Weirdly flowing water finally has an explanation: ‘quantum friction’

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, a new theory of small-scale friction…

Host: Shamini Bundell

And the latest from the Nature Briefing. I’m Shamini Bundell.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

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Host: Benjamin Thompson

First up on the show, reporter Nick Petrić Howe has been finding out about the mysteries of water flow at the quantum scale.

Interviewer: Nick Petrić Howe

There’s a mystery in the world of nanofluidics – the science of flow at the molecular scale.

Interviewee: Nikita Kavokine

So, it all started with reports of very fast flows of water through tiny, tiny carbon nanotubes. And so, the puzzling findings with these narrow tubes was the narrower the tube, the smaller the friction.

Interviewer: Nick Petrić Howe

This is Nikita Kavokine, a nanofluidics researcher at the Flatiron Institute in New York, USA. This faster flow through narrower carbon nanotubes is the opposite of what we’re used to at the macro scale. When water flows through a garden hose, say, the narrower the hose the more slowly it flows. So, what is going on at the tiny scale? Well you may have caught the word ‘friction’ there. This resistance to motion is a lot more important at the small scale. There is a greater proportion of the water in contact with the pipe at these tiny scales. But herein lies another mystery.

Interviewee: Nikita Kavokine

How is it possible to have friction when a surface is perfectly smooth?

Interviewer: Nick Petrić Howe

The carbon nanotubes at the heart of this mystery have what’s known as atomic smoothness – they have no defects – so there’s nothing really for the water to rub against to generate friction. But this week in Nature, Nikita and his colleagues have come up with a theory that they think can solve this conundrum.

Interviewee: Nikita Kavokine

It turns out that there can still be friction, and this is what we call ‘quantum friction’.

Interviewer: Nick Petrić Howe

Now, I know what you’re thinking and, no, they haven’t added the word ‘quantum’ and called it a day. At such tiny scales, quantum interactions between atoms are relevant. In fact, using quantum theory, the team were able to mathematically explain the fast flow in narrow tubes that has been seen in previous experiments. Their theory works like this. Water has a slight positive charge which fluctuates as it moves through the tube. This positive charge interacts with negatively charged electrons moving around in the solid wall of the carbon nanotube.

Interviewee: Nikita Kavokine

And it turns out that the interaction between the successive, instantaneous configurations of all these moving atoms, they produce friction still, even though the roughness on average is zero.

Interviewer: Nick Petrić Howe

So, even when things are perfectly smooth, at these tiny scales, friction, or in this case quantum friction, can still slow things down. Now, that’s one thing, but how does it explain the fact that in narrower carbon nanotubes the water moves more quickly? Why is there less quantum friction here? Well, carbon nanotubes are made from multiple layers. In the wider tubes, the layers are more well-aligned than in the narrower tubes. This alignment allows electrons to do something known as quantum tunnelling. This basically means that they can transiently move between these well-aligned layers. These layer-jumping electrons can work together to have a greater pull on the water. In other words, there’s more quantum friction. Whereas in the narrower tubes, the layers are less well aligned, so this doesn’t occur as much, so there’s less quantum friction. A similar rationale explains an equally strange finding in graphene and graphite.

Interviewee: Nikita Kavokine

The other striking experimental result is that friction of water is much lower on graphene than on graphite. Now, it turns out that in graphite, the electrons can move in between the layers and they can all oscillate in sync in between those layers. On the other hand, in monolayer graphene, the electrons are confined to the single layer. They cannot move perpendicular to the layer so there is very low quantum friction.

Interviewer: Nick Petrić Howe

Of course, there’s plenty of experiments to be done to confirm Nikita’s theoretical explanation, but it does explain previous experimental results well. For Radha Boya, a nanofluidics researcher who wasn’t associated with this study, one of the novel things about this new paper is that it takes into account the influence of the actual tube on the water.

Interviewee: Radha Boya

So, usually when people do simulations for nanofluidic channels, they usually worry about the fluids and the surface. But the confining material itself, the solid material, is not given so much importance. It is mostly thought of as a geometric barrier rather than contributing to the flows.

Interviewer: Nick Petrić Howe

So, until now, the influence of the material of the tube itself on the flow hasn’t really been studied. But if we understand this better, Radha says, we can then fine tune the flow of water through these tiny tubes by carefully selecting a material based on how much quantum friction it creates. For Nikita, this new paper on flows at the small scale represents a step change in our understanding.

Interviewee: Nikita Kavokine

Well, yes, I think this is really a paradigm change for fluid dynamics because usually in hydrodynamics, well, a wall is a wall. It’s simply a boundary condition. And here, we find that actually the water flows near the wall, they couple to the electron flows inside the wall, and so the very subtle properties of these electron flows determine how fluid flows near the wall. So, yes, this will, I think, completely change the way we consider fluid flows at the nanoscale.

