The Surprising Science Of Why Sneakers Squeak

- Hey there, it's Floor Lixman

and you're listening to Science Friday. March Madness is around the corner and of course that means a lot of this. (upbeat music)

“What is the science behind that signature sound?”

Look, if you're tempted to tag out, don't, because it turns out the physics is sticky and surprising and involves lightning bolts and earthquakes, I'm not kidding. So, lace up, get your mind off the bench

because joining me now to talk about this is the lead author on a new paper in nature that investigates the squeak. Dr. Adel, Julie is an experimental physicist at Harvard, Adel, why this question?

- Thank you for having me, Floor Lixman. Well, the reason why I asked myself this is because most of my projects are curiosity driven. I learned through my training during my PhD that even the most simple questions can be deceptive.

So, when I first arrived to the US, one of the first things that you do as an immigrant

“is to get a custom with the local people”

is that you go to their sports events and let me tell you that Boston people, the Bostonians are very much fans of their different sports. - We know, we know, we know, we know.

- And so, I went to a Celtics game for the NBA and you know, the two things that strike you, the first thing is how enthusiastic the crowd is. And the second one is this omnipresent sound

but never leaves your ears when you're in the stadium

which is the squeaking of basketball shoes when players are sliding, right? And so, for me, was why simple question? Why do basketball shoes squeak? - Okay, so run me through your setup.

How did you figure this out? - Well, one of my interns was a fan of basketball and he had these be-down basketball shoes. So, I asked him to borrow just one shoe, not the two. - I don't know, you needed a used shoe.

Like, I feel like there's the, you know, you couldn't buy a shoe, I love that. - You know, it's not that easy because if you go to your purchasing department and you tell them, I want new basketball shoes,

they start asking questions, you know? Even though the money that you got is from a grant that you wrote, it's yours sweat and they still ask your questions, you know, checks and balances.

- I got it, I got it, okay.

“So, you have this used basketball shoe and then what?”

- And then, you know, we're a friction lab. So, we're used to visualizing frictional interfaces and it's in and is quite simple to do. You take an acrylic plate, transparent, you put LED around it and you put black tape

all around that LED strip, you connect it to light and then you have this very simple, it powerful optical setup called total internal reflection. It's a great set up to visualize just the contact. - So, when you have no contact or lots of contact,

it's dark, when you have contact, it's bright, okay? Because it's reflecting it back, like a mirror. - Reflecting back. - And then you put in a camera, you put in a microphone, you synchronize the two, you take the beat down,

basketball shoe and you do a test. And we were recording with this very high speed camera

that can go up to a million images per second

and we saw something that we did not foresee. - Tell us. - All right, so what you see when you rub or when you slide the basketball shoe on the smooth dried surface is these ripples, think about them as wrinkles

so you're the soul of the shoe wrinkles and that wrinkled travels at supersonic speed. And the frequency of repetition of these fast traveling wrinkles sets the frequency of the sound. - Yes.

- So your shoe is not sliding uniformly. - No. - It's like sticking and slipping, is that right? - It's what we thought shoes did as they all stick in a block and they all move in a block.

So what does it mean slip pulses is that

“the whole interface, fictional interface”

between the shoe and the smooth surface, rigid surface is stuck. Nothing moves except the regions where this wrinkle is traveling. And so think about it as almost like when you take a rug

and then you give it a shake and you see that full travels through it. That's how I would try to convey what's going on at the frictional interface. - Your physicist was that cool and surprising?

We put that finding in context. - Okay, so usually what you think about rubbers are very boring. No disrespect to the rubber tribology community but it's usually boring, it's usually slow. And when you see these supersonic slip pulses,

it is quite exciting for us because usually these kinds of events, these kinds of phenomena you see mostly in geophysical settings. So for example-- - Like pointectonics?

- Yes, when you have an earthquake or when you have a rupture, the dynamics of this rupture versus the shoe squeaking or the shoe moving are share a lot of similarities

that we did not foresee. - Okay, so shoe squeaking across the surface is acting like two continental plates banging against each other? - Sliding against each other, yes.

Yes. - So it's like an earthquake on the basketball court. - I would say maybe like a shoe squeak, you know? (laughs) So it's a quake at a different scale.

Instead of being a hundreds of miles long, it's a few inches, so. - A shoe squeak. - Yeah. - Amazing.

