Science Friday - The ‘Wet-Dog Shake’ And Other Physics Mysteries

Episode Date: November 27, 2023

Ever wondered why your dog’s back-and-forth shaking is so effective at getting you soaked? Or how bugs, birds, and lizards can run across water—but we can’t? Or how about why cockroaches are so darn good at navigating in the dark?Those are just a few of the day-to-day mysteries answered in the new book How to Walk on Water and Climb Up Walls: Animal Movement and the Robots of the Future, by Georgia Tech mathematician David Hu.The book answers questions you probably won’t realize you even had, but they’re questions with serious answers that span the worlds of physics, fluid mechanics, and biology. Throughout the book, Hu demonstrates the extraordinary value day-to-day curiosity brings to science.But, while he explores how science can reveal wonders of the mechanisms in our world, Hu writes how his work has been the target of politicians for so-called “wasteful” science spending. One of the studies under attack, an inquiry into the average length of urination across the animal kingdom, might have had a laughable premise, but eventually led to serious attention by urologists and researchers working on treatments, prostheses, and artificial organs.“The concept of waste is based on the notion of a limited gas tank and a single known destination,” Hu writes. “People expect scientists to save gas as they go from A to B. But the real power of science is to take us to destinations that we have never been to.”To stay updated on all things science, sign up for Science Friday's newsletters. Transcripts for each segment will be available the week after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

If we're going to design for the future, one scientist says we should look at the critters around us. I mean, these animals are doing things that our machines are still not capable of doing. We're hoping that our robots can actually face the great outdoors. And to do that, they're going to have to become a lot more animal-like. It's Monday, November 27th, and it's also Science Friday. I'm sci-fi producer Rasha Auretti. To round out our celebration of the 2023 Ig Nobel's, the awards for science that make you laugh and then think,

We're re-sharing a conversation with two-time winner, Dr. David Hu. He won once for studying how and why wombats make cube-shaped poo, and again for discovering that almost all mammals empty their bladders in about 21 seconds, give or take a few. Ira talked with him about his book back in 2018. Never wondered why your dogs back and forth shaking is so effective in getting you soaked? How bugs and birds and lizards they can run across the water without fall. in, or how cockroaches are so darn good at navigating in the dark.

We're all in luck because all of these questions and more are answered in my next guest book, How to Walk on Water and Climb Up Walls, Animal Movement and the Robots of the Future. David, who is the author? He's a mathematician, professor of mechanical engineering and biology at Georgia Tech and Atlanta, and we have an excerpt up there at ScienceFrily.com slash walk on water. Welcome back, David. Hi, Ira. It's great to be back.

Nice to have you back. You just wonder about stuff around you, don't you, dear? Yeah, everyday world's a great window into evolutionary history. You know, all the animals around us, all the people around us, they do the same functions that, you know, all animals have done from small all the way up to large. They give us a really good idea of, like, you know, what is like to be really small or really big or really hairy. You've investigated something called the wet dog. shake and that's not a dance move I'm talking about. Tell us about that. When I first met my wife,

on our very first date, she brought this poodle that I basically had to learn to get to know and appease the next few years. And it was the first time I'd spend her time around dogs. And I noticed it was really, really good at shaking off water or any stickers I put on it. I mean, it was really, really fast. And I'd never seen it before. So I decided to high-speed film it. And what I saw in the high-speed film just amazed me. I don't know if you've seen a dog shaking of water under slow-mo, but they can generate huge amount of forces. They generate basically 12 times Earth's gravity. It's the same force that a Formula F-1 race car takes when it takes around a curve.

It's basically the limits of what the human being can take. There's this guy named Colonel John Stapp, a scientist who tried to test the limits of human acceleration. He strapped himself to a rocket sled and then slammed on the brakes. And he found out at about 10 Gs, 10 times Earth's gravity, your body's fine, but your eyeballs start detaching from the retinas. And so actually all these animals that are doing this wet dog shake, they're pushing the envelope of what their bodies can take. They're closing their eyes shut really tightly.

Just don't get their eyeballs sort of attaching. And it's amazing how much water they actually get rid of, isn't it? Yeah, we did these experiments. We weighed all sorts of animals before and after. shook off water. And in a single second, your average dog can remove 90% of its water. For a 60-pound laboratory retriever, that's about a pound of water. And it removes it all in a second.

And that's comparable to what your laundry machines do in about an hour. That's amazing. If you stand, if you've, you know, stood next to dogs, you understand how much water that is. They're really good at getting the water back on you. Yeah. Now, I know one of your first projects you studied, You studied how water strider bugs, these great bogs, they can walk on water. You see them on lakes all the time, how they can paddle through the water without having oarves on their feet.

