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Surface Tension and Adhesion

Video transcript

if you took a glass of water and a needle and you took that needle and you very carefully very carefully dropped it on the water it would stay there and it's not because it's floating this needle would not be floating on the water this needle is more dense than water and we know that if it's more dense than it should sink so it's not floating it's actually just sitting on the surface because there's surface tension water is a liquid that's capable of having a significant amount of surface tension and you know it's surface tension because if you were to come in here and exert a little force down breaking the surface tension or pushing this needle just below the surface then it would sink it would sink like a stone and just drop immediately to the bottom of the cup so why does water have this property of surface tension it has to do with the fact that the water molecules within this liquid are attracted to each other this water molecule can form hydrogen bonds with the other water molecules around it and it gets pulled toward them and there's a term for this we call this cohesion so the fact that water molecules and other liquid molecules are attracted to each other is called cohesion but what does this have to do with surface tension well the key is these water molecules would like to bunch together they want to group together if they can so what would this water molecule do I mean which way is he going to go how does he pick which one to group with is a problem here in the bulk of the liquid he can't decide or in other words just just say let's just say got pulled toward this molecule well it's also getting pulled to the left by all of this by this one's pulling it back to its original position this one's pulling it back to its original position because there will be a component of that force that will point in the direction of its original position as well as this one to the left so these are restricted these molecules here in the bulk of the liquid have too many other water molecules around them dictating where they need to be because if they tried to get displaced and pull them back to that position however at the surface there's no water molecules above the these are freer they're less restricted so that allows these water molecules on the surface to group together a little better form stronger tighter bonds closer spacing at the surface in such a way that they form a tension that's not present in the bulk of the liquid yes these water molecules down below will prevent them from just grouping into one big clump in the center but since they're less restricted they can form these tighter bonds here at the surface and this allows it to support a pressure from above so this allows it to support a certain amount of weight which allows the needle to rest on the surface a few practical applications of this one clinical if there's a bio present in urine you could detect its presence because it lowers the surface tension of urine so it gives you a test of whether the liver is metabolizing things the way it should another application is if you go camping and you're in the tent it's raining and the tent gets raindrops on it most tents will keep the water from seeping through but you're going to be tempted you're going to be sitting in here you're going to like that looks cool and you're going to touch it but you're not supposed to touch it because as soon as you touch it you may break the surface tension and once you break the surface tension that water is dripping into your tent from that spot that you touched it and you're probably not going to have a good night so resist the urge to break the surface tension on your tent if it's raining out and when you wash your hands when we use detergents if you washed your hands with just regular water and that's it sometimes the surface tension is too great these water molecules are too bound to each other they form too big of a clump it doesn't look like it looks perfectly smooth but on a microscopic level the water is not as diffuses it could be that's forming these clumps because the water has cohesion and it joins together but if you add a little soap to the scenario that breaks the surface tension it lowers the surface tension which means these water molecules don't clump together as much and if they're not clumping together they can get into the small cracks which kicks out the dirt in your hands and this water is better able to penetrate into the all the cracks and get where it needs to go so surface tension is due to cohesion between the water molecules at the surface of a liquid but water molecules aren't just attracted to each other they're actually attracted to the container too and other materials and that's called adhesion so the fact that water molecules are attracted to other materials as well is called adhesion so what happens is this water molecule isn't just attracted to the other water molecules it's attracted to the wall and these water molecules climb the wall a little bit so that's why you'll see when you fill a container with water or you're measuring an amount of liquid and a small burette it's not perfectly level with the surface it actually forms this kind of shape like that this is exaggerated but the sides will be a little higher than the middle so you have to be careful when you're measuring this is usually called the meniscus and it's caused by the adhesion the attraction of water molecules to the container that it's in this adhesive force this adhesion force is important it causes something called capillary action so let me get rid of this if you have a container with liquid or say water and you took another container you put it in here like a straw if you stick it in what you'll see is that because the liquid is attracted to the walls of this inner container it doesn't just stay at this level it'll rise above it pulls this up a little bit above the surface level of the water and if you took an even smaller diameter tube and put it in there the smaller the tube the greater this effect and you'd get this water rising to an even higher level within this tube due to the adhesion to the walls of this container and the name for this effect is capillary action which is important in a variety of biological and non-biological examples where fluid is being aided and transport partially by the attraction to the walls of the container or the tube that it's flowing in