The Surprising Science Of Why Sneakers Squeak

 FLOR LICHTMAN: Hey there. It’s Flora Lichtman, and you’re listening to Science Friday. March Madness is around the corner. And, of course, that means a lot of this.

[SHOES SQUEAKING]

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 Djellouli is an experimental physicist at Harvard. Adel, why this question?

DR. ADEL DJELLOULI: Thank you for having me, Flora. 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 to get accustomed with the local people is that you go to their sports events. And let me tell you that Boston people and Bostonians are very much fans of their different sports.

FLOR LICHTMAN: We know. We know, Adel. We know.

DR. ADEL DJELLOULI: And so I went to a Celtics game for the NBA, and there are two things that strike you. The first thing is how enthusiastic the crowd is. And the second one is this omnipresent sound that never leaves your ears when you’re in the stadium, which is the squeaking of basketball shoes when players are sliding. And so for me, it was, why? Simple question. Why do basketball shoes squeak?

FLOR LICHTMAN: OK, so run me through your setup. How did you figure this out?

DR. ADEL DJELLOULI: Well, one of my interns was a fan of basketball, and he had these beat down basketball shoes. So I asked him to borrow just one shoe, not the two.

FLOR LICHTMAN: I love that you needed a used shoe. I feel like there’s that you couldn’t buy a shoe. I love that.

DR. ADEL DJELLOULI: 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. Even though the money that you got is from a grant that you wrote, it’s your sweat, but they still ask you questions. Checks and balances.

FLOR LICHTMAN: Oh, yeah, I got it, I got it. So you have this used basketball shoe, and then what?

DR. ADEL DJELLOULI: And then, we’re a friction lab. So we’re used to visualizing frictional interfaces. And it 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, yet powerful optical setup called total internal reflection. It’s a great tool and a great setup to visualize just the contact.

So when you have no contact or loss of contact, it’s dark. When you have contact, it’s bright.

FLOR LICHTMAN: Because it’s reflecting it back like a mirror.

DR. ADEL DJELLOULI: Reflecting back. And then you put it 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.

FLOR LICHTMAN: Tell us.

DR. ADEL DJELLOULI: All right, so what you see when you rub or when you slide a basketball shoe on a smooth, dry surface is these ripples. Think about them as wrinkles. So the sole of the shoe wrinkles, and that wrinkle travels at supersonic speed. And the frequency of repetition of these fast traveling wrinkles sets the frequency of the sound. Yes.

FLOR LICHTMAN: So your shoe is not sliding uniformly.

DR. ADEL DJELLOULI: No.

FLOR LICHTMAN: It’s sticking and slipping? Is that right?

DR. ADEL DJELLOULI: It’s what we thought shoes did as they all stick in a block and they all move in a block. Instead, what we see are what we call slip pulses. So what does it mean slip pulses? It’s that the whole frictional 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 fold travels through it. That’s how I would try to convey what’s going on at the frictional interface.

FLOR LICHTMAN: You’re a physicist. Was that cool and surprising? Put that finding in context.

DR. ADEL DJELLOULI: 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 kind of events, these kind of phenomena see mostly in geophysical settings.

FLOR LICHTMAN: Like plate tectonics?

DR. ADEL DJELLOULI: 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 share a lot of similarities that we did not foresee.

FLOR LICHTMAN: OK, so a shoe squeaking across the surface is acting like two continental plates banging against each other?

DR. ADEL DJELLOULI: Sliding against each other, yes. Yes.

FLOR LICHTMAN: So it’s like an earthquake on the basketball court.

DR. ADEL DJELLOULI: I would say maybe a shoe quake.

FLOR LICHTMAN: [LAUGHS]

DR. ADEL DJELLOULI: So it’s a quake at a different scale. Instead of being hundreds of miles long, it’s a few inches.

FLOR LICHTMAN: A shoe quake.

DR. ADEL DJELLOULI: Yeah.

FLOR LICHTMAN: Amazing. OK, what else did you see? Because I know you were watching this in high speed. So you saw these waves. You find the shoe quake. What else do you find?

DR. ADEL DJELLOULI: This is why I love experiments, because it’s the ultimate simulation and it’s very surprising. It was 10:30 PM on a winter night. And Gabriele, my co first author and I, we were in the lab doing experiments. And we watched the movie after we do the experiment, and it’s several millions of images that we record. 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 assumed at first to be a glitch or the camera misbehaving. But then when you look closely and when you repeat experiments, you see systematically lightning as a trigger for these opening slip pulses.

FLOR LICHTMAN: Lightning?

DR. ADEL DJELLOULI: I don’t blush. 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 lightning, lightning under a show or lightning under rubber.

FLOR LICHTMAN: Shoe quakes and now shoe lightning.

DR. ADEL DJELLOULI: Exactly.

FLOR LICHTMAN: That’s happening every time. So it’s just a charge, an electrostatic charge that happens every time that you hear a

Squeak?

DR. ADEL DJELLOULI: Yeah, so think about it. When you have this kind of wool sweater, and then you remove, you probably or your audience probably has experienced this before, where you get a lot of tiny jolts 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.

When you slide a rubber on a 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 and triggers these open impulses.

FLOR LICHTMAN: Is the squeak the thunder to the shoe lightning?

DR. ADEL DJELLOULI: In a way, yes, but you need a certain condition to be met. So it’s not a general means for triggering these slip pulses, but it’s one of them that we have observed. Yes.

FLOR LICHTMAN: For me, this actually really makes me appreciate the squeak. 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 going to be thinking a little lightning bolt was created, and this is the physics that’s happening in an earthquake. That’s amazing.

DR. ADEL DJELLOULI: Yeah, and it’s several thousands of shoe quakes happening per second. So in a sense, it’s a super cool set of phenomena and a deceptively simple question.

FLOR LICHTMAN: I feel bad for the microbiome on those shoes. It must be really turbulent for them.

DR. ADEL DJELLOULI: 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 a smooth surface, and you slide it quite fast, let’s say, a mirror, you will hear squeaking. You will hear your hand hissing at quite a high pitch. And if you image your hand sliding on an acrylic plate with these LEDs and this high speed camera, you would see these hand quakes traveling at hundreds of meters per second and repeating tens of thousands of times per second.

FLOR LICHTMAN: OK. Adele, it seems obvious that you had fun doing this study. Did you have a favorite element?

DR. ADEL DJELLOULI: Yes. I would say it felt to me like a scientific Hercule Poirot story with a lot of unexpected twists. 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? And it’s a story about stubbornness, obsession, perseverance, and I would say, creativity.

FLOR LICHTMAN: Yeah, interestingly, that’s the story of many great sports achievements, too.

DR. ADEL DJELLOULI: I agree. Yeah, you need to be stubborn.

FLOR LICHTMAN: And creative and persistent.

DR. ADEL DJELLOULI: Yes, yes.

FLOR LICHTMAN: 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.

[MUSICAL SQUEAKING]

How long did it take to do “Darth Vader’s Imperial March” with rubber blocks?

DR. ADEL DJELLOULI: 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.

FLOR LICHTMAN: Adel, I can’t wait for your next study.

DR. ADEL DJELLOULI: Well, I’m probably leaving academia to produce something more, let’s say, impactful.

FLOR LICHTMAN: Really? OK, well, that’s a story for another time. Dr. Adel Djellouli is an Experimental Physicist at Harvard University. Thank you so much for joining me today.

DR. ADEL DJELLOULI: Thank you very much, Flora. This episode was produced by Rasha Aridi. We will catch you tomorrow. I’m Flora Lichtman.

[MUSIC PLAYING]

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