We don't understand yogurt

Many physicists dream of coming up with a unified theory of the universe. Rae Robertson-Anderson dreams of understanding ranch dressing, shampoo, and scrambled eggs. Guests: Rae Robertson-Anderson, a physics professor at the University of San Diego. (Find her TikToks at physics_mamma.) For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠ For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠ And please email us! ⁠⁠[email protected]⁠⁠ We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠ Help us plan for the future of Unexplainable by filling out a brief survey: ⁠⁠⁠⁠⁠voxmedia.com/survey⁠⁠⁠⁠⁠. Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

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• 0:49Speaker 01

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• 1:29Speaker 00

  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, and she told me that there is actually another state of matter out there: ranch dressing.

• 1:51Speaker 02

  Or shampoo, or scrambled eggs, or snot, or even your eyeballs.

• 1:57Speaker 00

  Also toothpaste and yogurt, ketchup.

• 2:00Speaker 02

  Anything that you think of, anything that's squishy or gooey materials that are sometimes somewhat like a liquid and somewhat like a solid.

• 2:09Speaker 00

  All of these are part of this extra state of matter.

• 2:12Speaker 02

  It's something called soft matter.

• 2:15Speaker 00

  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.

• 2:37Speaker 02

  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, that we encounter every day, like toothpaste and shampoo and yogurt. Like, we don't understand really how that works. So like, let's start there, and then see if we can build up to like, the unified model of the universe.

• 3:12Speaker 00

  So, this is Unexplainable. I'm Perd Pinkerton. And today on the show, why don't we understand yogurt? Why is soft matter so hard?

• 3:37Speaker 00

  Ray says that a big central mystery around soft matter materials 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.

• 3:51Speaker 02

  You know, when you first like, take off the lid, it looks very solid-like, right?

• 3:55Speaker 00

  If you tilt it to one side, it doesn't slide out or pour over the edge.

• 3:59Speaker 02

  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.

• 4:09Speaker 00

  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.

• 4:27Speaker 02

  If you shake it, then all of a sudden, it becomes much more fluid-like, and then you can pour it out.

• 4:35Speaker 00

  And then, just to make things interesting, some forms of soft matter are like the opposite world of this.

• 4:40Speaker 02

  So, Ublek is a perfect example.

• 4:42Speaker 00

  If you've never played with Ublek, 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.

• 4:57Speaker 02

  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, you know, covered in cornstarch and water. And so that's something, the more you try to deform it, the more it rigidifies.

• 5:26Speaker 00

  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.

• 5:53Speaker 02

  There's lots of theories that have been proposed and lots of different models that capture some of the properties.

• 6:01Speaker 00

  But not all of them.

• 6:04Speaker 00

  Ray 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.

• 6:41Speaker 02

  So this is where I 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.

• 7:15Speaker 00

  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.

• 7:37Speaker 02

  In a bowl of spaghetti, you might have, you know, 50 strands of spaghetti. In a material, you have 10 to the 23 spaghettis.

• 7:45Speaker 00

  That is 100 sextillion spaghettis that Ray is supposed to keep track of.

• 7:53Speaker 00

  And then different types of soft matter can have different molecular bowls of spaghetti, right, with different things inside them.

• 8:03Speaker 02

  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?

• 8:17Speaker 00

  Oh, so it really is like, did you add butter to the spaghetti?

• 8:23Speaker 02

  Exactly.

• 8:23Speaker 00

  No, it's super slippery.

• 8:25Speaker 02

  Exactly.

• 8:26Speaker 00

  Or did you add honey for some reason? Because you're a sick animal.

• 8:30Speaker 02

  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 dante or maybe it's a mixture. And so like, all of these things you have to think about.

• 8:44Speaker 00

  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.

• 8:58Speaker 00

  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.

• 9:23Noam Hassenfeld?

  Support for the show comes from Sonos. Hey, Sonos, give me some music.

• 9:29Noam Hassenfeld?

  All right, one of life's great mysteries. You're listening to music in the living room, but you gotta grab something from the kitchen. You don't want to take the speaker with you, but you also don't want to miss that beat drop. And this is where I introduce you to the magic of Sonos. Sonos sent me a couple of speakers to try out, so I've got to move two here in the living room. And then I walk down the hall.

• 9:54Noam Hassenfeld?

  I get in here to the kitchen, and I got a brand new Sonos Play right here on the counter.

• 10:02Noam Hassenfeld?

  I can open the fridge, grab a seltzer.

• 10:11Noam Hassenfeld?

  And yeah, I didn't miss a second. And I can make each speaker louder or softer in the Sonos app, or if I'm using AirPlay, so I can make it louder in the kitchen without blasting whoever is still in the living room. It's all just easy, and it sounds amazing. These speakers are crisp, they've got great bass, and I'm especially impressed with the new Sonos Play. It packs this truly awesome sound in a much smaller size. So, I don't know, if I want to head outside for a minute. It's super easy, and yeah, pretty nice out. Visit sonos.com and build your system. Sonos, love the sound, feel the system.

• 11:03Speaker 00

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• 13:05Speaker 00

  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.

• 13:15Speaker 02

  Like our eyeballs and snot and saliva in our tongues and our skin.

• 13:21Speaker 00

  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.

• 13:48Speaker 02

  So cells can tense up and they can also fluidize to do things like divide and crawl and also like to form structures.

• 13:57Speaker 00

  And these cellular skeletons can get softer and more fluid-like, basically.

• 14:02Speaker 02

  Understanding soft materials can basically help us understand biology. How biology has over billions of years evolved to create soft, squishy, living materials.

• 14:16Speaker 00

  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.

• 14:35Speaker 02

  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 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.

• 15:01Speaker 00

  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.

• 15:11Speaker 02

  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.

• 15:23Speaker 00

  It's just amazing to imagine like, oh, this is my little peasant dress.

• 15:26Speaker 02

  Yes, exactly.

• 15:28Speaker 00

  With bullets.

• 15:30Speaker 02

  Exactly.

• 15:31Speaker 00

  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.

• 16:01Speaker 02

  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 healed the bridge before you even knew there was a crack.

• 16:13Speaker 00

  Honestly, Ray kind of has an endless list of natural goos that she wants to remake or remix in some way.

• 16:20Speaker 02

  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.

• 16:41Speaker 00

  She says if we could imitate this tube 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.

• 16:54Speaker 02

  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.

• 17:04Speaker 00

  You might have seen them on your showerheads 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.

• 17:15Speaker 02

  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 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 like I have many more questions that I want to answer than I have time to answer them.

• 17:49Speaker 00

  While I was talking to Ray, I was totally infected by her enthusiasm. But eventually, we did have to end our call.

• 17:59Speaker 00

  So I went back to my desk and did my job, essentially.

• 18:05Speaker 00

  I told my editors about the interview and I talked to my colleagues about it a bit. Is it like dark matter, but soft? Both 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.

• 18:46Speaker 00

  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.

• 19:28Speaker 00

  If you want to learn more about soft matter, you can read Rae Robertson Anderson's book, Bio-Polymer 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, Byrd 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

• 20:12Speaker 00

  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 or thoughts about anything at all, please write in to [email protected]. 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/members. You will get ad-free podcasts, unlimited access to Vox journalism, other perks. We are running a limited time membership sale.

• 20:54Speaker 00

  So, membership is 30% off right now. So, what a great time to join vox.com/members. You can also support us by leaving a nice rating or a review or just by telling people in your life to listen. Unexplainable is a part of the Vox Media Podcast Network. And we will be back next week.

• 21:21Speaker 01

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• 21:59Speaker 05

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