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.

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There Are Probably No Nanoparticles in Your Food… At Least, Not Intentionally

Recently, Mother Jones posted an article about “Big Dairy” putting microscopic pieces of metal in food. Their main source is the Project on Emerging Nanotechnologies and its Consumer Products Inventory, a collaboration between the Wilson Center and Virginia Tech. Unfortunately, the Mother Jones piece seems to misunderstand how the CPI is meant to be used. But another problem is that the CPI itself seems poorly designed as a tool for journalists.

So what’s the issue? The Mother Jones piece mainly focuses on the alleged use of nanoparticles of titanium dioxide (TiO2) in certain foods to enhance colors, making whites whiter or brightening other colors. First, the piece makes an error in its description of TiO2 as a “microscopic piece of metal”. Titanium is a metal, but metal oxides are not, unless you consider rust a metal (which would also be wrong). But another issue is “microscopic”. Just because something is microscopic, which generally means smaller than your eye can see, doesn’t mean it’s a nanomaterial. The smallest thing you can see at a normal reading distance is about a tenth of a millimeter, which is 1000 times bigger than the 100 nanometer cut-off we typically use to talk about nanoparticles.

A clear glass dish holds a bright white powder.

Titanium dioxide is a vivid white pigment, even as macroscopic particles.

And that’s what confuses me most here. As you can see above, titanium dioxide is white as a powder, but in that form it’s several hundred nanometers wide at minimum, if not on the scale of microns (1000 nanometers). In fact, nanoparticles of TiO2 are too small to scatter visible light and so they can’t appear white. A friend reminded me how sunscreens have switched from large TiO2 particles to actual nanoparticles precisely because it helps the sunscreen go on clearer. I’m not naive enough to think food companies wouldn’t try to cut a buck to help improve and standardize appearances, but I also don’t think food scientists are dumb enough to pay for a version of a material they can’t fulfill the purpose they’re adding it for. So TiO2 is probably used in some foods, but not on a nanoscale that radically changes it’s health properties.

But I don’t entirely blame Mother Jones. The thing is, the main reason I had a hunch the article seemed wrong is because one of my labmates at UVA has been working with TiO2 nanotubes for the last three years, and I’ve seen his samples. If I didn’t know that, and I just saw PEN include TiO2 on its list of nano additives, I would be inclined to believe it. PEN saw the Mother Jones piece and another similar article and responded by pointing out that the inventory categorized their inclusion of TiO2 in the products as having low confidence it was actually used. But their source is an environmental science paper including actual chemical analyses of food grade TiO2, so why do they give that low confidence? Also, PEN claims the CPI is something the public can use to monitor nanotechnology in products, so maybe they should rethink how confident they are in their analysis if they want to keep selling it that way.

The paper CPI references in the TiO2 claim is interesting too. That paper actually shows that most of the TiO2 is around 100 nm (figure below). But like I said, that’s kind of pushing the limit on how small the particles can be and still look white. It might be that the authors stumbled across a weird batch, as they note that in liquid products containing TiO2, less than 5% of the TiO2 could go through filters with pores that were 450 nanometers wide. Does the current process used to make food grade TiO2 end up making a lot of particles that are actually smaller than needed? Or maybe larger particles are breaking down into the smaller particles that Weir sees while in storage. This probably does need more research if other groups can replicate these results.

A histogram showing the distribution of particle sizes of TiO2. Categories go from 40 nanometers to 220 nanometers in intervals of 10. The greatest number of particles have diameters of 90-100 or 100-110 nanometers.

Distribution of TiO2 particle sizes in food grade TiO2. From Weir et al, http://pubs.acs.org/doi/abs/10.1021/es204168d?journalCode=esthag

Wine Tasting 101

NPR’s blog about food and science, The Salt, has an amusing story on wine tasting this week. Part of it is pointing out the actual science in wine tasting, which has recently been a victim of fights on the Internet*. Basically, The Salt’s post focuses on the actual chemicals present in wines and wants to help wine newbies detect by saying where we can find them in other foods. So here’s the quick lowdown

  • Whites aren’t aged in oak as often as reds.
  • Wine aged in American oak picks up more vanillin from the word than wines aged in French oak. Evidently American oaks have a higher concentration of the lactones than the French oaks. (I can’t find an explanation why, but that’s interesting) Vanillin, of course, is the primary chemical responsible for the flavor of vanilla.
  • Cabernet sauvignon and green peppers have the same chemical responsible for their smell. In cabernet, it’s strongest when the grapes aren’t ripe, so smelling green pepper would suggest a low quality wine. Aside: I definitely did not know that. I actually thought it wasn’t bad to have the green pepper scent. I also almost never drink cabernet, so maybe nothing should surprise me.
  • Diacetyl, a chemical commonly used in artificial butter flavorings (but also present in actual butter, potential chemophobes), develops in wines that have undergone a further fermentation process that converts the more sour malic acid (found in green apples) to lactic acid (found in milk).

NPR talks about sniffing all the foods with the same chemicals so you can “follow your nose”, if you will.

We went there.

They even suggest putting the good things in a cheap wine and comparing it’s smell to a more expensive one so you can find the similarities. But maybe I’m just taking the wrong lesson from this article, because I want to spray Pam into a wine glass and add some vanilla flavoring after pouring some Two Buck Chuck in and seeing if that tastes good.

*Just a quick thought on the “wine science wars”. I don’t think wine critics view themselves as scientific arbiters of wine, but I do think they present themselves as having far more precision than the studies suggest they do. Wine blogger Heimoff asks why we never see a headline saying that restaurant reviews are all junk science. Because they don’t claim to be picking up 12 distinct flavors from a single component, unlike a wine reviewer who honestly said a single wine had flavors of “red roses, lavender, geranium, dried hibiscus flowers, cranberry raisins, currant jelly, mango with skins, red plums, cobbler, cinnamon, star anise, blackberry bramble, whole black peppercorn” (perhaps take that review with a grain of salt, since the story sounds like it is coming from an ad almost).

I also think they do have a harder job than the restaurant critics. At a restaurant, you get to examine multiple things: the food, the service, the atmosphere, etc. Heck, even if you only focus on the food, that still leads to several different things to examine, whether that’s multiple courses at a Michelin-rated restaurant or just the multiple ingredients in a sandwich. A wine critic has one thing to look at, and they try to go into intense detail. But humans aren’t meant for analytical chemistry. I think things like this Salt article are perfect. It does show what people can appreciate in a wine and what the industry tries to create.