Science Friday - Phasing Out “Problematic” Plastics, Sticky Surface Science, Monarch Boom. Feb 4, 2022, Part 2

Episode Date: February 4, 2022

Phasing Out “Problematic” Plastics Plastic packaging is just about impossible to avoid. Getting takeout? You’ll likely wind up with a plastic container, or cutlery. Grabbing a coffee? Plastic stirrers and straws are hard to evade. These items are tough to recycle, and most sanitation systems aren’t equipped to process them. That means they go into the trash, or worse, waterways. Last week, the U.S. Plastics Pact released a much-anticipated list of “Problematic and Unnecessary Materials” that pact members should phase out by 2025. These items include cutlery, straws, and stirrers, as well as materials that include certain chemicals and pigments. The impact could be large: Pact members make up about third of America’s plastic packaging producers. Members include companies that use a lot of packing, like Target, Walmart and Aldi, as well as those that make raw plastic materials. The goal of the U.S. Plastics Pact is to help make America’s recycling system more circular, where materials in theory could be recycled in perpetuity. But some in the plastics industry say the timeline for phasing out these materials are too fast, or may cause a reliance on more carbon-intensive materials. Joining Ira to break down the potential impact of phasing out these materials is Emily Tipaldo, executive director of the U.S. Plastics Pact, based in Mount Pleasant, South Carolina.   The Science Of Slip Versus Stick We’ve all had the experience of that uncomfortably sticky feeling of syrup or jam residue on the breakfast table. Or a wad of chewing gum binding our shoe to the sidewalk. But what’s the science behind why some things stick, while other things slip? Many of the reasons come down to friction, says Laurie Winkless, a physicist and science writer based in New Zealand. Her new book, Sticky: The Secret Science of Surfaces, explores how different materials interact—from the toes of an acrobatic gecko scaling a sheer wall to the molecular magic inside the rapid fusion of super glue. Winkless joins SciFri’s Charles Bergquist to talk about surface science, and what makes something slippery, including the question of how the famously non-stick Teflon manages to stick to your kitchen frying pan.   How Long Will California’s Butterfly Boom Last? Like their brethren east of the Rocky Mountains, the western population of monarch butterflies has been declining steeply since the mid-1990s. Every November, volunteers set out through the mountains of California with one goal in mind: Count those western monarchs as they gather for winter hibernation. Unfortunately, the recent numbers have been bad news. Back in the 1990s, the western population numbered more than a million. But in 2018 and 2019, volunteers only counted about 20,000 and 30,000, respectively. In 2020, the count turned up a mere 2,000 butterflies. This year, though, the news was good: The 2021 Thanksgiving Count found nearly 250,000 butterflies in winter enclaves throughout California. How did the population bounce back so dramatically? And is this number a blip on the radar, or the start of better times for the beleaguered butterfly? Ira talks to UC-Davis entomologist Louie Yang about the intricate timing of milkweed and monarchs, and why ecologists remain uncertain about the fate of this charismatic insect.   Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

This is Science Friday. I'm Iroflato. Later in the hour, diving into the sticky world of surface science. But first, I don't need to tell you that plastic products are everywhere. It's hard to go through life without picking up a plastic something, right? Getting takeout, here's a plastic fork and a knife. Grabbing a coffee, take a little plastic stir or a straw with you. These items are tough to recycle, and most sanitation systems aren't equipped to process. them. That means they go into the trash, or worse, they wind up in waterways. Well, last week,

the U.S. Plastic Pact released a much-anticipated list of problematic and unnecessary materials, which PACT members should phase out by 2025. Now, this is a big deal because the Pact's members make up about a third of America's plastic packaging producers, companies that use a lot of packaging like Target, Coca-Cola, and Walmart, as well as companies that make raw plastic materials. Joining me today to talk about which materials will be phased out, and the possible impact of this is my guest, Emily DiPaldo, Executive Director of the U.S. Plastics Pact, based in Mount Pleasant, South Carolina. Welcome to Science Friday. Thanks, Ira. I'm so glad to be here.