How do they do that? Yeah, that's right. I mean, imagine if you're going for a crew race, you know, a rowboat race, and someone handed you these long, just chopsticks and said, go row your boat. And that's basically what these water starters do. They row without any blades just with these long, spindly legs. Each of these legs is about a width of a human hair. But what allows them to work is that they're covered in hair. These water striders, they're the hairiest animals on earth.

I mean, I've been to some swimming pools. I think I've seen the hairiest things on earth. But no, it's these water striders. They've got about 10,000 hairs per square millimeter. And it's such a corrugated surface that water can't actually penetrate it. So when they're standing on water, they're actually standing on a cushion of air that's trapped within their hairs. From beneath, they just look like a pincushion. And because they're just floating on air, you can blow them and they just glide like the water's ice.

This is Science Friday. I'm Iroflato. And we're revisiting a conversation I had in 2018 with two-time Ig Nobel winner, David Hu, about some of the surprising things animals can do, what those feats tell us about physics and maybe teach us humans how to do things. better ourselves. And we're looking for your questions, anything that tickles your curiosity in your daily life, something that might be solved by creating an experiment. Here's someone who I might be thinking like that. Sean in Cincinnati.

Hi, Sean. Welcome to Science Friday. Go ahead. Well, this has been bothering me for about 25 years, and I can't understand how it is that a fly can land on a ceiling. Dave, what do you think? Well, flies, ants, and a lot of insects, they have a couple ways to basically walk on walls or walk on the underside of ceilings. And a lot of these ways would be difficult for us, but there's actually new technologies that are making it possible.

So there are actually people that have built devices that are allowing them to climb on glass buildings like geckos and flies. So there's a couple ways that the flies do it. If you zoom in, imagine zooming in down to that little fly leg. First, they've got this little thing called an aerolium. These are a small balloon that pops up every time the fly puts its foot down, and on this balloon is a really thin layer of fluid. It's kind of like a glue. In fact, if you actually look really closely at your fly,

and they've done this for ants, you'll see its footsteps. you'll see a trail of little drops of goo that is left behind. It's the surface tension of that goo that allows it to stick. The same force that allows drops to cling to your ceiling or to your car windshield window. That's the same force that supports the flyweight because it's so light. So that's one way that the flies do it. They also have a series of hairs. The same hairs that the ghetto has.

There's an example of what's called Convericks, Virgin evolution, that two species, they don't look at all like one's way bigger. In fact, one, each the other. They have very hairy feet. And the gecko's hairs, for example, aren't just hairs. There's like Christmas trees with Christmas trees on the tips of the Christmas trees. They have a series of progressions that get more and more hairy. It provides this really large surface area that's really close to that your ceiling.

That provides a huge what's called VanderWals Force. You don't even feel it. When you pick up things, there's always a van der Waals force, intermolecular force between two objects. It's what allows pollen to kind of stick to your clothes. But when you have a really large surface area, it's enough to actually support the weight of these insects. And these engineers at Stanford have actually built versions of this, where they've engineered arrays of hairs and made sure that they can hold on to things.

If you remember, there was a big controversy about this. In Colbert report a while back, people were saying Spider-Man doesn't exist because people had shown that if you try to scale up Spider-Man, sorry, not Spider-Man, a spider's legs, it wouldn't be able to support the weight of human. And that's because when you have a large array of these hairs, They don't act as good as just one hair multiplied many times.

They start to lose their effect because the weight support is not equally applied to all these hairs. There's some parts that get too much weight and those hairs peel off. While these engineers have found a way to basically create these little kind of yokes like small, these things that are in front of cattle, there's ways to basically equally distribute weight, they applied these things to these hairs and are allowed to basically made these handheld plates that allowed a student to actually climb up a glass building like a spider. Yeah, yeah.

Let's go to Dave. Dave in Cheyenne, Wyoming has an idea for an experiment, right, Dave? Yeah, I do. I have always been interested ever since I saw it with the levitating frog in the high-intensity magnetic field. Yes, go ahead. I wonder if that could be done with human beings. Levitating a human being in a high-intensity magnetic field. What interesting idea?

What do you think, David? So that's used this idea called paramagnetism where, you know, organic things, like things made in water, can actually become like magnets. The big issue was that that magnetic field, which was, I think in Europe, was only big enough. for strawberries and a frog. They still haven't big, made one that's strong enough to levitate a person. But the frog, so that guy who won an Andrew Geim, he's a fellow Ignobe prize winner. And he tells me that the frog was fine. So it's possible a human could do it too, but they would need a much bigger, much bigger magnet. That was the most expensive magnet they had and it was just big enough for a frog. Or a really tiny human.

