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Video transcript

if you were to take a glass beaker so let me draw it right over here if you ever 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 squat the glass it forms a shape that looks something that looks something like that and so the first thing we might ask is well what would we call this thing and this right over here is called a meniscus meniscus and in particular this meniscus because it is 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 we might have a convex meniscus well sure you could have a convex meniscus if you were to 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 the and so let me actually let me just label this this is a convex convex meniscus but it's one thing to just observe this and and to name them to say hey this is a meniscus 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 hold on hold on a second here we've been talking about how how water has the how has its polarity it has partial negative end each water mole each water molecule has a partially negative and has partially positive ends at the hydrogens so let me write this down partial partial positive charges at the hydrogens and that causes this hydrogen bonding to form and water 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 in itself because glass actually has the the the molecules in glass actually are quite polar glass typically made up of silicon a silicon oxide lattice for every one silicon atom you have two oxygen atoms you see that right over here for every one for every one silicon you have two oxygen atoms and it turns out that the 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 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 form around the oxygens form around the oxygens so these are partial negative and partial positive at the silicon and so you can imagine what's going to happen at the interface and let me make this clear what's going on this what i am 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 just got knocked by another molecule had enough kinetic energy to jump up here but once it came up it came in contact with the glass surface right over here this the the glass molecules it's stuck to them because it's partially it's partially positive end it's partially positive end at the hydrogen with the let me do that in that green color the partially positive end at the hydrogens would be attracted to the partially negative ends of the oxygens in 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 then the oxygen and the hydrogen in the water so these things just keep bumping around maybe there's another water molecule it just gets 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 add adhesion and to an adhesion is the reason why you also see the water a little bit higher there and when you see 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 also 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 at 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 let's go back to water so let's say this is a big tub of water i fill it so i i fill the water right over here now 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're dealing with a plastic tube because the plastic does not have that polarity 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 get your hands on a 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 that's interesting why is that happening well this 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 it's really just this adhesion occurring more 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 it also had glass over here and actually would be very hard to find a something that's that thin that's on the order of only a few molecules but this is i'm not drawing things at scale you can imagine now okay maybe another water molecule could jump up here and stick to the glass there and then one just gets bumped the right way jumps up and jumps there and if if we didn't have a polar container if we didn't have a hydrophilic container well then 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 and then it gets bumped the right way and then it gets 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 everyday lives all the time but 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 that's a spill right over there you spill so maybe you spill some water as you spill some 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 if you held it vertically you will see the water start to be absorbed into the paper towel this 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 it is attracted to the actual paper towel
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