. That might sound weird to think of a fluid as being elastic. While you really don’t see elasticity in pure fluids (unless maybe the force is ridiculously fast), you do see it a lot in mixtures. Oobleck, the ever popular mixture of cornstarch and water, becomes elastic as part of its shear-thickening behavior. (Which it turns out we still don’t have a great physical understanding of.)

 

You can think of viscosity as the “liquid-like” part of a substance’s behavior and elasticity as the “solid-like” part. Lots of mixtures (and even some pure substances) show both parts as “viscoelastic” materials. And this helps explain the confusion when you’re younger (or at least younger-me’s questions) of whether things like Jell-O, Oobleck, or raw dough are “really” solid or liquid. The answer is sort of “both”. More specifically, we can look at the “dynamic modulus” G at different rates of force. G has two components – G’ is the “storage modulus” and that’s the elastic/solid part, and G” is the “loss modulus” representing viscosity.

Dynamic modulus of silly putty

The dynamic moduli of Silly Putty at different rates of stress.

Whichever modulus is higher what mostly describes a material. So in the flow curve above, the Silly Putty is more like a liquid at low rates/frequencies of stress (which is why it spreads out when left on its own), but is more like a solid at high rates (which is why is bounces if you throw it fast enough). What’s really interesting is that the total number of either component doesn’t really matter, it’s just whichever one is higher. So even flimsy shaving cream behaves like a solid (seriously, it can support hair or other light objects without settling) at rest while house paint is a liquid, because even though paint tends to have a higher modulus, the shaving cream still has a higher storage modulus than its own loss modulus.

I want to publish this eventually, so I’ll get to why we do rheology and what makes it distinct in another post.

Weirdly Specific Questions I Want Answers to in Meta-science, part 1

Using “meta-science” as a somewhat expansive term for history, philosophy, and sociology of science. And using my blog as a place to write about something besides the physical chemistry of carbon nanomaterials in various liquids.

  • To what extent is sloppy/misleading terminology an attempt to cash in on buzzwords? Clearly, we know that motive exists – there aren’t two major papers trying to narrow down precise definitions of graphene-related terms for nothing. But as the papers also suggest, at what point is it a legitimate debate in the community about setting a definition? “Graphene” was a term that described a useful theoretical construct for decades before anyone ever thought* someone could make a real sheet of it, so maybe it isn’t unreasonable that people started using it to describe a variety of physical things related to the original idea.
    • This contains a sort of follow-up: What properties do people use in clarifying these definitions and how much does it vary by background? Personally, I would say I’m way closer to the ideal of “graphene” than lots of people working with more extensively chemically modified graphene derivatives and am fine with using it for almost anything that’s nearly all sp2 carbon with about 10 layers or less. But would a physicist who cares more about the electronic properties, and which vary a lot based on the number of layers even in the lower limit, consider that maddening?
  • Nanoscience is very interdisciplinary/transdisciplinary, but individual researchers can be quite grounded in just one field. How much work is being done where researchers are missing basic knowledge of another field their work is now straddling?
    • For instance, when reading up on polymer nanocomposites, it seems noted by lots of people with extensive polymer science backgrounds that there are many papers that don’t refer to basic aspects of polymer physics. My hunch is that a lot of this comes from the fact that many people in this field started working on the nanoparticles they want to incorporate into the composites and then moved into the composites. They may have backgrounds more in fields like solid-state physics, electrical engineering, or (inorganic/metallic/ceramic) materials science, where they would have been less likely to deal with polymer theory.
    • Similarly, it was noted in one paper I read that a lot of talk about solutions of nanoparticles probably would be more precise if the discussion was framed in terminology of colloids and dispersions.

51cb2b3noc-l-_sx346_bo1204203200_

Oh my gosh, I made fun of the subtitle for like two years, but it’s true

  • Is the ontological status of defects in nanoscience distinct from their treatment in bulk studies of materials? This is a bit related to the first question in that some definitions would preclude the existence of some defects in the referent material/structure.
    • On the other hand, does this stricter treatment make more sense in the few atom limit of many nanomaterials? Chemists can literally specify the type and location of every atom in successful products of well-studied cluster reactions, though these are even pushing the term “nano” (though in the sense they may be too small).
    • Is this a reflection of applications of defects at the different scales? (More philosophically worded, are defects treated differently because of their teleological nature?) At the bulk level, we work to engineer the nature of defects to help develop the properties we want. At the nanoscale, some structures can basically be ruined for certain applications by the mislocation of a single atom. Is this also a reflection of the current practical process of needing to scale up the ability to make nanomaterials? E.g. as more realistic approaches to large-scale nanotech fabrication are developed, will the practical treatment of defects in nanomaterials converge to that of how we treat defects in the bulk?

*Okay, more like anyone cared a lot about it, since there are papers going back to the 1960s where researchers describe what appear to be atomic monolayers of graphite.