The Science Of Slip Versus Stick

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Up next, getting down and dirty with the science of surfaces. SciFri’s Charles Bergquist is here. Hi, Charles.

CHARLES BERGQUIST: Hey, Ira. What do you think when I say the word “sticky”?

IRA FLATOW: Well, maybe honey or duct tape. Oh, that goo that you find on the side of your kid’s car seat. That is sticky.

CHARLES BERGQUIST: Ugh, yeah. All places where you’ve got that kind of tacky feeling. But on a deeper level, sticky’s all about how different surfaces come together. I recently talked with Laurie Winkless. She’s the author of a new book called Sticky– The Secret Science of Surfaces, just out from Bloomsbury. And I asked her what stickiness meant to her?

LAURIE WINKLESS: For me, I think of stickiness, really, as related to friction. That does include things like gloopy, sticky liquids and adhesives. But for me, it also includes how other materials interact. So solid materials, for example, so two solids sliding along one another– that’s all to do with friction. And then you will have things like swimsuits or aircraft moving through a fluid, and the frictional interactions that happen on those surfaces, too. So I’m kind of using the word “sticky” as an all-encompassing topic, really.

CHARLES BERGQUIST: So would something like Velcro be sticky under your definition?

LAURIE WINKLESS: Yeah, I think it would. But as I said, it’s really broad. There are plenty of spider species and insect species that also use tiny hooks to move around and to grip surfaces, so perhaps I should have called it grippy instead of sticky. [CHUCKLES]

CHARLES BERGQUIST: What about something like suction cups or static cling?

LAURIE WINKLESS: Yes, because I do feel like they fit within this wider umbrella, so how two things join together, effectively. So whether that might be by Velcro or an adhesive, or, like you said, you could use a suction cup and air pressure, or static cling.

CHARLES BERGQUIST: So are there actually different kinds of stickiness, or does it, as you say, just all boil down to friction at the molecular level?

LAURIE WINKLESS: I think there are different types of stickiness. And something I tried to talk about very early on in the book is the fact that the word “sticky” really has no scientific meaning. There isn’t this magical scale at which sticky sits at one end and slippery sits at the other end. But there are other metrics that we can use to describe how things interact, so how molecules interact within a liquid, for example, or how two solids interact when they move along one another. So for me, I think friction is what sits underneath all of it. I’m happy to debate that, perhaps, but yeah, for me, that’s the thing that really sits underneath it.

CHARLES BERGQUIST: So if we back up and talk about some of those actual gloopy, for lack of a better word, sticky substances, what makes a syrup or jam sticky?

LAURIE WINKLESS: Great question. Usually, when we think about things like that, it’s the sugar molecules. But almost always it’s to do with the types of molecules that those compounds are made up with. So we usually measure how gloopy a liquid is by a number called viscosity. So that defines, really, the friction between the molecules as they move around in a fluid.

A liquid that has a very high viscosity tends to be very gloopy. And what we think of as, I guess, tackiness, that idea of adhesion, is a function of a liquid’s viscosity. So usually, gloopy liquids will often be able to form a kind of sticky layer on a surface and will often confer some sort of adhesion properties to it. Not always, but in general. So viscosity is kind of the number that we’re looking for in that instance.

And every fluid has a viscosity. You can think of water. It has a viscosity, but it’s a lot less gloopy than, say, ketchup or honey. They’re further up on that viscosity scale. And yeah, it’s all to do with this internal friction between these molecules.

And when we put it onto a surface, so say something like Super Glue, really what defines how sticky that is is how it interacts with other compounds, so how it interacts with the air. So as soon as the cyanoacrylates– that’s the name of the molecule that’s inside Super Glue– as soon as that gets out into the air, the water molecules, H2O molecules, in the air join on to these cyanoacrylates, and they really set it hard. They make these chains connect together and form a really solid surface, and they do it very, very quickly. So you kind of have a mixture of interactions within the liquid itself, so in this case, within the Super Glue, and these long, long, long chains, and then you’ve got how it responds to, in this case, the water in the air. So you’ve got a mixture of those two things that will define how sticky or how adhesive this particular fluid is.

CHARLES BERGQUIST: Interesting. There’s a fundamental difference, then, between something that is sticky like a syrup, or gloopy, I guess was the word that we’ve been using, and something that’s actively designed to be an effective adhesive. This interlocking internally is key.

LAURIE WINKLESS: Yeah, I think that’s true. And what was kind of interesting was I have often thought about– as someone who does lots of DIY, I’d kind of thought of paints and glues as kind of being in the same box. And in a way, they are, in that they are to do with molecular interactions between surfaces and whatever’s in the fluid.

