The Surprising Science Of Why Sneakers Squeak

The forces that cause sneakers to squeak also create mini-earthquakes (shoe-quakes, if you will) and tiny lightning bolts.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

  WNYC is supported by Bear. Science is about asking questions, testing, trying again. It's not easy, but it's relentless. It evolves, and that's why science delivers. Learn more at sciencedelivers.com.

• 0:14Ira Flatow?

  Science Friday is supported by MathWorks, creator of MATLAB and Simulink software for technical computing and model-based design. MathWorks, accelerating the pace of discovery in engineering and science. Learn more at mathworks.com.

• 0:33Flora Lichtman?

  Hey there, it's Flora Lixman, and you're listening to Science Friday. March Madness is around the corner, and of course, that means a lot of this.

• 0:45Flora Lichtman?

  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. Adele Jululie is an experimental physicist at Harvard. Adele, why this question?

• 1:18Speaker 03

  Thank you for having me, Flora. Well, the reason why I ask 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 accustomed with the 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.

• 1:59Flora Lichtman?

  We know. We know, Adele. We know.

• 2:02Speaker 03

  And so I went to a Celtics game for the NBA, and you know. 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, right? And so for me was, why, simple question, why do basketball shoes squeak?

• 2:36Flora Lichtman?

  Okay, so run me through your setup. How did you figure this out?

• 2:41Speaker 03

  Um, well, one of my interns was a fan of basketball and he had these beat down basketball shoes. So I asked him to borrow, um, just one shoe, not the two.

• 2:57Flora Lichtman?

  I love that you needed a used shoe. Like, I feel like there's, that, you know, you couldn't buy a shoe. I love that.

• 3:05Speaker 03

  You know, it's, 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? Uh, even though the money that you got is from a grant that you wrote, it's your sweat and, but they still ask you questions. You know, checks and balances.

• 3:28Flora Lichtman?

  I got, I got it. I got it. Okay. So you have this used basketball shoe and then what?

• 3:33Speaker 03

  And then, you know, we're a friction lab, um, so we're used to, um, visualizing frictional interfaces. And it's, and it is quite simple to do. You take a, an acrylic plate, transparent. You put LED around it and you put black tape all around that LED strip. Um, 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, a great setup to visualize just the contact. So when, when you have no contact or loss of contact, it's dark. When you have contact, it's bright. Okay?

• 4:16Flora Lichtman?

  Because it's reflecting it back, like a mirror.

• 4:18Speaker 03

  Reflecting back. And then you put in a camera, you put in a, um, a microphone, you synchronize the two. You take your beat down basketball shoe and you do a test. And we we 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.

• 4:43Flora Lichtman?

  Tell us.

• 4:44Speaker 03

  All right, so um what you 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 your the sole of the shoe wrinkles and that wrinkle travels at supersonic speed. And the the frequency of repetition of these fast-traveling wrinkles sets the frequency of the sound.

• 5:14Flora Lichtman?

  Hmm.

• 5:15Speaker 03

  Yes.

• 5:15Flora Lichtman?

  So your shoe is not sliding uniformly?

• 5:18Speaker 03

  No.

• 5:19Flora Lichtman?

  It's like sticking and slipping, is that right?

• 5:22Speaker 03

  It's a it's what we thought um 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 is that the whole interface, 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 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.

• 6:08Flora Lichtman?

  You're a physicist, was that cool and surprising? Like, put that finding in context.

• 6:12Speaker 03

  Okay, so usually what what what you think about rubbers are very boring. Uh, no, no disrespect to the rubber tribology community, but it's usually boring, it's usually slow. And when you see these supersonic slip pulses.

• 6:31Speaker 03

  It is quite, it's exciting for us because um usually these kind of um events, these kind of phenomena you see mostly in geophysical settings. So um for example.

• 6:44Flora Lichtman?

  Like plate tectonics?

• 6:46Speaker 03

  Yes, when when you have an earthquake uh or when you have a rupture, the dynamics of this rupture versus the shoe squeaking or the shoe moving have share a lot of similarities that we did not foresee.

• 7:02Flora Lichtman?

  Okay, so uh shoe squeaking across the surface is acting like two continental plates banging against each other?

• 7:11Speaker 03

  Sliding against each other, yes, yes.

• 7:14Flora Lichtman?

