The Fluids That Flow Through Our Lives

23:10 minutes

Think back to the start of your day. You check the time, perhaps on a liquid-crystal display. You take a shower, enjoying the water. You grab some coffee, or maybe a glass of juice. And then you hop aboard a gas-powered vehicle of some kind. What do those things share in common? Liquids.

Read an excerpt from “Liquid Rules.”

They’re all around you, of course—but how often do we take a moment to think about how liquids work? What makes one slippery and another sticky? Why does one make a good salad dressing, but another a good rocket fuel? Materials scientist Mark Miodownik has tackled those questions, and others, in his new book Liquid Rules: The Delightful and Dangerous Substances that Flow Through Our Lives. He joins Ira to pour out some knowledge.

Further Reading

  • Listen to the last time Mark Miodownik was on Science Friday, where he discussed the world of solid materials.
  • Read an excerpt of Miodownik’s last book, Stuff Matters.

Segment Guests

Mark Miodownik

Mark Miodownik is author of Stuff Matters: Exploring the Marvelous Materials that Shape our Man-Made World and Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives. He’s also director of the Institute of Making and professor of materials and society at University College London in London, England.

Segment Transcript

IRA FLATOW: Think back to the start of your day. You checked the time, a shower, some coffee, maybe a glass of juice. Maybe a gas-powered vehicle was involved. What do these things share in common? Of course, chances are liquids of some kind were all involved. 

They’re all around us. How often do we take a moment to think about how the liquids work and what makes one liquid different from another? What makes one slippery, another sticky? What makes one a good salad dressing, but another good rocket fuel? 

Well, my next guest has taken some time to think about those and other questions. And he tackles them in his new book, Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives. Mark Miodownik is the author of Liquid Rules. He’s also director of the Institute of Making at the University College London. He’s professor of materials and society there. Welcome back to Science Friday. 

MARK MIODOWNIK: Thank you. Hello. 

IRA FLATOW: Great book. Let’s start with some definitions. What makes something a liquid? 

MARK MIODOWNIK: Yeah. Well, I mean, this is the strange thing is you’d think that would be a very easy question to answer, wouldn’t you? And actually, doing the research for the book, that’s the one question I found increasingly difficult to answer. The normal definition is a liquid is a substance that will flow. And if it flows into a container, it takes the shape of that container. 

But quite quickly, your listeners are probably thinking, well, that works for water, or beer, milk, orange juice, gasoline. But is the gel I use on my hair? Is that a liquid? Or is peanut butter a liquid? And you start thinking about these other substances. And they seem to sit somewhere between a solid and a liquid, right? 

IRA FLATOW: Right. Right. Let me get our listeners in. 844-724-8255, if you’d like to talk about liquids. 844-724-8255. This is Science Friday from WNYC Studios, talking with Mark Miodownik, author of Liquid Rules. It’s interesting you brought up peanut butter because we don’t think of peanut butter as a liquid. 

I know. And in fact, I didn’t either until I was going through airport control. And I got it confiscated from my luggage. And actually, that was one of the kind of spurs of writing this book. Because of course airport control, the security, they are really up on liquids, aren’t they? And we’ve all lived through this. 

Because they didn’t seem to be bothered about liquids about 15 years ago. And then suddenly, whoa, liquids were enemy number one. And we all get frisked for them. 


MARK MIODOWNIK: And peanut butter, they confiscate. 

IRA FLATOW: They confiscate. In fact, your book is based on a sort of a flight. You take a transatlantic flight. And you talk about all the liquids you encounter on the flight. 

MARK MIODOWNIK: Yeah. Because once you get frisked at the airport for your liquids, they take them all off you. Then they try and sell them all back to you, don’t they? You go into this cornucopia of liquids– booze. They try and give you coffee. 

There’s a toilet there. You can get rid of your liquids. They give you– there’s perfume. There’s creams of all sorts. Now, are creams liquids? They are actually liquids. 

And then you get on the plane. And then you get the pre-flight safety briefing. And then the whole safety briefing fails to mention the one liquid that you absolutely rely on to get anywhere on a plane. It’s under your seat. It’s in the wings. It’s aviation fuel. 

And if you saw a plane the way you see a disposable lighter– you know these transparent, disposable lighters you can buy? And you can see the liquid fuel in them. That’s what a plane would look like, if it was transparent. There’s a lot of liquid in the bottom of a plane. 