But if you think about the fundamental difference between a paint and an adhesive, a paint is a coating. It’s a top layer on something. It only has to stick to one surface. But an adhesive is there to kind of be the middle of a sandwich, the meat in the sandwich, as it were. It’s to join two things together.

So those two compounds have a very different job to do, even though they can be described using many of the same metrics, really. So there is definitely a difference between, like you said, something that is sticky or gloopy or viscous, and something that has an actual job to do. And in the case of an adhesive, that will be to actually bond something together.

CHARLES BERGQUIST: Right. So we all have had this experience that some glues are better than others for specific purposes. You’ve got glue that’s good for paper, or glue that’s good for woodworking or plastics. What makes any given glue good or bad?

LAURIE WINKLESS: It’s really to do with the actual surface that you’re dealing with. So like you said, if you’re trying to bond two pieces of paper together, there are lots of glues out there for you to do that. But if you’re trying to, I don’t know, like, bond two very low-friction plastics together, you might want to look at a different compound. And really, that was something, when I talked to people working in adhesives, and also in paint, knowledge of the surface was a really important thing. That’s the thing to think about.

So you’re thinking about what the surface is, maybe what the surface chemistry is, what molecules might be on the material that I could get my glue to react with. How rough is that surface? Do I need to think about how my glue actually flows over the surface? So you know, you’ve got lots of the questions around the surface itself.

But then you’ve also got questions about how it’s going to be used. You know, what environments will it be exposed to? What kinds of temperatures? What kind of forces? Is it trying to hold something together that would otherwise peel apart? Or are you trying to hold something together that will withstand a lot of compressive forces, so a lot of squishing forces? Or tension forces, so it being pulled rather than peeled?

So really, when you’re trying to design an adhesive for a specific purpose, all– you know, these designers, these manufacturers, are asking all of those questions, which is why there really isn’t one glue that works for everything. Adhesion is a property, as they say, of the system. It’s to do with all of the surfaces that are going to interact with this adhesive, and how those surfaces are going to be used in the future. So it is an incredibly complex thing to define and to find the exact right compound to bond two items together.

CHARLES BERGQUIST: So broadening that idea out, is then there are no such thing as stickiness or slipperiness as sort of a universal property, but everything is sticky or slippery in relation to another thing?

LAURIE WINKLESS: Yeah, precisely that. That’s exactly it. And you know some of your listeners may have heard of this term called the coefficient of friction. And this is a number that gets quoted a lot. And if you’re really keen to find coefficients of friction, there are many websites that have long lists of tables that give you that number.

But often what I’ve seen is people will– and not in technical documents, but kind of in daily life– people will talk about the coefficient of friction as if it’s something equivalent to the density of a material. But it’s not, because the coefficient of friction can only be defined between two specific things, so steel on ice or rubber on asphalt, for example. It is very specific to those materials.

And these numbers that we use– and engineers and physicists use them all the time– they’re very, very useful when you’re trying to understand how surfaces interact. They are measured experimentally. We don’t have a way to kind of gather together everything we know about two specific materials, like its lattice patterns or its crystal grain boundaries and all of those things that we can know about materials at a very small scale. We don’t have a way to translate that through a mathematical model or from first principles idea to then get this coefficient of friction.

This is just a number that’s been measured experimentally again and again and again, and these numbers are averages. So it’s not something that we have this kind of perfect key to open this door. It’s always been, when it comes to friction between solid surfaces, it’s always been something that we’ve just measured experimentally rather than predicted and modeled and then produced.

CHARLES BERGQUIST: So I’m thinking of a smooth drinking glass, which is, to my fingers, sort of itself slippery. And if you put a little bit of water on it, again, to my fingers, it feels slipperier. But at the same, time, I can stick my paper napkin to the side of the glass now. What’s going on?

LAURIE WINKLESS: [LAUGHS] Usually, the answer to these things is that we humans are pretty slippery. We’re quite moist. We always have a layer of water on us, anyway. So when we pick up a wet glass, really we have a very, very thin layer of water on our skin and a much thicker layer of water on the glass. So we end up having a very low friction interaction, really.

But when we’re sticking– when you say you put a piece of paper onto that, the paper wets. The paper is what’s called hydrophilic, which means it’s water loving. And that water absorbs into the body of the paper and holds the paper in place.

CHARLES BERGQUIST: I’m Charles Bergquist, and this is Science Friday from WNYC Studios. I’m talking with Laurie Winkless, author of a new book called Sticky– The Secret Science of Surfaces. You can find an excerpt from the book on our website at sciencefriday.com/sticky.

