We don't understand yogurt
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As a kid in school, I learned about four states of matter. Solid, liquid, gas, and then plasma. But recently, I spoke to a physicist named Ray Robertson Anderson who

And she told me that there is actually another state of matter out there.

Ranch dressing. Or shampoo. Or scrambled eggs. Or snot. Or even your eyeballs. Also toothpaste and yogurt. Ketchup. Anything that you think of, anything that's like squishy or gooey materials that are sometimes somewhat like a liquid and somewhat like a solid. All of these are part of this extra state of matter. It's something called soft matter.

So soft matter is basically stuff that is more bendy or squishy than a solid, but more able to hold its shape than a liquid can. And while scientists figured out some of the rules for solids and liquids centuries ago, like back in the 1600s, soft matter plays by its own rules. And physicists like Ray are still trying to figure out what those rules are.

All these things all around us, you don't really think about. They're so complicated and there's so much to understand about them. You know, a big goal of physics is to come up with like a unified model to understand how the universe works. And that's great and that's wonderful. But we don't understand yogurt. So like we don't understand how these things, you know, for that we encounter every day, like toothpaste and shampoo and yogurt. Like we don't understand really how that works.

works. So like, let's start there and then see if we can build up to like the unified model of the universe. So this is Unexplainable. I'm Bird Pinkerton. And today on the show, why don't we understand yogurt? Why is soft matter so hard? Ray says that a big central mystery around soft matter materials is

is that they do stuff that's just profoundly odd. So take our yogurt example.

Imagine that you have like a little single serving container of yogurt from the grocery store. You know, when you first like take off the lid, it looks very solid like, right? If you tilt it to one side, it doesn't slide out or pour over the edge. And if you stick a spoon in it, you can like scoop out an actual piece of the yogurt and it's solid. Okay, so then if you stir it though, it becomes more liquid-like. After you've stirred it, if you tilt it, you can actually pour it. It's become like a fluid.

And this is a very common phenomenon, right? You see it in Nutella and ranch and paint and mayo. It's what happens when you have a ketchup bottle where the ketchup won't budge if you just flip it over. If you shake it, then all of a sudden it becomes much more fluid-like and then you can pour it out.

And then just to make things interesting, some forms of soft matter are like the opposite world of this. So oobleck is a perfect example. If you've never played with oobleck, I highly recommend that you do so as soon as possible. It's very simple to make. You just mix cornstarch with water. But the result is this weird milky fluid that's hard in some situations and then soft in others. If you punch it.

It stiffens up and your hand cannot go through that fluid. But then if you try to slowly put your hand into it, you can fully submerge it. Then if you try to pull your hand out, it'll get stuck. So it's a fluid, like it fills a container. But then if I had a bowl of it and I tried to throw it at you, it would not leave the bowl. So you wouldn't get covered in cornstarch and water. And so that's something, the more you try to deform it, the more it rigidifies.

All to say that there are lots of examples of different soft matter materials doing all kinds of soft-to-hard-to-soft behavior, right? It's happening all around you, all the time, in different forms. And yet, while a lot of great research has been done,

There is still a lot that scientists like Ray would like to understand about how these materials are pulling this off. There's lots of theories that have been proposed and lots of different models that capture some of the properties. But not all of them.

Bray says the problems start when you zoom in to the actual molecules that make up different materials. So a solid basically has all of its molecules packed together in like a neat, tidy structure. And a liquid has its molecules kind of bouncing around like little balls in a ball pit.

But soft matter materials are kind of in between. Like, in a soft matter material, some of the molecules can be connected to each other in, like, strings or chains, say. And the chains can be kind of slipping and sliding around. So this is where, like, I like to describe soft matter as a bowl of spaghetti. Now, if I try to pull one strand of spaghetti out from the bowl,

It's hard. It might snap back. But now if you if you pull very gently, you might be able to unthread it. And also, if you take the whole bowl and pour it, it can pour out like a fluid, like the spaghetti altogether can move and rearrange. But when you try to pull one relative to the other, it has some like elasticity.

And so to understand exactly what is happening with a soft matter material, like why it is acting the way that it is, physicists like Ray basically have to tease out how all these strands of molecular spaghetti are interacting, like how they're all touching or sliding against each other.

And that's hard. In a bowl of spaghetti, you might have, you know, 50 strands of spaghetti. In a material, you have 10 to the 23 spaghettis. That is 100 sextillion spaghettis that Ray is supposed to keep track of.

And then different types of soft matter can have different molecular bowls of spaghetti, right? With different things inside them. Like spaghetti and SpaghettiOs. And you have meat sauce. And so then you have to take into account like, oh, how's the meat sauce interacting with the, you know, is it making the spaghetti more easily flow past each other? Or is it causing it to be sticky? Oh, so it really is like...

Did you add butter to the spaghetti? Exactly. No, it's super slippery. Exactly. Or did you add honey for some reason because you're a sick animal? No, seriously. Yeah. And then depending on like, you know, if you change, say, like the temperature or the pH, then maybe your spaghetti was overcooked or maybe it's al dente or maybe it's a mixture. And so like all of these things you have to think about.

So to summarize, figuring out how any given soft matter material does what it does requires a lot of work, right? A lot of throwing spaghetti against the wall. But Ray says that learning about soft matter could also teach us a lot about ourselves. Because it turns out that you and me and everyone we know are basically just a whole bunch of soft matter in a trench coat of skin.

Which, by the way, is also soft matter. More on that after the break.

