We don't understand yogurt

Summary

Podcast Title: Unexplainable
Host/Authors: Noam Hassenfeld, Julia Longoria, Byrd Pinkerton, Meredith Hoddinott
Episode: We Don't Understand Yogurt
Release Date: June 11, 2025

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Introduction to Soft Matter

In the June 11, 2025 episode of Unexplainable, host Bird Pinkerton delves into the enigmatic world of soft matter, a state of matter that defies the traditional classifications of solid, liquid, gas, and plasma. Inviting listeners to explore the peculiar properties and scientific mysteries surrounding everyday substances, Bird sets the stage for an engaging discussion with physicist Ray Robertson Anderson.

Bird Pinkerton introduces the concept by reminiscing about the classical states of matter:

“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.” [00:56]

Understanding Soft Matter

Ray Robertson Anderson explains that soft matter encompasses materials that are "bendy or squishy," exhibiting properties between solids and liquids. These materials include everyday items like yogurt, toothpaste, ketchup, and even biological substances such as our skin and saliva.

“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.” [01:41]

Unlike the well-understood behaviors of solids and liquids established centuries ago, soft matter operates under its own set of complex and not yet fully deciphered rules. Ray emphasizes the significance of understanding these materials as a stepping stone towards a unified model of the universe:

“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.” [02:04]

Everyday Examples of Soft Matter

To illustrate the peculiar behavior of soft matter, Bird and Ray discuss common examples that exhibit both solid-like and liquid-like properties depending on external stimuli.

Yogurt: A Classic Example

They examine yogurt's dual nature:

• Solid State: Upon opening a yogurt container, the product maintains its shape, allowing a spoonful to be scooped out effortlessly.

  “You can scoop out an actual piece of the yogurt and it's solid.” [03:18]

• Liquid State: Stirring the yogurt alters its consistency, making it pourable.

  “After you've stirred it, if you tilt it, you can actually pour it. It's become like a fluid.” [03:35]

Oobleck: The Non-Newtonian Fluid

Another fascinating example is Oobleck, a mixture of cornstarch and water that behaves differently under varying conditions:

• Solid Behavior: Punched or subjected to sudden force, Oobleck hardens, preventing penetration.

  “If you punch [it] stiffens up and your hand cannot go through that fluid.” [04:24]

• Liquid Behavior: Slow insertion allows complete submersion without resistance.

  “If you try to slowly put your hand into it, you can fully submerge it.” [04:24]

The Molecular Complexity of Soft Matter

Bird explains the molecular intricacies that make soft matter so challenging to study:

“In a material, you have 10 to the 23 spaghettis, that is 100 sextillion spaghettis that Ray is supposed to keep track of.” [07:09]

Ray elaborates using the metaphor of a bowl of spaghetti:

“Soft matter as a bowl of spaghetti. If I try to pull one strand out, it's hard; it might snap back. But poured altogether, it flows like a fluid.” [06:07]

This analogy underscores the difficulty in modeling soft matter due to the vast number of molecular interactions and the dynamic nature of these materials.

Implications for Biology and Engineering

Understanding soft matter extends beyond academic curiosity, impacting fields like biology and engineering.

Biological Significance

Ray highlights that human biology is fundamentally composed of soft matter:

“Life is made up of a whole bunch of squishy stuff, like our eyeballs and snot and saliva in our tongues and our skin.” [11:59]

On a cellular level, soft matter principles apply to the behavior of cells, which have molecular skeletons that can transition between rigid and fluid states to perform functions like division and movement.

“Cells can tense up and they can also fluidize to do things like divide and crawl and also like to form structures.” [12:05]

Future Engineering Applications

Ray envisions engineering breakthroughs inspired by soft matter, such as:

• Adaptive Bulletproof Vests: Vest materials that remain flexible under normal conditions but harden instantaneously upon high-speed impacts, mimicking Oobleck's behavior.

  “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.” [13:19]

• Self-Healing Infrastructure: Bridges that automatically repair cracks by utilizing cell-inspired materials that fluidize to fill gaps and then solidify.

