Why is Blight so far?
It sounds so simple.
They had no idea.
But now the data speaks.
I find this not only refreshing, but at some level astounding.
Welcome back to The Nature Podcast.
This time, why do shoes squeak?
And a clue to why some people's brains age better.
I'm Nick Petrichow.
And I'm Dan Fox.
This recording contains a tricky science problem.
Squeaky shoes.
More specifically, the squeaks that happen when a soft surface quickly slides over a hard one.
An everyday occurrence, but one without a satisfactory explanation.
Well, maybe until now.
Researchers think they have an answer involving physical phenomena more commonly associated.
I was expecting something quite boring, to be honest, and it turns out that it's a combination of earthquakes and lightning.
This is Adel Jalouli from Harvard University in the US, one of the authors of a paper on the origin of squeaks published in Nature this week.
He was on holiday when we spoke to him, so you may hear the occasional squeak of a parrot in the background.
It's not just soft soles on hard floors that demonstrate this behaviour.
As Gabriele Albertini from the University of Nottingham in the UK, another of the authors of this research, explains.
For sure your bicycle brakes that are rubber pads onto metal, they're very much the same problem.
Some biomedical implants that involve joints, they tend to squeak too.
If you take a hand or a finger and you slide it against a smooth surface dry, the type of sound that you hear.
This is squeaking as well.
With these squeaks being so common, you'd imagine they'd be well understood.
But actually... We knew very little, OK?
So we had an initial hypothesis that something called Shalamach waves.
These are wrinkles, the same as when you run your finger on your skin, you see these wrinkles and these wrinkles slide under your finger right.
These are basically Shalamach waves.
But these Shalamach waves, they're super slow.
The rate at which you can produce them is too low in comparison with the sound that you hear.
So okay, we had this.
Maybe they were involved, maybe their rate of generation somehow changes, but we discovered something completely new.
To understand the true cause of the squeaks, the team needed to look at the frictional interface, the point of contact between the two surfaces.
In this case, they looked at rubber blocks that had patterns on them similar to what you'd find on the bottom of your shoes.
And to really understand what was happening here.
They looked at these blocks as they slid over an acrylic surface quickly.
So people for a long time have looked at like rubber glass interface and took images of those interface.
But basically all the study have been looking at very, very small velocity of sliding.
The main thing that we did different but like crucially was to slide fast enough to have squeaking and measure at a very high frame rate.
And so we designed this very simple system inspired from DaVinci from the 15th century.
Basically, it's a weight that you drop and that slides an object.
And so now we have these controlled normal force that we can apply.
And we put LEDs everywhere in this acrylic plate that we attach to our lifting weights.
And then we have this high-speed camera that could image a stationary object as the interface was sliding.
Recording this super-fast footage of hard acrylic sliding across rubber blocks allowed the team to make several new observations.
At the interface, we see some local stick-slip dynamics.
So what does it mean?
It means that at the point of the interface lifts, so physically open and detaches, and then this region that is open and detached travels very quickly, at the speed of sound of the rubber itself.
These opening and detaching regions are called opening pulses.
Imagine lifting the edge of a rug up and down to send a ripple across it.
Most of the rug stays in contact with the floor, but the wave moves and lifts that area of the rug off the ground as it goes.
This is exactly what happens at the interface, right?
And it's happening at the speed of sound or even faster than that.
By measuring the repetition rate of these opening pulses using their high-speed video, the team were able to show that the rate of the pulses matched the frequency or pitch of the audible squeak they had recorded at the same time.
They could even reproduce the sound of the squeaks using data from the video of the pulses.
But they also found that this wasn't the case if they removed the pattern of ridges from their rubber blocks.
The main discovery is that geometry matters.
Small details matter in friction.
And specifically in this case, when you take a flat rubber and you slide it, you'll hear a sound that is akin to taking a tape and open it.
Okay?
And the moment you put these fine details very, very tiny details, at the interface, all of a sudden you hear a musical note.
A note.
So main frequency and its harmonics.
Most people thought of slip pulses of this magnitude and speed in the geoscience field.
So think about earthquakes, for example.
Without a pattern, the opening pulses travel in multiple directions and trigger a variety of vibrations in the block, making a noise but not a clear squeak.
The ridges channel the pulses, and so the block's vibrations, into a single frequency or pitch.
That's what happens with the grip on the soles of squeaky shoes too.
And that wasn't the only surprising finding.
The researchers saw that the pressure applied to squeeze the two surfaces together didn't change the pitch of the squeak.