IRA FLATOW: And you spend an interesting amount of time, and it was very fascinating to learn about the kerosene, the jet fuel, how that was invented and sort of let go. 

MARK MIODOWNIK: Yeah. I mean, that was invented 1,000 years ago by a Persian chemist, alchemist, who stumbled across it. And he was sort of investigating and discovering lots of new chemical processes. And he looked at this tar that was bubbling up outside Baghdad. 

And he did all sorts of chemical processes on it, distilled it, found this very clear liquid you could get from it, which turned out to be kerosene, put it into an oil lamp, and found that it was a smokeless fuel. And at the time, of course, that was very significant for him. Because smokeless fuels were associated with a myth of the jinn, or the genie. 

And the genie in the lamp is a recurrent myth in Middle East about the magic of something that can burn with a smokeless fuel. So he was very, very excited about discovering this thing. But at the time, they didn’t have the industrial processes to actually take advantage of it. So it sort of got lost in time for 1,000 years. 

IRA FLATOW: And now we use it for jet fuel and all kinds of stuff. It’s very, very important. And in your book– we only have about a minute to the break. I want to get into the ideas of why you can have a liquid that will explode, gasoline, or why the liquid is just like alcohol, and why some is sticky, why some glue, why– it’s all very fascinating. 

I’m talking with Mark Miodownik, author of Liquid Rules. And we’re going to talk about this book when we come back. Our number, 844-724-8255, if you have any questions about what makes a liquid a liquid or any of your favorite liquids. And I’m going to get into– Mark, you talk about alcohol and wine a lot in this book. 

MARK MIODOWNIK: Of course. How could I not? 

IRA FLATOW: You spend a lot of time with the booze in there. So we’ll– 

MARK MIODOWNIK: I did the research. 

IRA FLATOW: That’s good. 844-724-8255. We’ll be back after the break. So stay with us. 

This is Science Friday. I’m Ira Flatow. We’re talking with Mark Miodownik, author of the new book Liquid Rules. He’s also director of the Institute of Making at University College of London, professor of materials and society there. 

We have so many people who want to talk about liquid. So I’m going to go right to them. Because they’re better than my questions. Let’s go to Kyle in Augusta, Georgia. Hi, Kyle. 

KYLE: Hi. How you doing? 

IRA FLATOW: Hi, there. Go ahead. 

KYLE: I’ve been told that glass is a moving liquid. And I just wanted to ask our expert on this. 

IRA FLATOW: Yeah. No, that’s a good one. Because that’s a persistent myth, that glass, which we all see as a solid– and it looks solid enough, doesn’t it, when you tap your finger on it? And if you actually bash your head into it, it definitely feels like a liquid. 

Now, the thing is that the myth I think comes from when people look at old buildings, churches, and cathedrals from 1,000 or 2,000 years ago, they see often that the glass is thicker at the bottom than at the top. And so there’s this sense that it might have flowed down over 1,000 years. And the reason that people then sort of wonder if that might be possible is because if you look inside the glass at the structure of the atoms that make it up, they have a liquid structure. 

And what I mean by that is a random structure. It’s not crystalline. And so all the evidence seemed to stack up that glass was flowing– not in seconds and minutes, but over 10, 20, 100 years. But since then, it’s been discovered that that really doesn’t happen, that although glass can flow over geological times, it’s not flowing at that rate. 

And the reason for the thickness of the glass in these situations is probably that the early ways of making glass were to pour it on as a puddle when it was molten onto a plate and then cut it up. And there would be thicker bits in the middle, as you can imagine. Imagine pouring toffee. 

It’s very hard to get it very flat. And so they arranged the slightly thicker bits at the bottom. So this is essentially the story so far. 

IRA FLATOW: I remember my engineering professor said, yeah, it does. It’s called creep. It flows. So we can change that now, right? 

MARK MIODOWNIK: Well, the thing is that things do flow. Most solid things will flow over long periods of time or if you increase their temperature. Creep happens in jet engines of the plane that you– and that happens over hours. And you’ve got to be really careful because the temperatures are very high. 

But it also happens under our feet. The rocks that we stand on, the solid rocks that we think of as solid earth, they are creeping. And that’s the origin, of course, as I talk about in the book, of earthquakes. Because they creep towards each other. And then, of course, the stresses build up. And that releasing of that stress is an earthquake. 

