The WALKING WATER Mystery (in SPACE and SLOW MOTION!) - Smarter Every Day 160

Hey, it's me Destin and welcome back to Smarter Every Day! I have a problem. There is a specific water phenomenon that I see happening all around me, but I have no idea how it works. I've been trying to figure it out for years. In fact, I put a video on this channel in 2011, asking people to help me figure it out!

There's something interesting happening here I don't understand. It looks like water droplets are standing on water droplets. It looks like water is hydrophobic. It looks like when a bead of water skates across the hot griddle, it creates a layer of water vapor underneath it. Only, that's not what it is. If you have any information, please let me know. I’d appreciate it. You've seen this, right? If you watch the world with a critical eye like I do, you'll see this everywhere. For example, if you've ever allowed yourself to watch a drip coffee maker, when the conditions are just right, you'll notice that coffee beads up on top of coffee. What does it do that sometimes?

When you turn the shower head on, the water will blast to the other side of the tub instead of making the bottom of the tub wet. These pearls of water will dance on top of the water that's already in the bottom of the tub. Don't let this weird you out; it's just water and food coloring. But the interesting thing is if you get the stream in just the right location and interacts with the porcelain and the water that's in the toilet in just the right fashion, you'll get these little drops that bead up.

I don't understand what the conditions required to make it happen are, but I get really excited about it and I want to understand. Seriously, one time we landed an Apache helicopter on the pad at work, and the environmental control system was leaking condensate down onto the asphalt, and the beads were happening there. That was the moment I realized I have to know why this is happening. In order to figure it out, we're going to Science Garage to see my friend Don Pettit, who has out-of-this-world experience in the area of surface chemistry. Let's go get Smarter Every Day.

It's water dancing on water, but we're gonna do it here on earth, and then in space, that OK? Yeah, and we're going to water down this one! Yeah, and we're going to water down this one! [laughs] That's bad. If you know who Don Pettit is, you understand why I came to him. It's rare to find a person that not only understands how to capture beautiful photos but also understands the fundamental science behind each image. Seriously, who else do you know that collects books on surface chemistry and wedding phenomena? You own these references? Yeah. Really? Well, I mean when you're into this kind of stuff, you need it, right?

Anyways, scientists all over the world have created elaborate setups in their labs to try to observe these dancing drops, and they've got some really great data. But I knew if I got the right type of camera in Don's hands and just turned him loose, we would not only get the data, but we would do it beautifully. It's a Phantom v2511, which is a very, very fast camera, and it's really loud. But we're gonna turn on the science here and give it a go. After only about an hour of working through the lenses and lighting with Don, he quickly came up with a setup to observe a beautiful single drop of water landing on, bouncing, and then coalescing into a larger pool of water.

Alright, so we have a drop balancing on a still surface of water, and you'll notice that it isn't coalescing. Coalescence is when you have two pieces of water that touch and then merge to become one piece. Now you notice there's this delay time where the drop is sitting there before coalescence; that's called the 'Residence Time.' Scientists have a pretty good understanding of what's going on here. As a drop touches the surface, scientists believe there's air trapped in-between. As the drop rests, that air starts to seep out of that gap, and then coalescence occurs.

The National Institute of Health did this really cool study where they verified the air gap theory by reducing the air pressure around coalescing water drops, and as you'd expect, the lower the air pressure, the lower the residence time, which is really cool. Anyway, you can plainly see on the video that Don captured that as the drop comes to a rest, it starts to slowly bob up and down, and then it's like a little water bridge is built in that air gap, and then coalescence occurs.

So this is my question in the phenomenon I'm seeing: I'm seeing a steady state bouncing of a drop, which means there's impacts happening over and over. Why is that not punching through that air gap and causing coalescence to happen easier? In order to understand this, we actually have to look at that impact, which means this is about to get awesome. This is a device that's been shipped to me from another YouTube channel, Ben at NightHawkInLight, and he has drops of water that he puts on water which is sitting in a speaker. So, we're about to basically recreate the setup that Ben did.

So, full disclosure, you've done this in space, right? I have! But we're going to use this high-speed camera to understand the phenomenon here on earth, and then you're going to teach me about what you discovered in space, right? Yeah, we're gonna, we're gonna start on earth and end up in space. [phone ringing] Hello? Hey Ben, it's Destin, how are you? Yeah, and this is Don. We're in Science Garage here having fun. Awesome! Don, it's cool to hear from you. What frequency and amplitude worked out best for you? [Beep] If we were to turn that speaker off right now, they would go away, right? Well, let's see.

Okay. See? They're there, but they coalesce very quickly. So, okay, keep doing that. Don't change anything; we'll change one variable. OK. That's me turning on the vibration of the speaker. So what I think is happening is the water is trying to settle and go down and press against the surface. Uh-huh, that's right. But because it's being pushed back up due to that vibration, it adds energy back into the system and then pushes it away.

