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Surface Tension and Adhesion

David explains the concepts of surface tension, cohesion, and adhesion. 

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  • leafers ultimate style avatar for user Noel B Jacob
    Why does capillary action increase when the diameter of the vessel becomes thinner? Also, I understand that adhesion helps the water molecules stick to the walls of the container, but how does it help the water molecules to keep climbing higher?
    (28 votes)
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    • male robot johnny style avatar for user Dododeda
      Here's the answer to the second part of the question: Adhesion doesn't really play a large role in water molecules climbing higher. The property that allows water molecules to keep climbing higher is COHESION (water molecules sticking together due to hydrogen bonds). This is what makes transpiration in plants possible, and that process involves water moving upwards against gravity from the roots to the leaves.
      (8 votes)
  • old spice man green style avatar for user Tushar
    Can somebody help me with a question that i had in my mind lately?
    I was wondering why shuttlecocks' heads point wherever they are going. I thought about it for a while and came to the conclusion that it was because the rest of it's body suffered more drag than the head and therefore, just after it would leave a racket, it's head would start tuning towards where it was going because the rest of the body would start decelerating faster than the head because of air resistance and therefore would kind of be pulled forward by the head which would be suffering lesser deceleration due to drag.
    But if this is true, then that would mean that if you were to smack a shuttle by the back part instead of the head with a racket in a corner of space which was relatively free from any material media, the shuttle's head would not turn in the direction of motion of the shuttle which would be kind of funny and awkward to watch.... So, am i right?
    Well, please don't go about criticizing this question because of it's irrelevance, i posted it here because it is regarding fluids and how one would affect a shuttle's flight.
    Thank you a ton in advance.
    (9 votes)
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    • male robot hal style avatar for user Charles LaCour
      Your analysis is fairly good but it is not so much about the difference in the drag between the body and head as it is because of the torque caused by the position of the center of mass and the drag on the body. The center of mass of the shuttlecock is very far forward with the drag on the body produces a force acting in a backward there are only two orientations where the net force is acting through the center of mass and they are with the head pointing backward or forward. The head pointing backward is an unstable equilibrium because any deviation from pointing strait back will resulting in a torque that will rotate the shuttlecock until the head is point forward which is a stable equilibrium because any deviation will produce a torque that will make the head orient forward again.
      (11 votes)
  • leafers sapling style avatar for user Atharva  Mokashi
    What is concave and convex meniscus and why does it happen on the respective liquids ie Mercury (convex) and water (concave)
    (2 votes)
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    • blobby green style avatar for user Teacher Mackenzie (UK)
      I think its because the mercury is a metal and the (metallic) bonds between the atoms are stronger than the bonds between water molecules.
      This means that water can "stick" to other materials (such as glass in the tube) by cohesion but for the mercury molecules, the adhesion is much stronger so they can not adhere easily to other materials; The mercury tries to form a sphere hence the convex meniscus.
      (9 votes)
  • male robot hal style avatar for user ajmayberry
    I'm confused with capillary action. If you have water in a cup and the sides climb up the walls, it seems gravity is keeping it from going any higher. So what happens in the absence of gravity, like in space? How would water stay in a glass container?
    (4 votes)
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  • duskpin ultimate style avatar for user Amy
    Is Surface Tension what causes strands of wet hair to stick together in air? I assume it must be - the water molecules are attracted to each other and also to the hair molecules.
    (4 votes)
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  • spunky sam blue style avatar for user Vidit Chawla
    Will water ( or any other liquid ) flow out of the tube if the tube is of insufficient length
    (2 votes)
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  • male robot hal style avatar for user JW David Asher, FC1(SW) USN Ret
    Near the end of the video you discuss capillary reaction inside of the glass tube inserted in your container. I am sorry I am not prepared to readily answer by experimentation for myself - but does that same capillary reaction create another meniscus between your added glass tube and the original container?
    (3 votes)
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    • starky seedling style avatar for user deka
      yes it will do

      in fact, we don't need a pair of walls to create a capillary reaction, only with 1 wall there would be a part higher than the left in liquid because of adhesion

      so, right and left outside of the tube, we can find higher slope of water (but a bit lower than that of inside it), not to mention next to the walls of a container too
      (1 vote)
  • blobby green style avatar for user starace829
    what is free surface of matter ? why do solids have infinite of free surfaces and liquids only one ? please do answer
    (3 votes)
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    • starky seedling style avatar for user deka
      1. definition of free surface
      : surface that is free from contacting with a container

      2. number of free surface
      1) solid: no need to have a container to be solid by definition
      > thus every surface it has is free from a container
      > infinite free surfaces
      2) liquid: need to have a container with 3 or more walls except for the uppper one (given the gravity affecting from downward)
      > only the upper surface is free from contacting walls of a container
      > 1 free surface
      3) gas: only a fully walled container can contain a gas, otherwise it will run away from the container
      > no free surface from contacting a container
      > 0 free surface

      3. quiz
      how many free surfaces a water droplet in 0-gravity has? then which phase of matter it may be in with that number of free surfaces? is it still a liquid?
      (1 vote)
  • duskpin tree style avatar for user Oishika Chandra
    Surface tension is defined as the force acting per unit length of the liquid surface. Which direction does it act?
    (3 votes)
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    • purple pi purple style avatar for user luke.roeven
      It acts relative to the curvature of the water. If you have a droplet on a coin the force acts inwards keeping it on the coin. A better way to explain it could be that it makes the water form t object with the smallest surface area. I don't like this explanation so much because it doesn't explain why when a water jet falls vertically it creates a water bell (http://www.phikwadraat.nl/water_bells/). Think of it as the surface (skin) of the water curves to form the smallest bubble, and it acts it whichever direction is easiest. This might be completely wrong, but this is kinda how I look at surface tension. Just look a bit online and you should find a more satisfactory answer.
      (1 vote)
  • male robot donald style avatar for user Keshav Garg
    How many free surfaces are there when a needle floats on liquid at rest?
    My teacher says there are two: one on each side of the needle while I believe that there is only 1 free surface because the liquid-air interface is just once.
    (2 votes)
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Video transcript

- [Voiceover] 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, then 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? That's a problem. Here in the bulk of the liquid, he can't decide, or in other words, let's just say he got pulled toward this molecule. Well, it's also getting pulled to the left by all of this, by this one 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, it'd pull them back to that position. However, at the surface there's no water molecules above them. 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 bile present in urine, you can 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 rain drops 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 be 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's too great. These water molecules are too bound to each other. They form too big of a clump. It doesn't look like it. It looks perfectly smooth, but on a microscopic level, the water's not as diffuse as it could be. It'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 smaller 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 in a small burette, it's not perfectly level at 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 just 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 in transport partially by the attraction to the walls of the container or the tube that it's flowing in.