Surface Tension and Adhesion | Fluids | Physics | Khan Academy
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Surface Tension and Adhesion | Fluids | Physics | Khan Academy


– [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.

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67 thoughts on “Surface Tension and Adhesion | Fluids | Physics | Khan Academy

  1. I know what this has to do when relating it to medicine. It's significant to understand this concept because surfactant is the chemical that causes surface tension to occur in the lungs and makes it possible to breathe normally; without it the person's lungs would not be able to expand to take in breath in the first place and no way would the person be able to live outside the body without it.
    That ought to tell people something but I won't hold my breath waiting for it to dawn on them.

  2. A few misconceptions there Vicki, surfactant isn't a chemical its the name given to a family of chemicals like detergents that lower surface tension, a surfactant cannot cause surface tension in the lungs or anywhere else, it reduces it. The lungs expand due to the action of the diaphragm muscles causing partial vacuum to expand and squeezing to contract, which sucks air in and out. This has nothing to do with surface tension in the lungs.

  3. This 'explanation' of surface tension sounds nice, but in fact it is wrong. The density of a liquid decreases upon approaching the surface, which implies that the average distance between adjacent molecules at the surface is LARGER not 'smaller' than in the bulk. Usually Khan academy does a rather good job at simplifying complex effects – but this story is just incorrect. Interested readers may look into books such as Rowlinson and Widom's Theory of Capillarity.

  4. But what about the lateral and bottom molecules in a glass … They also experience as unblanced force so why not the water squeeze in the container …Why doesn't it bocome a big drop in the glass?

  5. Thnk u so much 4 this video….this is best explaination any1 can ever get….once again thnx 4 the video😘

  6. " and your gonna be like oh thats so cool and youre gonna touch it, youre not supposed to touch it " looooolll i LOOOOVE IT

  7. sir if an object is floating ….then what does it mean ….like whether it would be moving on the surface of water or just stay stationary at that place if, the density of the obj. is smaller than the density of water………….

  8. Ok, but why do also completely unpolar oils have surface tensions? They cannot form hydrogen bonds or have electrostatical interactions.

  9. Lol am I the only one who still finds this confusing and possible needs a computer generated 3D animation to demostrate the effect to better understand this stuff.

  10. Why with no water molecules on top creates surface tension? Explanation above doesn't work out. Becoz, even if they were present their contribution towards surface tension was none.

  11. i have a question, and excuse my ignorance, anyone can chime in.

    is there a way to make water remain in tension or increase the cohesion so that water can contain itself? its a bizarre question and making it happen would probably break a lot of rules about physics and chemistry.

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