And of course, the continents drifting over, again, millions and millions of years, that is this sort of flow of rocks, the creep of rocks. So it’s a real thing, creep. But it does take long periods of time, unless the temperatures are very high. 

IRA FLATOW: Let’s talk about the most basic of all liquids, and that is water. 

MARK MIODOWNIK: I mean, yeah, that’s amazing. Water is a– 

IRA FLATOW: It’s really amazing, isn’t it? 

MARK MIODOWNIK: I mean, the thing about water, it’s this thing called the universal solvent. It’s the only liquid, well, pretty much the only liquid we know of, that will dissolve organic carbon-like molecules as well as absorbing ions and things like salts. And so this means that it’s a melting pot of lots of different molecular types. 

So normally, you sort of think of oils as being very separate. And of course they are. But actually, certain carbon molecules have a little bit of polarity, as it’s called, which is that the structure of the electrons is not uniform. And this gives them a slight charge at one side or the other. And water’s the same. It’s also polar. 

And so it will basically absorb pretty much anything, water, in some way. So it’s this thing called the universal solvent, the melting pot of the universe. And that’s why they think that life evolved from the oceans and why, when you look for life in the stars and on planets, that you always look for liquid water. 

IRA FLATOW: Let’s go to San Antonio and Gabriel in Texas. Welcome to Science Friday. 

GABRIEL: Thank you. My question was if he was familiar with the pitch drop experiment of Thomas Parnell in the University of Queensland in Brisbane? And then the other question was, what role does viscosity play in defining something as a liquid? Is it possible to have something that has a viscosity but is not a liquid? 

MARK MIODOWNIK: Yeah, that’s a great question. So the pitch drop experiment is this amazing experiment which was carried out– so pitch, you can call it tar. And this is the stuff that we make roads from. Thomas– the Parnell experiments, he put some pitch, this tar. 

And we all drive on this stuff. It looks pretty solid, right? We’re driving on it. It can’t really be a liquid. But anyway, he asked this question. What would happen if you just put a blob of it and just waited for it? And he had a funnel. 

And he wondered whether if it was a liquid, well, then a little blob would come out. This funnel, a drop would come out. Anyway, years went by. And this material did sort of settle in the funnel. And then a drop started to form. And then, plop, it came out. But he had to wait about a decade. 

And in fact, that experiment’s been running for about 100 years now. And there’s been about, I think, nine drops. So this is an incredible thing. Each drop, if you pick it up, it’s a solid. But clearly, it was formed in a liquid process. 

So then you start thinking, well, actually, the tar on the roads is a liquid. It’s just a very viscose liquid, an incredibly viscose liquid, a million times more viscose than honey. And so then you bring up this thing about viscosity. 

And I think, essentially, people talk– viscosity is only really useful as a term when you talk about flow. And it’s how fast things flow. And if things aren’t going to flow over any time period that you can measure them, then you’re not going to give something a viscosity. It’s going to be a solid. 

IRA FLATOW: What turns something that’s a liquid into a solid? For example, you talk about Krazy Glue in your book. What is it that’s going on? I mean, suddenly you squirt it out. It’s a liquid. And it sits there a while. And suddenly, it’s a solid. 

MARK MIODOWNIK: I mean, the glues that we now have in our lives are just so amazing. I really think that if there’s one thing people should just sit in awe of in a DIY shop, they should just be in awe of the liquids on this. Because you can glue anything together now. And often, it’s stronger than if you bolted it together. And it really is a huge step forward. 

And you asked this thing about Super Glue. And what is this happening? If you take a drop of Super Glue, it just looks like a drop of water in a way, sticky water. But 10 seconds later, your fingers are stuck together. It’s so effective. 

In that case, what’s happening is water is causing the individual molecules of the Super Glue, which is a cyanoacrylate molecule, to bond together into a polymer. So it becomes a plastic. So you’re turning something that is a sort of fluid liquid into a plastic in real time in about 10 seconds. 

And the thing that turns it from one to the other, the thing that bonds the molecules together, is water. And you don’t need very much. In fact, the breath, the water in your breath, is enough. So if you just breathe on whatever you’re gluing together before you join the two pieces, that’s enough water to polymerize it, as it’s so called. 