Does scale come into play here? Are there things that are super sticky at a molecular level but not when you scale them up, for instance?

LAURIE WINKLESS: Yeah, I mean, we know– we have lots of examples in the nano scale world of a material behaving entirely differently when you have just a few atoms of it compared to the bulk version of the material. So like, gold is a good example. On the nano scale, it’s very reactive. It’s used to catalyze other chemical reactions. But macro scale gold is– doesn’t react with anything, really, which is why we use it in jewelry that hangs around for a long time.

So the moving from the nano to the macro world is always really tricky, and it always has some– it can have some interesting implications, I guess. But yeah, we do see some materials that are particularly useful in the nano scale. And I guess I’m thinking about lubricating materials, so kind of trying to reduce friction between materials. So things like molybdenum disulfide– this is really commonly used, increasingly used, in the space industry to lubricate very kind of precise machined components. And it works particularly well in the space environment.

But that’s not something necessarily that you would want to make a big paste of molybdenum disulfide and smash it into your car engine parts. That really works best when it’s an extremely thin layer. So it gives you the kind of highest, easiest movement between surfaces when you just have a few layers. You know, I’m talking about five, 10, 15 atomic layers of molybdenum disulfide. That’s where you get the lowest friction. And you don’t really see that when you scale it up. So yes, scale certainly does have a role to play.

CHARLES BERGQUIST: Going on with the idea of lubricants, the sort of cartoon image always seems to be sort of a layer of super tiny ball bearings between the surfaces. Is that accurate, or is there something more complicated going on here?

LAURIE WINKLESS: [LAUGHS] Yeah. Yeah, it kind of is accurate in some cases, but usually those ball bearings are coated in something else. So the ball bearings themselves are lubricated with some other sort of compound.

The thing about lubrication is that we’ve been doing it for a long time. We’ve been lubricating contacts since the Industrial Revolution. And arguably, we’ve been doing it for much longer. There’s some examples of Roman chariots, for example, using waxes and animal fats to lubricate the way that their chariot wheels moved, so putting it onto the axle of a chariot wheel to make the chariot wheels’ motion smoother. So we’ve been doing lubrication for a long time.

But even in the time of the Industrial Revolution, it was very much an experiment, like, let’s chuck these compounds together and see if we can get a low-friction surface. And let’s see if it will work in my engine part or my machine, whatever my machine is. That process has changed a little bit in more recent years. Definitely, lubrication engineers are much more scientific in their approach to designing the right type of compound for their specific purpose.

So again, it comes back to this idea of adhesion being a property of the system. You will want to choose your lubricant based on the precise materials that you have in your system. And sometimes they are liquid lubricant, so they are kind of a liquid or a paste. Increasingly, they’re dry lubricants, so they will just be like a dust, effectively. And you can think of graphite on your pencil nib. If you kind of scribble on your page and then rub your finger over it, you’ll see the friction is much lower if you put graphite there. So graphite lubricants, these kind of powder, dust-based lubricants– very, very common.

So you’ve got dry and wet lubricants, you’ve got lubricants that can withstand incredibly high temperatures, incredibly corrosive environments. You have lubricants that work particularly well in the space industry. So definitely, our ability to lubricate mechanisms has improved drastically in the past few centuries.

And now that our technology is starting to kind of miniaturize, I guess, and we’re starting to make increasingly small devices, there’s a real growing interest in trying to design lubricants or low-friction surfaces that behave like that on the nano scale. So could we just use a single layer of graphene, for example, to reduce the friction between a tiny micro-electromechanical system? So now that we’re going down to that sort of scale, we really have to understand the fundamentals of how these materials work and how we can reduce friction. So that’s a big, big push now in the lubrication sector.

IRA FLATOW: We need to take a break. We’ll be back with more sticky situations, and some slippery ones, too, like the mysteries of your non-stick pan, after this.

This is Science Friday. I’m Ira Flatow. Continuing our conversation with Charles Bergquist about sticky versus slippery. So Charles, it all comes down to friction– sandpaper, sharkskin, things like that?

CHARLES BERGQUIST: On some levels, yes, in many ways. But there’s still a lot scientists have to learn here. I asked Laurie Winkless about the science of surfaces at different scales.

LAURIE WINKLESS: We know a lot from a practical point of view. We’ve been manipulating surfaces for a very, very long time. We are experts at it. There are examples from ancient Egypt, which suggests we’ve been lubricating surfaces for millennia.

So on that level, we really do understand friction. And there are companies and research organizations all over the world whose job it is to understand and manipulate friction and to produce products that help us to do that. And of course, all the paints and adhesives manufacturers. So we know what is going on on the macro scale.