Host: Benjamin Thompson

That was Nikita Kavokine, from the Flatiron Institute in New York in the US. You also heard from Radha Boya from the University of Manchester, here in the UK. To find out more about flows at the tiny scale, check out the paper. We’ll put a link to that in the show notes.

Host: Shamini Bundell

Coming up on the show, dogs large and small and the genes that make them grow. Right now, though, it’s the Research Highlights with Dan Fox.

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Dan Fox

Soap bubbles aren’t normally known for their longevity. But a few extra ingredients have allowed researchers to make a bubble that lasts over a year. Typically, a soap bubble bursts within a few minutes, when the liquid in its shell evaporates or drains due to gravity. Longer-lived bubbles have been reported, but these required carefully controlled environments to maintain and still shrunk over time. Now, researchers have produced bubbles with a shell made of water, miniscule plastic particles and glycerol. The team monitored how the bubbles’ mass and shape evolved over time, and report that they remained essentially unchanged for up to 465 days – more than 200,000 times longer than normal. They suggest that the plastic particles prevent gravity-induced drainage, while the glycerol counteracts evaporation by absorbing water from the surrounding air. Pop over to Physical Review Fluids to read that research in full.

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Dan Fox

Hippos can recognise each other’s honking voices, and if they hear a group of hippos they dislike, they react by aggressively spraying dung. Little is known about hippo communication because, in the wild, the animals are very difficult to tag and identify. To study how hippo pods communicate, a group of researchers played recordings of hippo ‘wheeze-honk’ sounds for seven pods in the Maputo Special Reserve in Mozambique. They measured each group’s response to the broadcast voices of hippos that were members of the same group, neighbours or strangers. The animals acknowledged the voices by making their own sounds and spraying dung, an aggressive response. But they were usually less aggressive when responding to the voices of other pods that lived on the same lake than when responding to strangers living on other lakes. The authors say that hippo communication should be taken into account when animals are relocated for conservation purposes. For example, biologists could prepare a settled pod for new neighbours by playing recordings of the incoming animals’ voices. Read that research in full in Current Biology.

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Host: Shamini Bundell

Finally on the show, it’s time for the Briefing Chat, where we discuss a couple of stories from the wider world of science featured in the Nature Briefing. So, Ben, what have you found for us to discuss this time?

Host: Benjamin Thompson

Well, Shamini, I’ve got a story from Nature that’s based on some analysis published in The Lancet, and it’s looking at the global death toll associated with antimicrobial resistance and, in this case, the researchers were looking at bacteria that were resistant to antibiotics, and, well, it makes for some sobering reading.

Host: Shamini Bundell

Gosh, and, yeah, so, we do occasionally hear about antibiotic-resistant strains of things, but globally what is the kind of scale of the problem then?

Host: Benjamin Thompson

Well, this is analysis that looked at 2019, and I guess the top line is really that they estimate that almost 5 million people died from an illness in which resistant bacteria played a part and, of that, 1.27 million deaths were caused directly as a result of a resistant infection. And kind of putting that into some context, those numbers are kind of higher than the yearly deaths caused by either HIV/AIDS or by malaria.

Host: Shamini Bundell

Oh, wow, I mean, I feel like that’s bigger than I thought. So, that’s far beyond just MRSA, for example?

Host: Benjamin Thompson

Yeah, that’s absolutely right, Shamini. So, there’s been a bunch of studies looking at antimicrobial resistance, but maybe few have tried to look at a global scale. And, in this case, what they did is they took loads of different datasets from different places and put them together and did some modelling to come to these numbers. And you’re right, there are a bunch of bacteria that are involved. In fact, six bacterial pathogens were responsible for three quarters of these deaths. E. coli and Staphylococcus aureus were the top two, and E. coli was responsible for an estimated 200,000 deaths.

Host: Shamini Bundell

And is this all because we hear about basically misuse of antibiotics, things like if you don’t finish a treatment or antibiotics being overused for livestock. Are those the kinds of things that are causing these massive numbers?

Host: Benjamin Thompson

Certainly, they are some of the reasons, Shamini, that they talk about in the report. But there are other things as well. Lack of access to sanitation in places or lack of access to diagnostics or surveillance and what have you, so these are things that can drive the spread and increase in resistance. And what is interesting as well is that when this is looked at at a global scale, there are some discrepancies that leap out as well. For example, antimicrobial-resistant deaths were estimated to be the highest in sub-Saharan Africa and the number of resistant infections were estimated to be higher in low-income countries compared to wealthier countries. And again, I think some of the reasons behind that are the things we’ve discussed and also lack of access to the latest drugs and vaccines and what have you.

Host: Shamini Bundell

And does the fact that this is now a much bigger picture global view give any assistance in sort of where to target this problem, where to focus?