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Spaghetti. Lots of oregano. Spaghetti.

One pretty compelling reason to figure out why soft matter is the way that it is and how it does what it does is that life is made up of a whole bunch of squishy stuff. Like our eyeballs and snot and saliva in our tongues and our skin. But also on a more fundamental level,

Ray says you can think of parts of our cells as soft matter. Because our cells, right, the little blobs that make up not just our eyes and our snot, but our muscles and parts of our bones and our hearts and our minds.

Our cells have a kind of skeleton in them. And Ray says that that cellular skeleton can sometimes be hard, giving our cells shape. So cells can tense up and they can also fluidize to do things like divide and crawl and also like to form structures. And these cellular skeletons can get softer and more fluid-like, basically. Understanding soft materials can basically help us understand things

biology, how biology has over billions of years evolved to create soft, squishy living materials.

Ray also wants to take things even one step further. She says if we could figure out how soft matter like our cells or even soft matter like just oobleck, if we could figure out how these things really work on a molecular level, we could engineer all kinds of wild soft matter inspired stuff. One example that I like to give is, you know, like think about a bulletproof vest. So a bulletproof vest, what it needs to do is take,

tense up when something hits it at a very, very high speed, right? So if a bullet's hitting, you want it to tense up. But, you know, if you're just walking around, you know, if like when the wind blows or something like you don't need it to be very rigid and very tense all the time. If we figure out how all the molecules in something like oobleck are coming together to do what they do,

then maybe we could make a vest that was flowy and flexible under most conditions. But then if it was hit by a bullet, something very, very fast, then it would tense up, just like your oobleck. But the minute the bullet drops on the ground, then it returns and becomes fluid-like again. It just is amazing to imagine, like, oh, this is my little peasant dress. Yes, exactly. Right?

Exactly. And then Ray says, if we can figure out how cells do their version of hardening and softening, then maybe we could make our own simplified versions of cells. These materials that would react to their environment and shift into something more solid or fluid depending on different conditions.

She dreams of making a self-healing bridge. So basically like a bridge where if it started to develop a crack of some kind, the cell-inspired materials around that crack would react and become more fluid. And basically fill in that crack. And then once they fill it in, they sense that it's been filled in and then it solidifies again. So you basically like heal the bridge before you even knew there was a crack.

Honestly, Rae kind of has an endless list of natural goos that she wants to remake or remix in some way. There's this tube worm, this marine tube worm that I studied for a while that actually has this mucus that's bioluminescent. And it will like trap food with it, but it also allows like nutrients to pass through. And it lives in these tubes and it makes the tubes with the mucus. So it's crazy. It does this with all this one type of goo.

She says if we could imitate this to worm mucus, we might be able to build our own nets that let good things like nutrients pass through them no problem, but harden up to trap bad stuff that we want to filter out of the ocean, like microplastics. There's so, you know, like I'm collaborating with somebody recently where we're looking at biofilms. And biofilms are basically like when you have just a layer of bacteria on something. You might have seen them on your shower heads or your faucets. They also grow on ships, apparently.

And Ray says that if we could learn how they work, we might be able to prevent them. There's so many questions that we don't understand and that I would love to study. Like anytime when I go to like a conference or whenever I'm talking to somebody and it's like, oh yeah, I'm studying this. I'm like, oh, that's so cool. I want to study that. You know, like there's things like, you know, like spider silk and, you know, like even things like eggshells and like these materials that are just like

totally fascinating and that we don't understand. And so that's kind of what keeps me excited, just knowing that I have many more questions that I want to answer than I have time to answer them. While I was talking to Rae, I was totally infected by her enthusiasm. But eventually, we did have to end our call.

So I went back to my desk and did my job, essentially. I told my editors about the interview and I talked to my colleagues about it a bit. Because I was excited and also because I wanted to get a sense of how I might structure this episode. But I was mostly in work mode until a little later in the day when I got hungry.

and I popped into our office kitchen. And there, in one of the weird, wonderful coincidences that happen in life sometimes, someone had left a bottle of ranch dressing on the kitchen table. And I will admit, it felt like a sign from the universe. I'm not sure why the universe would choose to send signals through bottles of mildly spicy ranch dressing. That's maybe a question for the unified model down the line.

But in that particular moment, it was like all the joy and wonder of the conversation with Ray fully sank in. Because somehow, looking at this bottle of ranch, it really hit me that the whole world is brimming over with mysteries. And even this ridiculous condiment contains riddles that we have not yet solved. ♪

If you want to learn more about soft matter, you can read Rae Robertson Anderson's book, Biopolymer Networks. Or for slightly more accessible content, she suggests that you visit her TikTok, which is physics underscore mama. That's mama with two Ms, where she makes physics videos with her kids.

This episode was produced by me, Bird Pinkerton. It was edited by Jorge Just. Meredith Hodnot runs the show. Noam Hassenfeld made the music for this episode. Christian Ayala did the mixing and the sound design. Melissa Hirsch checked our facts. Julia Longoria is the fact that toads and frogs can absorb water through their skin. And I am always, always, always grateful to Brian Resnick for co-creating the show.

Thanks also to Colm Kelleher for his time and to Uri Bram for his help. If you have deep thoughts about ranch dressing or shallow thoughts about ranch dressing, we're happy

Thoughts about anything at all, please write in to unexplainable at Vox.com. Your emails make my day. If you want to support the show and help us keep making it, perhaps, please join our membership program. That's at Vox.com slash members. You will get ad-free podcasts, unlimited access to Vox journalism.

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