Somewhere Galvani is laughing about that experiment. I want you to recount for us because you have a great story in your book. You open your book about a story about you as a new father. Let me put it that way. An event that happened to you as a new father that launched into an investigation of yours. Did I give you enough information? Yeah, we can talk a little bit about what it's like to be urinated on. And so, yeah, when you change kid diapers, they sometimes they play games, and one of their games they play is let's wait to the diapers off to urinate.

And there was a time that happened, and I was kind of shocked. And one of the things I was shocked by was how long it takes little kids to urinate. I mean, any parent that's waiting for a kid in the bathroom, you think if you're smaller, everything should be faster, but no. Urination takes about the same amount of time. And for my measurements, it was about 21 seconds for about a 10-pound kid. And those were comparable to my own measurements of my own urination time. And it just struck me because if you're 10 times smaller, you should have 10 times less urine. Urines are a byproduct of the blood, urea, and the bladder should be 10 times smaller if you're smaller.

So I couldn't wrap my head around it. And, you know, I got a PhD in fluid mechanics. I couldn't understand why it takes. the same amount of time if you're smaller. So I said some undergraduates, one of them who's now a professional urologist, which I'm super, he just lifelong learning, just couldn't stop, couldn't stop it, couldn't stop the fun. And they went to the zoo and I tell them, you know, bring this stopwatch and bring this dirty old bucket and this camera.

And, you know, don't come back until you've taken all the animal urination videos and measured all the urination volumes of every animal in the zoo. and they took me seriously and they actually did come back in a few weeks they smelled disgusting and splattered in urine and they told me the very worst was the rhino they were kind of traumatized by the rhino

but they I said just tell me one data point and I'll know everything I need to know and that's the elephant just tell me how the elephant goes to the bathroom and they said the elephant doesn't listen to anything they tell it to do

it just wakes up in the morning but when it does wake up it takes a long urine And they put this kitchen garbage can. It's about 20 liters, very large garbage can last for a week. And it fills the entire can. And I said, how long does it take? And they said, well, that bladder is about 100 times as big as your wife's dog.

And it takes about 21 seconds. And I said, that's the most amazing discovery I've ever made. That's pretty much this is the pinnacle of my career. and it turns out it's because they have this long pipe in their body. So doctors and veterinarians have long known that in the body there's this thing called the urethra. My kids call it the pee-pipy pipe. And I have to tell my kids, you know, boys have the pee-pipe and girls have a pee-pipy pipe. It's just, you know, in different places.

And it actually has the same aspect ratio. The length-width is the same for mice all the way to elephants. And to put it in perspective, when an elephant, you know, elephant urinates, a female elephant uses a peepe pipe that's about a meter long and about the width of my fist. So if you imagine that peeve pie is like a highway, you've got, you know, we wish we had this in Atlanta. We have like 20 lanes for the urine to come down. And moreover, the length of that pipe, it uses this effect that dates back to the 1850s called Bernoulli's Law, where basically if you've got a long pipe underneath a sort of a vessel, that pipe can actually

amplify the force of gravity and increase the speed of urine. So much that when an elephant urinates, it's like five showerheads going on at once. Wow. You've given us more to talk about tonight over a beer than any time in the recent past on size fly. Yeah. You would get really clean from five showerheads, but not five urine showerheads. That would be less clean.

Yeah, we'll quote you on that. While we're on the topic, while we're talking about wet things and on the topic of water, There's a chapter about another question I never knew I had until I read it in your book, and that is, how do mosquitoes fly through the rain? You know? I mean, if it's raining really hard and tiny little mosquito, why doesn't it get pummeled and knocked away and smashed by all that water? One would think that with the mass of water, it should be devastating for them, but not so.

Yeah, it should be devastating. And if you, you know, take a picture of a mosquito in a rainstorm, you'll see this water drop. They're about the same size, but the mosquitoes don't, you know, it's long, gangly legs. So the water drop actually weighs 50 times as much as a mosquito. It's like you getting hit with a Volkswagen Beetle. It's a huge, huge difference in weight. But, you know, that's the amazing thing about nature.

It's taken advantage of this, you know, huge David and Goliath story. And it's taken advantage of the mosquitoes really lightweight. And this is how it does it. Like when you go in a rainstorm, you stick out your... hand, a raindrop hit your hand and splashes. And you can feel it. It's a hard force. And the reason the force is so high is because you're ricocheting the raindrop.