Host: Benjamin Thompson

Well, I think, Shamini, it’s interesting because it gives a sense of scale to this problem, and I think for a long time people have been saying that resistance to drugs is a serious issue that needs to be addressed. And in this work, they say that back in 2016, another report came out suggesting that by 2050, their might be 10 million deaths a year as a result of resistance, but this new estimate suggests we’re already well on the way to that and things might be accelerating quicker than anyone realised. So, policy work and healthcare work needs to be done sort of right now to really get on top of this.

Host: Shamini Bundell

Well, hopefully we’ll be hearing more about some actions that are being taken to tackle this and we can report on those in the podcast. I have something a bit lighter for you this week. I have a fun story that I read in Nature based on a Current Biology paper, and I very much enjoyed reading this one because it’s all about why some dogs are really little and some dogs are really massive. Fun fact: dogs differ more in size than any other mammal species on the planet.

Host: Benjamin Thompson

That’s the sort of fact that I come to these Briefing chats for. I always love when we do our animal stories. Okay then, so the difference in size for a dog. So, I’m imagining a chihuahua and maybe a Great Dane or something at the other end of the scale. What’s causing the difference between the two?

Host: Shamini Bundell

Well, I think the general idea has always been, look, humans domesticating the dogs have obviously created, in a way, all sorts of different sort of shapes and features in these pets. So, people thought, okay, a lot of this difference is human-driven. But these particular researchers wanted to know a little bit about the genetics behind it, and this is obviously very complex because there’s never going to be a single gene that affects size. But they have found one particular bit of DNA, one particular variant, that they’re very excited about and they think is really key for some sort of broad size distinctions.

Host: Benjamin Thompson

And what is this segment of DNA then? What is it doing?

Host: Shamini Bundell

So, it’s just this little bit of DNA that itself doesn’t code for a protein. It just makes a non-coding RNA, but that RNA is involved in controlling a gene, essentially. There’s an IGF1 gene and IGF1 protein, which is a growth hormone. So, there are two alleles, sort of different versions, of this little stretch of DNA, and there’s essentially a big one and a small one. And if you have double small, you’re more likely to be a little dog, and if you have double big, you’re more likely to be huge, and if you have one of each, then you might be more likely to be a medium-sized dog.

Host: Benjamin Thompson

And presumably then they must have sequenced the genomes of a lot of dogs then to find this out. What are we talking about in terms of the sort of experimental setup?

Host: Shamini Bundell

Yeah, so we’ve got a lot of genome sequencing. But interestingly enough, not just domestic modern dogs. We’ve got 230 modern dog breeds. We’ve got some ancient dogs. We’ve also got wolves, cayotes, other canids. And really interestingly, and this quite surprised the researchers, was that the actual alleles that they found didn’t originate in domesticated dogs. They’re found in these other species. So, for example, things like cayotes and foxes tend to have two of the sort of small-bodied version of this bit of DNA whereas wolves, they think maybe slightly later, evolved the big version.

Host: Benjamin Thompson

I mean, it sounds like things are kind of wrapped up, I guess, when it comes to doggy body size. Is this kind of the end of the story here now, Shamini, or is there more genetics that needs to be done?

Host: Shamini Bundell

I mean, I can’t think of the last time that scientists said, ‘Right, we understand everything now. Our research here is done.’ No, there’s lots more that they’re keen on unpicking in terms of what’s actually contributing to body size because, as we said, this is one factor, so the IGF1 gene maybe accounts for 15% of the variation between the body size of different dog breeds. And there’s a nice quote from the researcher at the end of the article here saying basically that there’s no one mutation that makes a wolf chihuahua-sized.

Host: Benjamin Thompson

Very nice. Well, Shamini, thank you for bringing that one to the Briefing chat today. And listeners, we’ll have links to both of those stories in the show notes as always, and we’ll also have a link on where you can sign up for the Nature Briefing. If you’d like stories like these delivered directly to your inbox then that’s where you should go to sign up.

Host: Shamini Bundell

That’s all for this week’s show. But before we go, I just want to mention there is a video I’ve made that is out now on our YouTube channel. It’s about something called the Leidenfrost effect, which is the thing that makes little water droplets skitter over very hot surfaces. You might have seen it if you’re sort of cooking in a wok. And for a long time, scientists have actually been trying to overcome this phenomenon as it makes cooling things down very difficult. So, in this video, researchers may have done just that by creating a fancy armour that defends against the Leidenfrost effect. So, we’ll pop a link to that in the show notes for you.

Host: Benjamin Thompson

Yeah, I’d recommend watching that because you can see some of Shamini’s cookware live in action being demonstrated.

Host: Shamini Bundell

My poor wok.

Host: Benjamin Thompson

Well, let’s leave it there then. As always, you can keep in touch with us on Twitter – we’re @NaturePodcast – or you can send us an email – we’re [email protected]. I’m Benjamin Thompson.

Host: Shamini Bundell

And I’m Shamini Bundell. Thanks for listening.