But applying more pressure did affect how the opening pulses got started.
If you slide it and you put it under sufficiently high pressure...
Tribal charge happens.
So, you know, just like rubbing a balloon on your head, that gives you this electrostatic charging.
And what happens is that it discharges.
And when it discharges, it creates a mini explosion or a micro explosion in the contact area.
And that's enough to create pockets of high pressure that nucleate in opening slip bolts.
This tiny spark lifts the rubber from the surface and starts a pulse rippling through the material.
Further investigation showed the pitch of these lightning-initiated squeaks is defined by the height of the rubber block.
Understanding this gave the team an idea for a demonstration.
Well, we're talking about squeaking.
Squeaking is a noise.
It's a note.
What do you do with notes?
You can speak as we're doing now.
You can also make music.
This is the team using rubber blocks of different heights to perform the Imperial March by John Williams.
It's an excellent showcase of also the robustness of our finding, because when you play these blocks by hand, sliding them on top of a glass plate, you cannot control the small torques and angles in the same precise ways that we do in the experiment, but they can have a very reproducible sound.
Even if these squeaking rubber blocks don't catch on as a new instrument, Adele and Gabriele think this work could have an impact in a variety of fields, be that earthquakes, metamaterials or engineering.
And then the natural question that arises is like how can we then design and optimize intricate shapes of those contact ridges and patterns so that we can better control the sound, suppress the sound, control the friction resistance and so on?
That was Gabriele Albertini from the University of Nottingham in the UK.
You also heard from Adele Jalouli from Harvard University in the US.
For more on that paper, including a link to a video where you can see those shoes squeak away.
Check out the show notes.
Coming up.
Researchers think they've found a debated process in the human brain which could help with better brain ageing.
Right now though, it's time for the Research Highlights with Shamni Bundel.
A runaway black hole is barrelling through space, leaving a trail of newborn stars in its wake.
Back in 2023, a paper reported the discovery of a bright linear feature that the authors suspected to be the wake of a black hole that had escaped its home galaxy.
But the feature's identity couldn't be pinned down.
Now, information gathered by the James Webb Space Telescope suggests that this feature, which is 200000 light-years long, is the trail left by a black hole at least 10 million times as massive as the Sun.
At the tip of the feature is an intense shockwave, which reveals that the object is moving at nearly 1,000 kilometres per second.
Other observations show the signatures of young stars which can be born in cosmic shockwaves.
According to the research team,
Follow the trail to that research over in the Astrophysical Journal Letters.
A four-legged robot can crawl up walls made of steel glass, aluminium or rough wood, all thanks to a gecko-inspired adhesive.
Robots that can both stick to a surface and move across.
It could be useful for applications like spacecraft maintenance.
But designing a sticky substance for this sort of job is tough, as the robots have to be able to adhere and detach with every step.
Now a team of researchers have designed a temperature-responsive adhesive from a polymer covered with molecular hairs which mimic the numerous tiny hairs on a gecko's foot.
Above 32 degrees Celsius.
The adhesive is a liquid allowing strands of the hairy polymer to spread out and conform to every nook and cranny on a wall.
Below 25 degrees Celsius, the adhesive crystallises and the hairs grip the surface.
Multiple cycles of warming and cooling of the adhesive on the robot's feet allowed it to take a step roughly every 11 minutes.
But the researchers are working on an adhesive formulation that heats and cools more rapidly, so the robot can climb faster.
Head over to the journal Matter and stick around to read that research.
A debate around whether neurons can grow from stem cells in a specific part of the human brain may now be resolved, a finding which could help us age better.
The hippocampus is a part of the brain that plays a key role in learning and memory.
But the precise mechanism of how this happens in humans has been difficult to figure out.
In mice and non-human primates.
Neurogenesis occurs there, the growth of new neurons from stem cells.
And this process has been shown to be very important for learning and memory in these animals.
In humans though, only hints of this hippocampal neurogenesis have been shown, and these haven't convinced everyone in the scientific community.
A new paper in Nature may change that, as the team think.
They've shown this progression of stem cells to neurons in the human hippocampus by looking at examples of brain tissue from deceased individuals.
They believe this finding could one day help us maintain our cognition as we age.
I reached out to one of the authors, Orly Lazarov, and asked why they were interested in hippocampal neurogenesis.
Hippocampal neurogenesis is probably one of the most profound forms of plasticity.
And many studies in rodents and in primates have shown that hippocampal neurogenesis plays different roles in learning and memories.