Well, we're so glad to have you. Thank you. Let's get right into this. let's talk about the materials that will be phased out. I've mentioned some that people might be familiar with, like straws and those little coffee stirs, but what else is on the list of problematic plastics? There are 11 materials on the list that we released, and some, as you just mentioned, are formats, so like cutlery, straws, stirs. Some are actual plastic resins or the plastic types that you would think about.

So PVC, polystyrene, and PETG. And then some of the items are additives or things that are added to provide some performance enhancement for plastic packaging, like non-detectable pigments, thinking about carbon black or pigmented PET. So again, talking about colorants that are added, or things like oxodegradable additives, which are added sort of with an eye toward plastics being able to break down, but oftentimes they just break down into smaller plastic pieces or microplastics. And what makes these ones that you've chosen so problematic? The U.S. Plastics Pact, our primary reason for being is the creation of a circular economy for plastic packaging.

So we're working together as an initiative to build this circular economy. And as part of that, we are constantly asking ourselves, you know, are these things reusable, recyclable, or compostable? And we have very tough definitions for those terms. So if something that's used in plastic packaging or the plastic packaging itself is not reusable, recyclable, or compostable today, or if there isn't that trajectory in place for it to become one of those three things in just the next couple of years by 2025, then we have to ask ourselves, is there really a place for this in the future? And is there the support behind it to bring it into circularity, to make it reusable, recyclable, or compostable? And if the answer is no to those things, then we're

looking at a potential contaminant to the system, something that our system can't manage and may wind up as litter or in the environment. Now, I said that the companies that make up the U.S. Plastics Pack make up about a third of plastic packaging producers. And some of these companies are very familiar, like Walmart and Clorox and Nestle. There are also recycling associations and companies that make raw plastic materials, as you say. Does that mean we can, expect that those companies will not produce any of these plastics on the problem plastic lists by 2025. You're on the right track.

So it means that those companies or organizations who've signed on to participating with the U.S. Plastics Pact will work over the next couple of years to eliminate these 11 materials from their supply chain. So if it's relevant to their business and part of their supply chain, then, yes, they are meant to take voluntary action to eliminate these things, hopefully completely doing so by 2025. But that's what we will be measuring year over year to track that reduction progress. Now, you said voluntary action. Are these companies going to be held accountable? and how would that work? Yes, they will be held accountable.

So one of the key tenants of the U.S. Plastics Pact is that each year, each participant is required to report. So we are reporting progress toward our 2025 targets that we have set for ourselves. There are four targets. And the very first target is spot on about our elimination. and reduction goals. So we will be measuring year-over-year progress as to how all of the signatories to the U.S. Plastics Pact are taking steps and eliminating these items

from their supply chain. And then that will be reported publicly. So we're able to track and hold folks accountable for how they're doing in terms of eliminating these items. Yet about two-thirds of plastic packaging is made by companies that are not in the Plastic Pact. Do you expect that some of these will phase out these problem plastics on their own and join with you someday? That's a great question.

So while we do have sort of limited breadth and depth in terms of the signatories to the U.S. Plastics Pact, you had mentioned earlier, some are national or multinational companies with really large footprints, which means they have lots of suppliers and lots of customers sort of at both ends. And I would highly anticipate sort of the positions and something like the elimination list from the U.S. Plastics Pact to really penetrate those supply chains of the U.S. Plastics Pact signatories. So it will have waves throughout the broader plastic packaging value chain. Now let's talk about some of the pushback that you're getting. I understand that the American Chemistry Council has pushed back saying that phasing out these materials will lead companies

to rely on materials with a higher carbon footprint. More recyclable materials like metal do have a high carbon footprint. What's your response to that criticism? Oftentimes the upstream industry will raise the issue of what happens when you switch to an alternative material and potentially drive up carbon emissions. And while it is valid to consider how carbon emissions or greenhouse gas emissions are impacted by switching materials, there are a number of other factors that you have to consider alongside carbon. We can't have carbon tunnel vision. So, So we need to think about a circular economy because, again, that's what we are looking to build as the U.S. plastics pact.

And by building a circular economy, we need to think about keeping materials in the loop. So how are we doing that? How are we being regenerative? How are we also looking at other environmental impacts? So it's much bigger than just assessing the carbon footprint. You know, I can see that you walk a tight rope and you find yourself between a rock and a hard place because I know that some big players in the plastic industry say that these goals are too aggressive. And some environmentalists say they're not aggressive enough. How do you walk that line?