  “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.” [14:15]

Ongoing Research and Enthusiasm

Ray expresses her excitement and the endless array of puzzles soft matter presents:

“There's so many questions that we don't understand and that I would love to study.” [15:00]

Her enthusiasm is infectious, inspiring listeners to appreciate the hidden complexities in everyday substances.

Conclusion: Embracing the Mysteries of Soft Matter

Bird wraps up the episode by reflecting on the ubiquitous presence of soft matter and its myriad mysteries:

“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.” [16:55]

Listeners are encouraged to delve deeper into the subject through Ray's book, Biopolymer Networks, and her accessible content on TikTok (@physicsmama).

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Credits:

• Produced by: Bird Pinkerton
• Edited by: Jorge Just
• Showrunner: Meredith Hoddinott
• Music: Noam Hassenfeld
• Mixing & Sound Design: Christian Ayala
• Fact-Checking: Melissa Hirsch
• Special Thanks: Brian Resnik, Colm Kelleher, Uri Bram

Additional Resources:

• Book: Biopolymer Networks by Ray Robertson Anderson
• TikTok: @physicsmama

Engage with Us:
Share your thoughts on soft matter or any intriguing condiments at unexplainableox.com. Your emails make our day!

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Support the Show:
Join our membership program at vox.com/members for ad-free episodes, unlimited access to Vox Journalism, and more perks. Enjoy a limited-time 30% discount!

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Stay Tuned:
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Transcript

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[Bird Pinkerton]

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.

[Ray Robertson Anderson]

Ranch dressing or shampoo or scrambled eggs or, or snot or even your eyeballs.

[Bird Pinkerton]

Also toothpaste and yogurt, ketchup, anything that.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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

[Ray Robertson Anderson]

It's something called soft matter.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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 like, 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. So like, let's start there and then see if we can build up to like the unified model of the universe.

[Bird Pinkerton]

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 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.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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

[Ray Robertson Anderson]

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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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

[Ray Robertson Anderson]

So Oobleck is a perfect example.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

If you punch 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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

So this is where 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 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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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?

[Ray Robertson Anderson]

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? And so it really is.

[Bird Pinkerton]

Like, did you add butter to the spaghetti? Exactly. No, it's super slippery.

[Ray Robertson Anderson]

Exactly. Right?

[Bird Pinkerton]

So like honey for some reason, because you're a sick animal.

[Ray Robertson Anderson]

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.

[Bird Pinkerton]

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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

Stuff, like our eyeballs and snot and saliva in our tongues and our skin.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

And these cellular skeletons can get softer and more fluid.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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. And very tense all the time.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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.

[Bird Pinkerton]

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

[Ray Robertson Anderson]

Exactly, bullet. Exactly.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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

[Bird Pinkerton]

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

[Ray Robertson Anderson]

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 nutrients to pass through and it lives in these tubes and it makes the tubes with the mucus. So it's crazy. Like it does this with all this one type of goo.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

They're 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.

[Bird Pinkerton]

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.

[Ray Robertson Anderson]

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 they're 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 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.

[Bird Pinkerton]

While I was talking to Ray, I was totally infected by her enthusiasm. But eventually we did have to end our call. I mean to work. So I went back to my desk and did my job, essentially. Hey, I told my editors about the interview and I talked to my colleagues about it a bit.

[Thumbtack]

Is it like dark matter but softer?

[Bird Pinkerton]

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. 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 Ray Robertson Anderson's book Biopolymer Networks or for slightly more accessible content, she suggests that you visit her TikTok, which is physicsmama, that's mama with two M's, 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 fax. Julia Longoria is the fact that toads and frogs can absorb water through their skin and I am always always always grateful to Brian Resnik 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 unexplainableox.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'[e-mail] members you will get ad free podcasts, unlimited access to Vox Journalism. Other Perks we are running a limited time membership sale so membership is 30% off right now. So 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.

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