[SOUND] Water is really fascinating. You see, water has an oxygen molecule in the middle and it has two hydrogens. Well, the hydrogen basically is just a bare proton. So it turns out that the unpaired electrons from oxygen and the positive protons here make water into something we call polar. It actually has a dipole moment, a very strong dipole moment. And this makes water incredibly useful. Of course, our bodies are mostly water, but water can dissolve things. Being polar, it's a wonderful cleaning agent. As anyone knows, if you try to get dirt out of anything, eh, put a little water on that and you'll clean it up. This fact that water is very polar also gives it a very high surface tension. Let me explain this concept. So if we have a whole bunch of atoms or molecules and they're in a solid, every one of these has a whole bunch of neighbors. And that's fine, it's like every other molecule. But if we get up here on the surface, it has unpaired things. It you had bonds, it has something that can't bond to anything else, because it's on the surface. This gives the atoms that are on the surface a different energy state than the ones that are in the bulk. Especially if you have a very polar molecule. What happens now is that these want very much to get into a lower energy state. And to do that, because everything rolls downhill, you'll actually get curvature. If take a group of water, the droplet forms a sphere. The surface tension is so high that to minimize that surface area so I have the fewest number of atoms that are in this higher energy state, the way to enclose a volume with the minimum amount of surface is to make a sphere out of it. So water likes to form a sphere. And if I have a droplet and I don't have a normal amount of gravity, some people say, no, water droplets look something like this. Well, that's because they're falling this way in a rainstorm and you've got gravity heading on here, and you have aerodynamics to try to make sure that you minimize drag, and that's why a water droplet looks like this when it's falling. But if we can catch one just as it's formed, before it's maybe moving, maybe this was bouncing up and then to get the height of its trajectory, you get a nice perfect sphere. The fact that water acts like this, can do some very interesting things. This is water droplets on a lotus leaf. And you might notice that they don't appear to stick. They have almost that same total spherical shape. And that's not normal. This surface is extremely hydro, yeah, for water, phobic, for fear of. The droplets aren't afraid, it's just that they don't like this surface. They'd rather get that minimum energy by being in a sphere. And the lotus leaf makes this surface hydrophobic in a very cool way. If we look at an electron micrograph of this, an actual electron microscope picture, you can see that the lotus leaf has a lot of these tendrils sticking up. So if we think about what's going on here, imagine you have a surface that has these little tips going up. Now, how is a water droplet going to interact with this? Well, if I have a perfectly flat surface, the water will sit there and it'll make some droplet shape, but it will still have quite a large amount of contact area. This would be the minimum energy it would go to. But if I have to get this tip the water will, if it had to, the droplet. These are very close together. This is an electron microscope, right? That's 200 microns. This droplet is maybe many millimeters, a centimeter, right? So the droplet on that scale is enormous. It's not going to try to be able to flow into each of these things. That curvature would be too high. This would have way too much surface energy. So it won't do this, it won't fill in. It will just balance on these little tips because of their spacing and their sharpness. And, therefore, it will turn into the whole sphere. So I can demonstrate the between water's property, it's a very polar substance, and let's say something like alcohol, which is not. So a paperclip's clearly more dense than water. It's made out of metal, metal sinks, it doesn't float in water. Or does it? Can't we utilize the surface tension of the water and actually have the paperclip, Float? Now if you look in close, you can see the bending of the water. The same thing for the bug's feet, of a water bug on a surface. And the slightest bit of extra weight will overcome that surface tension force and the paperclip will sink. Now let's try this with a nonporous substance, I brought some isopropyl alcohol. Isopropyl alcohol should have much lower surface tension. Let's see if we can get my paperclip to float on it. Hm, not that time, but doesn't look like it's quite as full. Isopropyl alcohol is diluted with some water anyway, so a little bit more water is only going to help the surface tension. But I do want to make it a fair comparison. It looks as though, It looks as though I was right, alcohol is not polar. It doesn't form as much round droplets, you can even see that from the stuff that's spilled here. And let's try it even with a lighter weight paperclip. Even though I'm doing the same thing, trying to get it very much on the surface and tipping it, it just sinks right in. Surface tension depends on the material that you're using. The planet is mostly covered with water. We're mostly water. And many creatures on the planet, which are also mostly water, have adapted to life on it or in it. Particularly, you may have seen water bugs who actually can walk on water. Their big advantage is that they are extremely lightweight. So the surface tension of the water can actually overcome the gravitational force of the bug. The bug is denser than water and it's got water with stuff in it. So the bug