  So it's like an earthquake on the basketball court.

• 7:17Speaker 03

  I would say maybe like a shoe quake, you know? So it's a quake at a different uh scale. Instead of being hundreds of miles uh long, it's a few inches, so.

• 7:30Flora Lichtman?

  A shoe quake.

• 7:32Speaker 03

  Yeah.

• 7:32Flora Lichtman?

  Amazing. Okay, what else did you see? Because I know you were watching this in high speeds. You saw these waves, you you find the shoe quake. What else do you find?

• 7:42Speaker 03

  You know, uh, this is why I love experiments is because it's the ultimate simulation and it's very surprising. It was ten thirty P M on a winter night and um Gabrielle, my co first author and I, we were in the lab uh doing experiments and you know, we watched the movie after we do the we do the the experiments and it's uh several millions of images uh 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.

• 8:27Speaker 03

  But then, when you look closely and when you repeat experiments, you see systematically lightning as a trigger for these opening slip pulses.

• 8:41Speaker 03

  And you know.

• 8:41Flora Lichtman?

  Lightning.

• 8:42Speaker 03

  I don't blush. I don't I don't I don't get I don't get this rush uh usually. Uh but at that moment, I was like, this is so cool. This is really one of the coolest moment in my life to to share that kind of happiness and that kind of um. I don't know, uh excitement to see lightning, lightning under a shoe or lightning under rubber, right?

• 9:12Flora Lichtman?

  Shoe quakes and now shoe lightning.

• 9:14Speaker 03

  Exactly.

• 9:15Flora Lichtman?

  That's happening every time. So it it's just a a charge, an electrostatic charge that happens every time that that you hear a squeak?

• 9:22Speaker 03

  Yeah, so think about it, uh you know, when you you have this kind wool sweater and that you remove. You probably or your audience probably has experienced this before where you get a lot of tiny jolts or uh zips. Uh 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 a rubber on a smooth surface, you create this imbalance by rubbing.

• 10:07Speaker 03

  And then that same rubbing creates this discharge that think about it as a mini explosion. Uh that drastically increases temperature locally, which increases the pressure. And triggers this open impulses.

• 10:24Flora Lichtman?

  I mean, is the squeak the the thunder to the shoe lightning?

• 10:28Speaker 03

  Uh uh in in a way, yes, but you need a certain um condition to be met. So it it's not a general means for triggering these uh slip pulses, but it's one of them that we have observed, yes.

• 10:44Flora Lichtman?

  For me, this 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.

• 11:07Speaker 03

  Yeah, and and and it's like several thousands of shoe quakes happening per second. So it's in a sense, it's a super cool set of phenomena under a deceptively simple question.

• 11:21Flora Lichtman?

  I feel bad for the microbiome on those shoes. It's just, it must be really turbulent for them.

• 11:27Speaker 03

  Yeah, and and let me tell you, Flora, so we were very curious to see how uh, 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 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 um handquakes traveling at hundreds of meters per second and repeating tens of thousands of times per second.

• 12:11Flora Lichtman?

  Hmm. Okay. Adele, it seems obvious that you had fun doing this study. Did you have a favorite um element?

• 12:21Speaker 03

  Yes, I I would say it's um

• 12:25Speaker 03

  it felt to me like a a scientific Hercule Poirot 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, you know, to the drawing board and say, hey, what did we miss? Right? And it's a it's a story about stubbornness, obsession, perseverance, and and I would say creativity.

• 12:58Flora Lichtman?

  Yeah, interestingly, that's like the story of many great um sports achievements, too.

• 13:03Speaker 03

  I I agree. Yeah, you need to be stubborn.

• 13:06Flora Lichtman?

  And creative and persistent.

• 13:08Speaker 03

  Yes, yes.

• 13:09Flora 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.

• 13:26Flora Lichtman?

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

• 13:32Speaker 03

  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.

• 14:04Flora Lichtman?

  Adele, I can't wait for your next study.

• 14:07Speaker 03

  Well, I'm probably leaving academia to produce something more, let's say, impactful.

• 14:18Flora Lichtman?

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

• 14:27Speaker 03

  Thank you very much, Flora.

• 14:29Flora Lichtman?

  This episode was produced by Rasha Aridi. We will catch you tomorrow. I'm Flora Lichtman.