Cohesion and adhesion
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Capillary action and why we see a meniscus
- If you were to take a glass beaker, so let me draw it right over here. If you were to take a glass beaker and you were to fill it up with water, you might expect that the surface of the water would be flat. But it's actually not the case and I encourage you to try it. You might have even observed this before. The surface of the water will not be flat. The surface of the water will actually be higher near the glass than it is when it's away from the glass. It forms a shape that looks something like that. And so the first thing we might ask is what'll we call this thing. And this right over here is called a meniscus. Meniscus. And in particular this meniscus, because the fluid is higher near the container than it is when you're away from the container, we would call this a concave, concave meniscus. And you might say, "Well if this is a concave meniscus, "are there any situations where might have "a convex meniscus?" Well sure, you can have a convex meniscus. If you were take that same glass beaker, instead of filling it with water if you filled it with say, mercury. If you filled it with mercury, you would get a meniscus that looks like this where there's a bulge near the center when you're further away from the container than when you're at the container. And so let me just label this. This is a convex, convex meniscus. But it's one thing to just observe this and to name them. To say, "Hey this is a meniscus." So this is a concave meniscus. But a more interesting question is why does it actually happen. And so you might imagine this concave meniscus is because the fluid is more attracted to the container than it is to itself. And you might be saying, "Wait, wait. "Hold on, hold on a second here. "We've been talking about how water "has this polarity, it has partial negative end. "Each water molecule has a partially negative "and has partially positive ends at the hydrogens." So let me write this down. Partial positive charges at the hydrogens. And that causes this hydrogen bonding to form and that's what kind of gives water all of these special properties. "You're telling me that it's more attracted to the glass than it is to itself?" And I would say, "Yes, I am telling you that." And you could imagine why it is going to be more attracted to the glass than itself, because glass actually has, the molecules in glass actually are quite polar. Glass, typically made up of silicon oxide lattice. For every one silicon atom, you have two oxygen atoms. You see that right over here. For every one silicon, you have two oxygen atoms. And it turns out that the electronegativity difference between oxygen and silicon is even higher than the electronegativity difference between oxygen and hydrogen. Silicon is even less electronegative than hydrogen. So the oxygens are really able to hog silicon's electrons. Especially the ones that are involved in the bonding. So you have partial charges, partial positive charges form at the silicon and then you still have partial negative charges form around the oxygens. Form around the oxygens. So these are partial negative. And partial positive at the silicon. And so you could imagine what's going to happen at the interface. And let me make this clear what's going on. This, what I am circling right now, that is the water. This right over here, that's the water molecules. And what we see over here, what we see over here, these are the glass molecules. So this is the glass right over here. And sure the water is attracted to itself because of the hydrogen bonds. But it has some kinetic energy, remember these things are jostling around, they're bouncing around, we're in a liquid state. And so you can imagine all of a sudden, maybe this, let me see, maybe this character, this water molecule right over here. Maybe a moment ago it was right over here but it popped up here. It just got knocked by another molecule, it had enough kinetic energy to jump up here. But once it came up, came in contact with the glass surface right over here, the glass molecules. It stuck to them. Because its partially positive end, its partially positive end at the hydrogens. Let me do it in that green color. The partially positive end at the hydrogens would be attracted to the partially negative ends of the oxygens in the glass. And so it'll stick to it. This is actually a stronger partial charge than what you would actually see in the water because there's a bigger electronegativity difference between the silicon and the oxygen in the glass than the oxygen and the hydrogen in the water. So these things just keep bumping around. Maybe there's another water molecule that just get knocked in the right way. All of a sudden for, you know, a very brief moment it gets knocked up here. And then it's going to stick to the glass. And this phenomenon of something sticking to its container, we would call that adhesion. So what you see going on here, that is called adhesion, adhesion. And adhesion is the reason why you also see the water a little bit higher there. When you talk about something sticking to itself, we call that cohesion. And that's what the hydrogen bonds are doing inside the water. So this right over here, that over there, that is co-, that is cohesion. So that's why we have things, why we observe a meniscus like this. But there's even more fascinating properties of adhesion. If I were to take, if I were to take a container of water. If I were to take a container of water. And just to be clear what's going on here with the mercury, the mercury is more attracted to itself than it is to the glass container, so it bulges right over there. But let's go back to water. So let's say that this is a big tub of water. I fill it. So, I fill the water right over here. And let's say I take a glass tube, and the material matters. It has to be a polar material. That's why you'll see the meniscus in glass, but you might not see it or you won't see it if you were dealing with a plastic tube because the plastic does not have that polarity. But let's say you were to take a glass tube, a thin glass tube this time. So much thinner than even a beaker. So you take a thin glass tube and you stick it in the water, you will observe something very cool. And I encourage you to do this if you can get your hands on a very thin glass tube. You will notice that the water is actually going to defy gravity and start climbing up this thin glass tube. And so that's interesting. Why is that happening? Well this phenomenon which we call capillary action. Capillary, capillary action. The word capillary, it'll refer to anything from you know, a very, very narrow tube and we also have capillaries in our circulation system. Capillaries are our thinnest blood vessels, those are very, very, very, very thin. And there's actually capillary action inside of our capillaries. But what we're seeing here, this is called capillary, capillary action. And it's really just this adhesion occurring more intensely because more of the water molecules are able to come in touch with the polar glass lattice. And so you can imagine we have glass here. If you also had glass over here. And actually it would be very hard to find something that thin that's on the order of only a few molecules. But this is, I'm not drawing things in scale. You can imagine now okay, maybe another water molecule could jump up here and stick to the glass there. And one just gets bumped the right way, jumps up and jump there. And if we didn't have a polar container, if we didn't have a hydrophilic container, well then the thing might just jump back down. But because it went up there, it kind of just stuck to it. And then it's vibrating there and then maybe another water molecule gets attracted to it because of its hydrogen bonds. Then it gets bumped the right way. And then it gets bumped with the higher part of the container but then it sticks there. And so it starts climbing the container. And that's what capillary action is and it's not just some neat parlor trick, we actually probably use capillary action in our every day lives all the time. Beyond the fact that it's actually happening in your capillaries in your body that allows you to live, but if you have a, if you spill something on your counter. So let's say that's a spill right over there. You spill some maybe, you spill some water, or you spill some milk. And if you take a paper towel. If you take a paper towel. In fact, if you took a paper towel like this. If you held it vertically, you will see the water start to be absorbed into the paper towel. This kind of absorption action that you see, that actually is capillary action. It's the water going into the small little gaps of the paper towel, but that's because it is attracted to the actual paper towel.
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