Surface tension

0:00- [Instructor] Check out these insects. They can literally walk on water. They're called water striders. You can similarly make a paper clip made of metal just sit on water. That is so cool, right? So what's going on? Well, something's happening on the surface of the water, but what exactly?

0:19Well, let's find out. To do that, we will zoom in all the way until we can see individual water molecules. Each circle over here represents a water molecule. I've drawn a circle because it's easier to model it that way. And yes, these water molecules are all randomly moving, but we're gonna ignore that.

0:37We're gonna simplify it and assume that, you know, they're pretty much stationary 'cause that'll make it easier to understand what's happening on the surface. This is what modeling is all about. We ignore the unimportant stuff so that we can simplify things and we can focus on what's important for us, okay?

0:52All right, now if you were to concentrate on any one of the water molecules which are inside the bulk of the liquid, let's say this one, then it is being attracted by all the molecules in the neighborhood, right? This is called cohesion, but what's important is that it's being pulled in all the direction, and so the forces kind of balance out.

1:12Imagine a toy car being pulled into opposite directions with equal forces. That toy car is not going anywhere. Similarly, the forces over here get balanced out, and this would be true for all the molecules which are in the bulk of the liquid. But what about the molecules which are at the surface, like say the one over here?

1:30Well, then it's only being pulled on the side and down. There is nothing to pull up on it because there are no water molecules on the top. I mean, there are air molecules, but the force between the air and the water is negligible, so we can ignore it, but this means the forces do not all balance out, and this has some pretty cool consequences.

1:50So if you imagine, you know, hypothetically, if you were to sort of like pull this water molecule up, then look, the forces are going to pull that water molecule back down. And if you were to push this water molecule down, well, then look, the forces are going to pull that water molecule back up, so and since this is true for all the molecules on the surface, it kind of means that the molecules on the surface act like a stretched film, kind of like a skin, right?

2:18Like if you stretch a skin, it'll come back down. You know, if you push it down, it'll come back up. That surface acts like a stretched skin, right? And this phenomena is what we call surface tension. Surface tension is where the surface of any liquid acts like a stretched film.

2:36And what causes surface tension? Well, it's caused by the attractive forces, cohesion, but where? Well, cohesion exert on the surface molecules of the liquid by the molecules that are below them, and this now can explain why water striders can walk on water. They have long legs like this, and they're pretty light.

2:57And since the surface of the water acts like a stretched film, look, the water strider, because of its weight, it deforms the surface, but it doesn't break it. So it is the surface tension. That is the reason why water striders can literally walk on water. Similarly, when you place a metallic clip, a paper clip, carefully on the surface of the water, again, look, it is the surface tension.

3:18Because the surface of the water is acting like a stretched film, it is able to support the weight of that clip. Now we may think that this has something to do with the density, but it's not. If I just drop the paper clip, look, it's sinks. So what's really going on over here?

3:34Well, the trick is to place the paper clip flat and evenly. This way the surface tension spreads out over a larger area, making it more likely for the cohesive forces at the surface to balance the downward force, which is the paper clip's weight over here. On the other hand, when we drop the paper clip or keep it on its edge, the update applies enough down force to break the water surface tension, causing it to sink.

3:59So, look, this isn't about the difference in density at all but rather a balancing of forces at the liquid's surface. And yes, because the surface of the liquid acts like a stretched film, this is the reason why they form spheres. Remember, the surface of the molecules are all being pulled inwards tightly together, which acts to minimize the surface area of a liquid.

4:22And how do you minimize the surface area? Well, for a given volume or for a given amount of water over here, the geometric shape that has the minimum surface area is a sphere. So look, surface tension naturally pulls a liquid into a spherical shape to minimize the surface area and the associated energy.

4:41So putting it all together, what is surface tension? Surface tension is where the surface of any liquid acts like a stretched film, and this happens because of cohesion, the attractive forces that are exerted on the surface molecules by the molecules below it.

4:59What I find fascinating is that there are certain creatures evolved to take advantage of this very specific property of water, which is pretty cool.