But there are a lot of other things that people don’t realize are glues that polymerize. So for instance, oil paint, all those masters, the Rembrandts, the Renaissance paintings, all of that oil paint, that’s all the same process. That’s a polymerization reaction. Those are plastic paintings. 

IRA FLATOW: They’re plastic? 

MARK MIODOWNIK: Yeah. Because you put the oil on. And it’s reacting with the oxygen in the air to become a plastic. And that’s what it is. And that’s why you have to wait. Anyone who’s an oil painter will know this, how frustrating it is. Because you have to wait for it to polymerize, i.e. harden. 

It’s a chemical reaction with the oxygen in the air. It becomes a layer of plastic. And then you can put the next one on. If you forget, if you don’t wait for it to harden, i.e., you don’t let enough oxygen get to it, then when you put the next layer on, it will just mix with the previous layer. And you’ll get a mush. 

IRA FLATOW: Did the masters know that they were making plastic when they were putting their– 

MARK MIODOWNIK: What they knew, which is really amazing, and what they had discovered, is that if you do wait, and you get this layered structure, what you’re doing is you’re doing what nature does with beetles. You can get incredible coloring by essentially reflecting different layers at different bits of the painting. And so they had 11 or 12 layers. 

And you can get really deep blacks or luminous whites by having all these different layers. And you see it in nature. But they sort of rediscovered that. And that’s the appeal of that technique. 

IRA FLATOW: Let’s go to Pennsylvania. Let’s go to– is it Janelle there? 

JANELLE: Yeah, hello. 

IRA FLATOW: Hi, there. Go ahead. 

JANELLE: I was calling to see, is it true that most metal, like steel, [INAUDIBLE] in a liquid form before [INAUDIBLE] form [INAUDIBLE]? As far as making rings, making frames for cars and stuff like that, is that all melted down to really hot temperatures into liquid form and then put into a mold? Or how is that formed? 

IRA FLATOW: What is melting all about? 

MARK MIODOWNIK: Yeah. I mean, I think you’re asking about how the kind of wheel hubs and all of the bits of the metal car– do they start out as liquid metal? And the answer is yes. And what is melting? I mean, you start with a solid. 

And in the case of metal, it’s individual atoms, let’s say, of steel, which is what most of the cars are made of. So you have iron atoms. And they’re all irregularly arranged in a crystal and trillions of them in every direction. So what does that mean? 

If you look down, there’s just long lines of them all in the right cubic array. And then little bits of carbon are stuck in between. That’s the magic ingredient. And then, if you– that’s a solid. And that’s a great solid. And we all rely on it in our lives– our bridges, our tunnels, our railways, our cars, everything. 

But if you heat it up, those atoms start to vibrate. Well, they’re vibrating anyway. But they vibrate more and more and more. And at some point, the energy of vibration overcomes the bonds holding them together. Now, you’d thought that would just make them all fly apart and become a gas. 

But actually, there’s an intermediate state, where they’re bonded together, but there’s still so much heat that they can’t really stay next to their neighbor for very long before zooming off somewhere. And so they’re constantly being pulled together and then kind of allowed to mush around. And that’s the liquid state. 

So liquids are the state between total chaos and total kind of prison for atoms. Atoms are either in a prison, in which case they’re that solid, or they’re flying around the atmosphere. They’re a gas. And the liquid state is somewhere in between. And as I said in the book, the liquid state kind of has elements of both. 

IRA FLATOW: Yeah. You spend– before I run out of time, I want to get to one of– it looks like one of your favorite subjects, and that’s wine. 


IRA FLATOW: As a liquid. Why– you got very interested in that. And let me just remind everybody that this is Science Friday from WNYC Studios. I’m Ira Flatow, talking with Mark Miodownik, a really interesting new book, Liquid Rules. OK. Tell us about wine, why you find it so fascinating, and how you’ve done your homework on it. 

MARK MIODOWNIK: So I mean, the thing about wine is– and especially if you get on a plane, and they give you some wine, it’s always hard to resist, isn’t it? And you’re drinking this drink. And the truth is it’s a poison. I mean, ethanol, which is the alcohol in wine, it really poisons your body. 

But it poisons your body in a pleasing way. And basically, there’s a certain percentage of it in there that, for most people, it puts your body into a state where you’re sort of intoxicated. And the word intoxicated is the clue, right? Toxic. It’s toxic for you. 