Okay, what else did you see? Because I know you were watching this in high speeds. You saw these waves, you find the shoe quake, what else do you find? - You know, this is why I love experiments

is because it's the ultimate simulation and it's very surprising. It was 10 30 p.m. on the winter night

and Gabrielle, Michael, first author and I,

we were in the lab, doing experiments. And you know, we watched the movie after we do the experiment and it's several millions of images that we recall. So we go one by one to try to look at what's going on and what we saw there was something very surprising

that I assume that first to be a glitch or the camera I miss behaving. But then when you look closely and when you repeat the experiments, you see systematically lightning

as a trigger for these opening slip pulses. And you know lightning. I don't blush, I don't get, I don't get this rush usually. But at that moment, I was like, this is so cool. This is really one of the coolest moments in my life

to share that kind of happiness and that kind of, I don't know, excitement to see light thing,

“lightning and the issue or lightning and the rubber, right?”

- Shoe Quakes and now Shoe lightning. - Exactly. - That's happening every time. So it's just a charge and electrostatic charge that happens every time that you hear a squeak. - Yeah, so think about it, you know,

when you have this kind of wool sweater and they remove, you probably, your audience probably has experienced this before where you get a lot of tiny jolt or zips, electricity zips.

Basically what you do when you rub two objects

against each other, you create an imbalance in electric charges. And when this imbalance becomes big enough when it has sufficient potential, it discharges to equilibrate or to balance back these charges.

Okay? When you slide a rubber on smooth surface, you create this imbalance by rubbing and then that same rubbing creates this discharge that think about it as a mini explosion

that drastically increases temperature locally which increases the pressure that triggers this opening process.

“- I mean, is this squeak the thunder to the shoe lightning?”

- Yeah, in a way, yes,

So it's not a general means for triggering these slip horses,

“but it's one of them that we have observed, yes.”

- For me, this actually really makes me appreciate the squeak. You know, as someone who doesn't really think of the squeaking as a value add to my basketball experience, now I do, because every time I hear it, I'm gonna be thinking,

a little lightning bolt was created and this is like the physics that's happening

in an earthquake, that's amazing.

- Yeah, and it's like several thousands of shoe quakes happening per second. In a sense, it's a super cool set of phenomena and they're a deceptively simple question. - I feel bad for the microbiome on the shoes.

It must be really turbulent for them. - Yeah, and let me tell you, Flora.

“So we were very curious to see how universal this is.”

Does it just happen for rubbers? Or can it happen to your hand, for example? And if you slide your hand on the smooth surface and you slide it quite fast, let's say a mirror, you will hear squeaking.

You will hear your hand hissing at a quite a high pitch. And if you image your hand sliding on a acrylic plate with these LEDs and these high speed camera, you would see these hand quakes

traveling at a hand is of meters per second

and repeat in tens of thousands of times per second. - Okay, Adele, it seems obvious that you had fun doing this study. Did you have a favorite element? - Yes, I would say it felt to me like,

a scientific acupuncture story, right? With a lot of unexpected twists and we had to challenge our assumptions all the time, confront our biases and more than once go back to the drawing board and say, hey, what did we miss, right?

And it's a story about stubbornness, obsession, perseverance and I would say creativity. - Yeah, interestingly, that's like the story of many great sports achievements, too.

“- I agree, yeah, you need to be stubborn.”

- And creative and persistent. - Yes, yes. - We can't end this interview without hearing some art that you made to go along with your study, which is quite unusual.

Let's play it and then you can tell us why. (upbeat music) - How long did it take to do Darth Vader's imperial march with rubber blocks?

- It took us three days, basically.

Three days to rehearse this improbable squeaky band. With three different people, each one of us had two or three notes that we needed to slide and you need to determine what is the length of sliding to get the proper tempo. And so there's a lot of coordination

to produce that, I don't know, 15 seconds of music, but it was a lot of fun, at least for us. - I can't wait for your next study. - Well, I'm probably leaving academia to produce something more, let's say, impactful.

- Really? Okay, well, that's a story for another time. Dr. Adel Julie is an experimental physicist at Harvard University. Thank you so much for joining me today.

- Thank you very much, Flora. - This episode was produced by Russia, a reading we will catch you tomorrow. I'm Flora Lake tonight. (upbeat music)

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