Something I didn’t quite realize is how much we are learning about friction at the nano scale. And at this scale, I’m talking about a few atoms, you know, one or two atomic layers on top of one another. I kind of thought it was still all a big giant mystery. But that hasn’t really proven to be the case. We are developing a fairly sophisticated understanding of friction down there at the atomic scale.

And like I mentioned, we are thinking about how heat is transferred through solid materials and things like that, really some of the nitty gritty details about friction. But what we lack is a model that joins the two. So we have all of this information at the nano scale and all of this inherent knowledge, really hard-earned knowledge, on the macro scale. But we don’t have anything that joins those two schemes. There’s no way for us right now to take what we know about a material on the atomic scale and use that to predict its frictional behavior or its adhesive behavior on the macro scale.

CHARLES BERGQUIST: Interesting. One of the things that people think of as sort of the classic example of slipperiness is Teflon. What makes it so slippery?

LAURIE WINKLESS: Teflon basically hates everything that is not Teflon. That’s the simple answer to that. So Teflon, which we would usually call PTFE, because Teflon is just its trademark– what it looks like is a long chain, a kind of a backbone of carbon surrounded by fluorine atoms. And the bond between those fluorine atoms and this carbon backbone is incredibly strong, like, famously one of the strongest in organic chemistry.

And that basically means that there’s no available bonds. There’s no kind of loose, wiggling bonds available to react with any other material. If you have a Teflon surface, and you put a different compound onto it, that compound just has no opportunity. There’s no way in to the Teflon structure. And that is really what makes it, like, ultra, ultra, ultra low-stick and why we’ve put it on our frying pans for such a long time.

CHARLES BERGQUIST: But how do they get it to stick to the frying pan?

LAURIE WINKLESS: [LAUGHS] The ultimate question. Lots of different ways. I struggled to find lots of detail on this. But you’ve kind of got two main categories.

The first one is that you– so if we’re thinking about a Teflon pan, we might start with something that’s made from aluminum, right? So one option is that we sandblast the aluminum, or we might stick it into an acid bath. And really, what you’re trying to do there is to roughen the surface, make it as rough as possible, pit some holes into it, lots of cracks. Kind of you’re trying to cause a fair bit of damage to that aluminum surface.

And then when you spray on a very thin layer of Teflon to that, it doesn’t react with the surface, but it kind of gets caught in all of that roughness. It gets caught in the bumps and the cracks and the holes that you’ve created. So it’s kind of clinging on like a mountain climber will cling on to a rock. It’s not really about chemistry. It’s more mechanical.

So once you’ve done that, then you’ve got a thin layer of Teflon. And then you just layer on more Teflon. And we know Teflon loves to stick to Teflon, so that’s one option.

The second way to do it is to actually try to break down this fluorocarbon bond, this bond between the carbon backbone and the fluorine atoms. And that takes a lot of effort, a lot of energy. You basically slam it with charged particles to try and knock some of those fluorine atoms away.

And the other option is you could try and replace the fluorine with something else. But again, that’s really hard. But either way, what you’re trying to do there is to make the Teflon available to do a bit of chemistry.

CHARLES BERGQUIST: So what are some of the other big unanswered questions in stickiness? Are there things that we just don’t get? Or is this pretty much a settled thing and we’re just tweaking the parameters of what we already know about?

LAURIE WINKLESS: It’s definitely less settled than I had realized when I set out to write this book. That’s for sure. There were a few topics that when I had started putting together the ideas for this book, I thought, OK, well, we’ll talk about geckos, because we totally understand how geckos can do what they do. And actually, I found in chatting to researchers working on gecko adhesion that there are still some unknowns in there. And I didn’t really expect that.

I also didn’t really expect us to not fully understand where ice– you know, why ice is slippery. And particularly, I’ve become very interested in curling, this kind of iconic sport of the Winter Olympics where you’ve got these people with a big curling stone and a broom sweeping in front of it furiously on the ice. I did not expect to find out that we don’t understand why a curling stone moves the way that it does. These were topics I picked because I felt like, this is good, these will have straightforward answers, you know? There have been heaps of things that I thought were neatly tied up in a bow that have not proven to be the case, which is kind of joyful for me.

CHARLES BERGQUIST: Well, this has been delightful. Thank you so much for taking time to talk with me today.

LAURIE WINKLESS: Thank you so much, Charles.

CHARLES BERGQUIST: Laurie Winkless is author of a new book called Sticky– The Secret Science of Surfaces. You can find an excerpt from the book on our website at sciencefriday.com/sticky.

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