That's a great question. We are sort of in a very interesting spot. And I think part of it is we don't. have time to waste. I think over the last 10 years or so, we have seen a number of multinational and national companies make aggressive sustainability goals, make packaging goals, and the dates by which those things are supposed to happen come and go, and we haven't seen progress. And it's really hard to know whether or not they're delivering. So in some sense, we have really aggressive goals in order to ignite a fire under companies and really push them to see what can we do in a

really short amount of time and come to a realization that a lot is within their control. So companies need to look at it from a place of strength that they have so much power over how their products are delivered to the market. and they have so many decisions that can be made in the right direction to help build a more circular economy for the materials that they are putting out into the market. And we can help find solutions in a pre-competitive way. So that's what the pact is about. We know we're pushing ourselves on a really aggressive timeline.

I'm so thankful that we have the transparency and accountability hook of requiring annual reporting. So we're tracking our progress and we'll know how we're doing as a group. And it's about our measurement, sort of our actions together as a pact. So we won't be pointing fingers or calling it gotcha on a particular company or an organization publicly, but more so again, telling that story as a pact. How are we moving together toward these aggressive targets we've set for ourselves? on a 2025 timeline. And I guess consumers themselves could become active and see what companies are in the pact

and what companies are not in the pact, then decide where they want to spend their dollars. Yes, exactly. I think if you're looking to support companies that are taking action and really pushing themselves to be part of something that does require transparency, that is working across all sectors of the value chain, including the public sector and nonprofits and universities. And that the pact is part of a broader global network, which is really exciting because we're measuring things the same way. We're gaining insights from other parts of the world and trying to sort of bring all of the good things that exist and that are growing in terms of

materials management and circular economy and putting them to work, again, in geographies across the globe. Thank you, Emily. Thank you very much. Emily DePaldo, executive director of the U.S. Plastics Pact, based in Mount Pleasant, South Carolina. And if you would like to see who is a member and who is not a member of the U.S. Plastics Pact, you can go to us.plasticspack.org. We're going to take a break. And after the break, what makes something stick and something slip? It all comes down to physics, a sticky situation after this. This is Science Friday. I'm Ira Flato. Up next, getting down and dirty with the science of surfaces. SciFrize Charles Berkwist is here. Hi, Charles.

Hey, Ira. What do you think when I say the word sticky? Well, maybe honey or duct tape. Oh, that goo that you find on the side of your kid's car seat, that is sticky. 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 Lori 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?

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. So would something like Velcro be sticky under your definition? 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. What about something like suction cups or static cling? 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 Valko or in a decent. or like you said, you could use a suction cup and air pressure or static cling. So are there actually different kinds of stickiness?

Or does it, as you say, just all boil down to friction at the molecular level? 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. You know, there isn't this magical scale at which sticky sits at one end and slippery sits at the other end. 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. 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? 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, you know, 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 as 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 superglue, 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 cyanide acrylates, that's the name of the molecule that's inside superglue, as soon as that gets out into the air, the water molecules, H2O molecules in the air, join onto these cyanacrylates 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.

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. Yeah, I think that's true. And what was kind of interesting was I have often thought about, you know, 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, you know, the meat and 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. 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? 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, 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. 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. Yeah, precisely that. That's exactly it.

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, 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, you know, the density of a material. But it's not because the coefficient of friction can only be defined between two specific things. So, you know, 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, you know, 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, you know, 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. 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? 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. I'm Charles Berkwist, and this is Science Friday from WNYC Studios. I'm talking with Laurie Winkliss, 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 slash 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? Yeah, I mean, we know we've have lots of examples in the nanoscale 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 nanoscale, it's very reactive. It's used to catalyze other chemical reactions. But macro scale gold doesn't react with anything really, which is why we use it in jewellery that hangs around for a long time. So moving from the nano to the macro world, it's always really tricky

and it can have some interesting implications, I guess. But yeah, we do see some materials that are particularly useful in the nanoscale. 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, ten, 15 atomic layers of molyptenum 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. 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? 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, you know, 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. You know, there's some examples of Roman chariots. And for example, you know, using waxes and animal fats to lubricate the way that their chariot wheels move. 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. 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're a liquid lubricant, so they're 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, you know, 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 nanoscale. So could we just use a single layer of graphene, for example, to reduce the friction between a tiny microelectro-mechanical 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. 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 Plato. Continuing our conversation with Charles Berkwist about sticky versus slippery.