You're actually throwing it back up in the air. It's splashing because it's hitting your hand. But when raindrops hit mosquitoes, they don't splash. That's the thing we discovered in the high-speed film. They just keep on going. And so if you don't actually, this is kind of a Zen thing. If you don't slow down the drop, if you don't, resist the drop, you don't get that much force. And so because you're not sort of exploding

the drop, you don't get that much force. And the mosquitoes, they just go along for the ride, sort of acts as like a stowaway on this drop. I mean, they're so hydrophobic. They'll eventually sort of split off, but they don't resist the force and they just survive that way. I'm Ira Plato. This is Science Friday from WNYC Studios. Talking with David Who, author of the new book, How to Walk on Water and Climb on Walls, Let's see if we can get a phone call in before we have to go. Let's go to Barb in Seattle. Hi, Barb.

Go ahead. Okay. So my question, going back to the fly theme that we had earlier, is those annoying little fruit flies like in your kitchen, little tiny things and you have a great big hand and you go to swap them. And they seem to magically transport to someplace else. Why can we not hit those little bitty things? How do they maneuver so quickly and so agile that we can't get them?

Good question. Yeah, David. Well, Sally, if you, if so, that has an evolved response for millions of years. I mean, those flies are tasty little protein, fatty treats. So if an animal could really get them, they would have. And the way they do it is all preparation. So if you actually, the fly has excellent vision. They can see what's called looming objects.

So your hand, as it's coming down from far away, it's increasing in size. And the flies can see that. And even before you're even close, they see this looming object, and they actually start preparing, far even inches, foot before your hand is even close.

And what they do is they actually, Michael Disson has filmed this at Caltech. They move their middle legs. They've got six legs. They move it in response, so they're actually, they feel the direction of this looming object, and they move their legs in response,

so they're ready to jump in the opposite. direction as soon as possible. And they do all this without the brain. It's basically all sort of autonomous. It's just an instinct for them. So just from this Lumen Response, they know what direction is coming from. And by the time your hand has even gotten close, they've prepared just take this catapult-like leap. And they just basically leap in the opposite direction before you even touch them.

Before we go, I want to touch on a serious turn. Your book takes at the end when you talk about how your research has been targeted by politicians for being so-called wasteful science. Tell us about that. That's right. About two years ago, my university told me to turn on the TV show Fox and Friends, and there's this huge game show where they put all the names of these scientific studies. And I found out that I was on this list of the most wasteful scientists for the entire country.

And not only that, but I was on the list three times, which made me responsible for 15% of the entire country. which I was kind of proud of because I'm just one person. I thought that's pretty good this year. This I can go for 30% next year. But basically they were saying that, and they don't just target me. They've targeted a lot of people who study animal movement. Sheila Pattox Fight Club for Shrimp, basically people who are studying how shrimp can use their

fist to break open mollusks, treadmills for shrimp. People that are basically studying animals and how they move. We're sort of easy prey for these attacks against science. You write that the concept of waste is based on the notion of a limited gas tank and a single known destination. People expect science is to save gas as they go from A to B, but the real power of science is to take us to destinations that we have never been to. It's hard for people who are not involved in science to know about that failure is an option in science. You know, making mistakes and failing is something you really welcome.

as opposed to other places in life. Yeah, and the study of these animals, these animals are, I mean, for example, the people have been calling, how do bugs escape a fly swatter? I mean, these animals are doing things that our machines are still not capable of doing. And this is important because we're hoping that our robots, I mean, these robots are trapped in factory floors.

They're doing repetitive tasks. We were hoping they can actually face the great outdoors, places where there's, you know, leaves and wind. and water. And to do that, they're going to have to become a lot more animal-like. They're going to have to learn to deal with different terrain, sand, water, rainstorms. We're going to have to, and the only way we can understand how to design those kind of things is to look at, you know, the things around us and how they're doing it.

Well, you do a very good... They're sort of our first step. Yeah, and you do a very good job of that in your book, How to Walk on Water and Climb Up Walls, animal movement and the Robots of the Future. Thank you, David. It's a great read. Thanks. Thanks for joining us. David Hu is a professor of mechanical engineering and biology at Georgia Tech in Atlanta, and we have an excerpt up on our website at Science Friday.com slash walk on water. Tomorrow we revisit a conversation with science legend Dr. Jane Goodall for more than 20 years ago. Join us. I'm Rasha Aureedi.