But actually, that then disables some of your systems, some of your nervous systems. And it essentially relaxes you. And that’s I guess one of the things we like about it. Not everyone likes it. And some people feel out of control. And some people abuse it. And of course it damages your body and damages your brain. 

So the question is, why do we put up with that? Why do we just not have grape juice? You know, you could have grape juice. Why is something with alcohol in it so much better as a drink? And I think the thing I wanted to say in the book is that actually wine, the thing it really does really well, apart from making you drunk, is it’s a fantastic accompaniment to meals. 

It really is the drink of choice for that. And the reason it is is because it cleanses the mouth really well. And it does it in two ways. One is the alcohol’s a very good solvent. They’ll take a lot of the fats away. And a lot of meals, if they’re fatty, they’re delicious for that reason– creamy, delicious chips, salmon, steaks, delicious. 

But it coats your taste buds. And it dulls the next bite. And if you want to get rid of that, then you want something that’s astringent in there. You want a tannin. And wines convey that in them, especially the red wines. 

And so the tannins, it’s been shown with experiments that tannins will cleanse your mouth of the fattiness feeling. So this thing about having a wine with a steak or a hamburger or some fish, this really is good. It actually increases your enjoyment of that meal. 

IRA FLATOW: I have one– let’s see if we can get one more call in. Tracey in Savannah, Georgia, welcome to Science Friday. Hey, Tracey. 

TRACEY: Hey. How are you this afternoon? 

IRA FLATOW: Fine. Go ahead. 

TRACEY: OK. So I’m asking you to solve a debate that’s been raging at the school where I teach for the entire school year. Is water wet? 

MARK MIODOWNIK: Aha, yeah. Now, that’s a good one, isn’t it? 

IRA FLATOW: What does that mean? What do you mean, Tracey? Why is that a debate in your school? 

TRACEY: Well, there is a group of people who say that water is wet. Because if you touch it, you get wet. But then there’s other people who say that it’s not wet. Because wet is what happens when you touch the water. 


MARK MIODOWNIK: Yeah. And I’m with them on this. So wetting is a phenomenon where a liquid– it’s how a liquid interacts with a solid. So when you’re in the shower, the liquid doesn’t just bounce off your body. It wets your body. 

And what does that mean? It means that the surface tension of the liquid means that it’s kind of– it sticks to your body. So this is where a lot of stickiness comes from. And the sticking to your body is the wetting. And of course that’s where it comes from. 

But water isn’t the only material that wets your body. Oils wet your body. And so, in fact, if you wanted things to be waterproof, then you created non-wetting surfaces. And one of the best ones, of course, is Teflon. It doesn’t wet many things. And so things, liquids, sit proud on them. 

IRA FLATOW: Well, while I have you there– and I’ve got less than a minute to go– I wanted you to talk about why you hate liquid soap so much. 


IRA FLATOW: 30 seconds. 

MARK MIODOWNIK: OK. Liquid soap came along as an advertising, as a marketing, ploy. We didn’t need it. We had bar soap. And bar soap’s fine. There’s nothing wrong with bar soap. And it doesn’t need a load of packaging either. 

Liquid soap comes along. And it’s too good, actually. You don’t need half as much as you get when you squirt that thing in your house. And when you put your hands under the water, most of the liquid soap goes down the drain without ever cleaning your hands. 

So because it’s a liquid, you basically lose most of it down the drain, unlike with a solid. And so we’ve invented this thing that you buy loads of. You hardly use it. And it goes straight into the environment. And then it pollutes it. 

IRA FLATOW: There you go, firing that one out. Very nicely done. And there’s so much very nicely done in your book, Mark. It’s a great book. Mark Miodownik, he’s author of the new book Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives. He only touched upon them today. Thank you for taking time to be with us today. 

MARK MIODOWNIK: It’s a pleasure. It’s a pleasure, yeah. 

IRA FLATOW: We’ll be looking for the follow-up. 

MARK MIODOWNIK: Yeah, of course. It’s gas. 

IRA FLATOW: Gas. Have a great weekend. That’s about all the time we have. BJ Leiderman composed our theme music. And if you missed any part you want to hear of this terrific interview we just had, you can ask your smart speaker to play Science Friday wherever you want. Every day now is Science Friday. 

Have a great weekend. Nice to be back. I’m Ira Flatow in New York.

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