So, Charles, it all comes down to friction, sandpaper, shark skin, things like that. On some levels, yes, in many ways, but there's still a lot scientists have to learn here. I asked Lori Winkless about the science of surfaces at different scales. We know a lot from a practical point of view. You know, 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 edesis manufacturers. So we know what is going on in the macro scale. Something I didn't quite realize is how much we are learning about friction at the nanoscale. 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, you know, 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 nanoscale 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. Interesting.

One of the things that people think of as sort of the classic example of slipperiness is Teflon. What makes it so slippery? 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 this 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 avail of it. 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, ultra low stick and why we've put it on our frying pans for such a long time.

But how do they get it to stick to the frying pan? Yeah, 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 aluminium, right? So one option is that we sandblast the aluminium, 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, what kind of you're trying to cause a fair bit of damage to that aluminium 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 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.

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? 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, okay, well, we'll talk about, you know, 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, we 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. And 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. Well, this has been delightful. Thank you so much for taking time to talk with me today. Thank you so much, Charles. 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 ScienceFriiday.com slash Sticky. Every November, volunteers set out through the mountains of California with one goal in mind to count monarch butterflies as they gather for winter hibernation. And the recent numbers have been

bad news. For example, back in the 1990s, the western population numbered more than a million. But in 2018 and 2019, volunteers only counted 20,000. And in 2020, the count turned up, and in 2020, the count turned up a terrifyingly small 2,000 butterflies. So you might imagine there was concern about how many butterflies might be found or not found this year. Well, drum roll please. The 2021 Thanksgiving count turned up 250,000. Yes, 250,000 butterflies in winter enclaves throughout California. What a victory. Okay. So how did the population bounce back so dramatically? And is this number a blip on the radar or the start of better times for the beleaguered butterfly? Here to help us unpack the news, Dr. Louis Yang, a professor and ecologist at the Department of Entomology and nematology, UC Davis.

Hey, welcome to Science Friday. Thank you. I'm glad to be here. Was it surprising to you to see such a rebound in such a small population? Absolutely. Yeah, I think it was astounding, actually. magnitude of this increase is unprecedented. I say that knowing that populations of butterflies and monarchs in particular are extremely variable. They have the capacity to increase tremendously in a given year. A single female monarch can lay hundreds of eggs. And so they do have the capacity to increase dramatically across multiple generations. But that cuts both ways. We've seen this

population decline dramatically in ways that were utterly unanticipated and surprising. And then we've seen it bounce back in ways that were even more surprising, I think. This more than hundredfold increase in the population in one year is a remarkable event. We still don't fully understand. Okay, let's talk about the why. How could they have rebounded so quickly in just one year? That's a great question, and it's a question that I think a lot of monarch researchers are working to understand right now. It seems reasonable to say that an increase of this magnitude would probably require a series of fortunate conditions throughout the breeding season that would sustain population growth across multiple generations.

Such as. You know, my research has really focused on understanding the seasonal dynamics of milkweed monarch interactions. How do monarch caterpillars develop on their milkweed host plants? And what are the conditions that allow them to develop well and survive to adulthood? and what are the conditions that cause lower survivorship? Some of the main factors that likely limit monarch populations are the kind of bread and butter of ecology that we often think about, things like resource limitation or competition, predation,

and also things like disease, and there are also some other drivers like environmental chemicals and changes in land use and other things that are certainly happening. But my research has really focused on understanding what are the limitations in the early season and what are the limitations in the late season. Let's talk about that. What are the early limitations? What kind of, you say resources. I take that to mean food. Can they get enough to eat? Would that be correct? And what does your research show you about the populations and their eating habits? Yeah, so monot butterflies lay their eggs on milkweed. There are several species of milkweed across North America and a tremendous diversity of milkweed.

host plants in the Western Range. And that's the main resource that they need to grow and develop. The caterpillars eat milkweed and they grow at a tremendous rate. So this year they could have had more food earlier in the breeding season? Yeah, we've been very interested in this idea of a potential phenological mismatch in the timing of milkweed availability and the timing of monarch demand for that milkweed. And that seems to be a pattern we're seeing some evidence of, especially in the early season. So maybe to take a step back, what we've seen over several years of experiments and observational studies is that if you experimentally introduce monarch eggs to host plants, milkweeds, at different points in the season, we see evidence for seasonal windows of opportunity.