So the question was if human neurogenesis does not exist, then what are the means that the human brain uses in order to perform those functions?
The second question was if it does exist, then how does it change as a function of cognition and whether it does play a role in the human brain similarly to the rodent?
So does it play a role in learning and memory?
And to To answer that we analyzed the different cohorts with different cognitive diagnosis in order to try and associate the level of cognition with the signature of neurogenesis.
So you thought that perhaps there would be differences between these different cohorts if in fact neurogenesis did exist.
Correct.
And so you had these different samples from the different cohorts.
You were looking for the telltale signatures of hippocampal neurogenesis.
What did you find?
Well, first of all, we were able to use methodology that helped us determine that hippocampal neurogenesis does exist in the human brain.
We're able to show that there is a developmental trajectory from neural stem cells all the way to immature neurons in the hippocampus.
That's the first finding.
We determined that in a cohort of young adults we obtain a substantial amount of neurogenesis in the young adults.
As soon as we determine the signature of neurogenesis in those brains, were able to apply the same signature into the other cohorts that are all aging right.
They're all at the range of 70 to 90 years of age, but they have very distinct cognitive functions, right.
We had groups that were preclinical Alzheimer's disease, a group that was diagnosed with Alzheimer's disease, and a group of aging individuals that are performing appropriately to their age in terms of cognitive function, and a group of super-agers that their memory is exceptional.
We observed that in the preclinical and in Alzheimer's disease cohort the level of neurogenesis was extremely low, almost nonexistent.
In the super-agers, we observed that there was increase in neurogenesis, and they also exhibited a unique signature that we interpreted as a resilience signature.
So in the super ages, these people who are performing well, even into their 80s on these cognitive tests, they have this specific signature.
So do they have a lot of neurons growing new in the hippocampus?
Do they constantly have a churn of new neurons?
Is that what's happening?
Yes, they had significantly more neurons immature neurons, new neurons compared to the other diagnostic groups.
And in addition to the numbers, they also had a unique signature.
So their profile, their gene profile was different than the other cohorts.
So it's not like they had young brains.
They had their own special way of dealing with ageing, this resilience signature, as you say.
Yes.
And were you able to figure out what is causing these differences?
Yes.
So the unique mechanism that we observed is mostly due to alterations, epigenetic alterations.
So these are alterations in the areas of the genes that allow genes to get expressed.
So there's different amounts of genes switching on and off between these different cohorts.
Obviously, the mind leaps to potential ways that we could utilise this knowledge.
Does this open up the possibility for maybe sort of pharmacological interventions like ways to help people age better?
Yes, correct.
If we're able to validate the signature functionally and show that indeed it does regulate cognitive function, the goal would be to develop therapies that would allow us to induce these processes pharmacologically.
And as you say, you need to validate this finding first.
How are you going about that?
So we're going to do that by looking into the function of those genes that we identified using patient-derived induced fluorocodon stem cells, and look at their function while we're modulating the expression of those genes, of those signatures that we identified.
So essentially looking to see if you can replicate this in cells in a dish.
Correct.
And, you know, this is obviously looking at samples from deceased individuals.
Do you anticipate that perhaps things will be different in a living brain?
Definitely possible.
This is one of the shortcomings of the study is that we're using an endpoint, right?
Postmortem tissue is one point in time following death. right?
So it's definitely limiting our capability to follow the progression of hippocampal neurogenesis over time and over disease progression.
Unfortunately, until we have new technology that would allow us to follow neurogenesis in live individuals, we're pretty limited to postmortem tissue.
And this is somewhat of a debated thing, whether humans have this hippocampal neurogenesis.
Do you think everyone in the community will be convinced of this new finding?
I sure hope so.
I think what we have done was a very thorough analysis of the gene expression and of the mechanisms that control gene expression.
Taken together, I think that this is a very, very comprehensive study that establishes not only the existence of hippocampal neurogenesis in the human brain, but also ties it to cognitive function and shows that hippocampal neurogenesis is altered with cognitive decline and it's in terms of the extent of neurogenesis it's greater extent in individuals with superior cognitive function and that together ties it to a direct connection between hippocampal neurogenesis and cognitive function.
That was Orly Lazarov from the University of Illinois Chicago in the US.
For more on that story check out the show notes for some links.
And that's all for the show.
Stay tuned on Friday for more from us with the briefing chat.
Until then, you can keep in touch with us on social media.
We're at Nature Podcast.
Or you can send an email to podcast at nature.com.
I'm Dan Fox.
And I'm Nick Perchichow.
Thanks for listening.