And I'm saying that that process keeps the small air gap between the drop; it helps maintain that. You can sit here and postulate things till you're blue in the face. Until you make an observation to see what that fundamental physics is, then you can back out what's going on. The high-speed of what's going on with that drop would do a lot to help answer these questions. This is quickly becoming one of the most awesome setups I've ever been a part of, would y'agree? Yeah, this is just totally awesome!

But you've been to space! Alright, here we are, the moment of truth. After years of waiting, we finally have everything we need in order to make an actual observation. Here we go. And there it is! Did you see it? When I first saw this footage, it stood out to me like a sore thumb. Instead of crashing into the drop like I anticipated, it's almost like those peaks and valleys make some type of moving catcher's mitt that gently catches the drop and lets it down gently without breaking that air layer. Now I want to zoom in and see if we can see that wave catch the drop. There it is! It's pretty clear what's going on here.

Think about it this way: if somebody threw you a water balloon, would you rush your hand up and try to meet it really quickly and risk bursting that surface tension? No, you would try to match the velocity of the balloon with your hands so you could decrease the collisional kinetic energy. In fact, there's an equation for how to calculate collisional kinetic energy, and that's what factors into whether or not the drop survives. If the velocity of the liquid and the small drop are about the same, then that factor goes to zero, and the collisional kinetic energy quickly decreases to zero.

Now that we understand this, we can infer that timing is a huge factor here, and there's probably a bunch of drops that just don't survive because they hit the wave wrong. In fact, that's exactly what we see in the high-speed. One more thing to think about is the fact that gravity is constantly pulling the drops back down to that vibrating surface, which means it always has to find the soft spot to land. The larger the drop, the greater the gravitational force pushing down on it.

But what if you did this somewhere where there was no weight on the drop? Oh, then it bounces off! I've got the video. Can we see it? Yeah! I discovered this phenomenon for myself occurring naturally in the woods of Alabama in 2011, the same year that Don Pettit launched to the International Space Station, where he accidentally recreated the phenomenon in a really remarkable way. He performed a very similar water speaker experiment, except in weightlessness the water forms a sphere. Just to make it interesting, Don put an air bubble on the inside of that water sphere, and then he played tones through the speaker just to see what would happen.

[Don]... and the air bubble is attached to the speaker cone, and so is the water and I'm at 60 Hertz. I'm going to start increasing the amplitude. Here we go! Wow. So we're getting a series of standing nodes on the interface of the air bubble with the water. I'm going to put a square wave in. OK... triangle wave. So Don played with different frequencies, amplitudes. He played different types of music like rock and roll, and then he played the cello. Don said whenever he played cello music specifically, little drops would break off on the inside of the sphere and start bouncing around on the inside.

A Cello!? There’s something about the Cello! [Music: Bach's Cello Suite No. 1 - Prelude] This video sets my brain on fire. We've learned two main mechanisms for coalescence. We have the weight of the drop that squishes that air boundary and makes it go. We have no weight, so we're left with collisional kinetic energy. Every collision that results in coalescence; everything is right there and you can just directly see what's happening. The sidewall that's vibrating? It makes the small drops go faster than the big drops because there's lower mass, but it's the same amount of excitation energy.

I really liked this video; it's beautiful on multiple levels, and I want you to think it's beautiful too. [Music: Bach Cello Suite 1] This has been a huge science goal for me for a really long time, so I want to say thank you to the Google Making in Science team for making this video possible. Thank you so much! If you would see really cool videos, check out the hashtag #sciencegoals. There's all kinds of videos being uploaded all the time.

For example, check out Ben at NightHawkInLight; obviously, he helped me make this video—it was his speaker setup that helped us do the stable bouncing droplets in this video. Or check out Diana, AKA PhysicsGirl, who teaches you how to make anti-bubbles in your own home using household items. Or you can go check out this cool video by Derek at Veritasium, who explains that the bouncing drop mechanism can be used to model quantum theory. He uses bouncing drops of silicone oil to reproduce the double-slit experiment.

Check out any of these other videos under the hashtag #sciencegoals to go see really awesome science content. A huge thank you to Don Pettit for sharing his knowledge of surface chemistry and his orbital footage in a really humble way; that was awesome! Thank you, Don! Last thing, the v2511, the Phantom that we used in this video—that's now a thing on Smarter Every Day, which has also been a science goal for a really long time. So if you're interested in subscribing, you've ever thought about it, now is a really good time to do that because the slow-mo is about to get slower; it's about the higher resolution, and it's about to get more fun.

Feel free to click my face if you'd like to subscribe. If not, that's it. I'm Destin, you're getting smarter every day. Have a good one.