We see evidence for a window of opportunity where they're developing quite well in the late spring or early summer, and that's followed by a mid-summer slump where they don't seem to develop very well. And then in the late summer or maybe early fall, there's a second window of opportunity. When we look more closely at the early season and late-season windows, we see that they're different. They're different in character. A recent observational study that we conducted before the big crash showed that the early season window was characterized by large numbers of eggs on relatively small numbers of milkweed plants resulting in high densities on those plants. And those eggs, relatively few of them, survived to develop as caterpillars and as adults.

So low survivorship on relatively small, early season milkweed plants. The late season seemed to be characterized by a different pattern, where we saw higher survivorship. The plants in the late season are much bigger. However, they've been busy ramping up their defensive traits across the season. By the end of the season, those plants are big and robust and well defended with things like toxic latex. And we think that the second window of opportunity actually coincides with the point in the season where the plant starts to draw back those resources. So during this period, the leaves get a little bit less well defended, and that seems to coincide with this second window of opportunity.

This is Science Friday from WNYC Studios. Let me ask you this about the changes in the population. Could climate and climate change help explain the timing of milkweed and how well the monarchs are doing? I think it's certainly related. I think we've seen that climate is fundamental to most of the factors that limit monarchs. Even if it's not the direct driver, it relates to all the drivers that seem to affect monarchs, or many of the drivers that affect monarch development.

In particular, the timing of milkweed emergence from the ground and the timing of milkweed senescence at the end of the season both seem to be strongly related to climatic drivers, maybe most strongly precipitation in the previous winter. There's also warming trends that seem to have strong effects on monarch development. So warming could influence when the monarchs begin their spring inland migration and it could also affect the rate at which caterpillars develop on these plants. And especially in the late season, we're starting to see some evidence that heat waves might have negative effects on monarch development.

We know that these monarch caterpillars experience sublethal thermal stress when the temperatures get too high. And very high temperatures can even be lethal to them. And those temperatures are temperatures we're starting to see during heat wave periods in California. over the summer. How many years will it take for you to know whether this is, you know, goes up and down now, up and down, up and down, or is this a blip on a radar screen that's just a one-year event coming back in great numbers like this? It will take time, but we have good hypotheses and we have good questions to ask. I actually think that the number of motivated folks studying these questions will have some answers for us soon.

What we've seen is a long-term trajectory of declining populations, even before the recent population variability. I think one thing to emphasize is that we really didn't have much data on how a population that is that small responds. The population for the last several years has been around 2,000 butterflies across all of Western North America, and that's a tremendously small population, considering that, you know, just a few years earlier, we had 200,000 butterflies, and not that long before that we were talking about numbers

that were projected to be in the millions. So we don't have a lot of data to understand how this specific population responds at such low population densities, but it is possible that there was some benefit to being at that low population. allowed them to increase in this one year, but it's also useful to recognize that this population was at that low density for several years, and it didn't increase in those previous years. So we shouldn't be complacent or assume that populations will always bounce back from low densities. The opposite could have been true. It could still be true in the future.

I think a lot of folks are breathing a sigh of relief that the population has increased as much as it has over the past year. And I think we all share that sense of relief and joy that the population has increased. But also, there is that note of caution that there is a lot about the dynamics of this population that we don't yet understand, and we're still working on. What a great place to stop, Dr. Yang. Thank you for taking time to be with us today. Thank you. Dr. Louis Yang, professor and ecologist at the Department of Entomology and nematology at the University of California at Davis. And that's about all the time we have for today.

In case you missed any part of this program, or you'd like to hear it again, subscribe to our podcasts or ask your smart speaker to play Science Friday. Of course, you can always get back to us, our email, SciFry at ScienceFri.com. Have a great weekend. We'll see you next week. I'm Ira Flato.