should generally sink. If I stuck this bug under the water, it would not float to the top, it would sink. But on the water, you notice that there's these little indentions, right? These indentions of where the bug is contacting the water is actually changing that surface energy, making it more curved, which is less, and that difference in energy is enough to offset the force of gravity. So a water bug can literally walk on water because it is so lightweight, much less gravitational force. And then that surface energy from the deformations of the water surface can support it. So you've got bugs that can go on water, what about something a little bigger? So let's take the basilisk lizard, this lizard can actually run across water. It's pretty amazing to watch. Because if you see it in real life or in real speed, it really just looks like this small gecko-like lizard running across the water. Very useful for it, it can catch prey in this manner. Let's look at a movie in slow motion. And you notice that as its feet go down it makes large pockets of water but they don't close up, if they ever closed up that lizard would sink. Instead, he's able to move his foot out fast enough using sort of a spring from the surface tension force of the water itself. We can also do this with a rock, a flat rock, large surface area. And if you throw it fast enough and when it contacts the angle of the water, the water effectively will be a hard surface and the rock can bounce. Just like if you took a rock on a sidewalk and you threw it the same way that you're going to skip a rock, you could actually get the rock to skip along the sidewalk and no one would think twice of that. A rock in water, a rock is clearly more dense, it will sink when it runs out of velocity on the next hit. But in the meantime, it was going fast enough to have the gravity versus the surface energy of the deformation of the water, let it bounce off like the water was a brick sidewalk. What about us? Could we as mere mortals walk on water? I'm not talking about Jesus, but I'm talking about physics here. Could we actually walk on water? No, but we could move very, very fast, if we could, then just like the rock or the bug or the basilisk lizard, we can do the same thing. Of course, that speed has to be very fast, say 20 to 30 meters per second. Something like 40, 50 miles per hour. Here we have a guy barefoot skiing. In my youth, I've actually done that, not, clearly, that well. But it's quite a rush to act as though this nice smooth lake is a solid surface that you're skipping along. It's really something when you fall. So I've actually been able to do this when I was young, I grew up on Lake Michigan, not very well and certainly not for very long. That's a lot of force on the bottom of your foot. Of course, usually the lake is cold so you're all numb anyway. And what's worse, of course, is when you are no longer skiing and you're skipping along like the rock at 50 miles an hour across the lake. Still, it's quite a rush. I like a good show as much as the next guy. But when I see magic show, being a scientist, I always try to figure out how did they do that, because I know it isn't really magic. When I saw a person walk across a swimming pool, I was just amazed. And then I figured, I had to make this everyday stuff video. Because here was a large pool, you have all sorts of people swimming in the pool, and you have this guy, very gingerly, very carefully, right? So here's your swimming pool, here's the surface of the water, here are all the swimmers in the pool, yay, they're playing, okay? And now you've got the person here, right? Not a very good stick figure, but he simply, slowly walks all the way across. People are splashing, some people swim underneath him. Some people are right in front, and then duck down as he steps over their head. Absolutely amazing. Now it's all about surface tension. So to make sure I have the surface tension just right, take off my shoes, all right? Pants out of the way, all right? And now I've got to step very, very carefully. This take the utmost of concentration. [MUSIC] >> [APPLAUSE] >> Unbelievable! >> How did he do it? Well, he cheated. All those swimmers in the pool? They're part of the act. And what there were is, I guess the bottom of the pool is here, okay? What there were, were these plexiglass platforms, all right? Plexiglass platforms with maybe a gap in them now and then. And the plexiglass platform, just under the surface of the water, so the person would go and walk, right? There's another one here. A foot here, a foot here, a foot here. Somebody would swim underneath, they went underneath the platform. Someone pokes their head up, there's a gap in the platform. He steps from here to here. Very, very simple. What the audience didn't realize, of course, is that all these random normal people, kids, everybody, different ages, some not even paying attention, right? This is all part of the act. So there are, of course, other ways to get pictures of people walking on water. Like this one. Which looks very impressive because this person right here is clearly heel to toe, normal walking. Except this is actually frozen water, [LAUGH] okay? If you look real carefully you can see a little snow in the background. Yeah, I know this guy's in a t-shirt, but there is a thin layer of ice on this or maybe it's just really clear ice, because they haven't fallen through yet. Or maybe it's just a really, really thin layer of water like a giant puddle as oppose to a pond. And rest assured, this is probably not a magic trick, just a really good photo. Don't try this at home, [LAUGH] okay? You're just going to get wet. And that's what you need to know about surface tension. [MUSIC]
