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- [Voiceover] If you took a
glass of water and a needle,

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and you took that needle
and you very carefully,

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very carefully dropped it on
the water, it would stay there,

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and it's not because it's floating.

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This needle would not be
floating on the water.

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This needle is more dense than water,

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and we know that if it's more
dense, then it should sink.

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So, it's not floating.

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It's actually just sitting on the surface,

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because there's surface tension.

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Water is a liquid that's capable of having

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a significant amount of surface tension,

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and you know it's surface tension because

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if you were to come in here
and exert a little force down,

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breaking the surface tension,

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or pushing this needle
just below the surface,

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then it would sink.

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It would sink like a stone

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and just drop immediately
to the bottom of the cup.

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So, why does water have this
property of surface tension?

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It has to do with the fact
that the water molecules

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within this liquid are
attracted to each other.

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This water molecule
can form hydrogen bonds

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with the other water molecules around it,

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and it gets pulled toward them,
and there's a term for this.

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We call this cohesion.

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So, the fact that water molecules
and other liquid molecules

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are attracted to each
other is called cohesion.

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But what does this have to
do with surface tension?

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Well, the key is these water molecules

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would like to bunch together.

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They want to group together, if they can.

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So, what would this water molecule do?

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I mean, which way is he going to go?

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How does he pick which one to group with?

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That's a problem.

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Here in the bulk of the
liquid, he can't decide,

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or in other words, let's just say

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he got pulled toward this molecule.

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Well, it's also getting pulled
to the left by all of this,

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by this one pulling it back
to its original position.

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This one's pulling it back
to its original position,

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because there will be a
component of that force

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that will point in the direction
of its original position,

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as well as this one to the left.

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So, these are restricted.

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These molecules here in
the bulk of the liquid

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have too many other water
molecules around them

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dictating where they need to be,

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because if they tried to get displaced,

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it'd pull them back to that position.

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However, at the surface

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there's no water molecules above them.

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These are freer.

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They're less restricted.

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So, that allows these water
molecules on the surface

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to group together a little better,

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form stronger tighter bonds,
closer spacing at the surface

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in such a way that they form a tension

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that's not present in
the bulk of the liquid.

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Yes, these water molecules down below

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will prevent them from just grouping into

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one big clump in the center,

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but since they're less restricted,

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they can form these tighter
bonds here at the surface,

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and this allows it to support
a pressure from above.

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So, this allows it to support
a certain amount of weight,

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which allows the needle
to rest on the surface.

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A few practical applications
of this, one clinical.

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If there's bile present in urine,

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you can detect its
presence because it lowers

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the surface tension of urine.

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So, it gives you a test
of whether the liver

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is metabolizing things the way it should.

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Another application is if you go camping,

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and you're in the tent.

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It's raining, and the tent
gets rain drops on it.

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Most tents will keep the
water from seeping through,

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but you're going to be tempted.

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You're going to be sitting in here.

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You're going to be like, that looks cool,

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and you're going to touch it,

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but you're not supposed to touch it,

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because as soon as you touch it,

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you may break the surface tension,

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and once you break the surface tension,

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that water is dripping into your tent

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from that spot that you touched it,

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and you're probably not
going to have a good night.

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So, resist the urge to
break the surface tension

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on your tent if it's raining out.

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And when you wash your hands,
when we use detergents.

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If you washed your hands
with just regular water

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and that's it, sometimes the
surface tension's too great.

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These water molecules are
too bound to each other.

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They form too big of a clump.

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It doesn't look like it.

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It looks perfectly smooth,
but on a microscopic level,

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the water's not as diffuse as it could be.

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It's forming these clumps,
because the water has cohesion,

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and it joins together, but
if you add a little soap

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to the scenario, that
breaks the surface tension.

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It lowers the surface tension,

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which means these water molecules

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don't clump together as much,

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and if they're not clumping together,

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they can get into the small cracks,

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which kicks out the dirt in your hands,

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and this water is better able to penetrate

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into the smaller cracks and
get where it needs to go.

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So, surface tension is due to cohesion

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between the water molecules
at the surface of a liquid,

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but water molecules aren't
just attracted to each other.

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They're actually attracted
to the container too

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and other materials, and
that's called adhesion.

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So, the fact that water molecules

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are attracted to other materials
as well is called adhesion.

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So, what happens is, this water molecule

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isn't just attracted to
the other water molecules,

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it's attracted to the wall,

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and these water molecules
climb the wall a little bit.

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So, that's why you'll see

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when you fill a container with water,

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or you're measuring an amount
of liquid in a small burette,

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it's not perfectly level at the surface.

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It actually forms this
kind of shape like that.

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This is exaggerated, but the sides will be

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a little higher than the middle.

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So, you have to be careful
when you're measuring.

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This is usually called the meniscus,

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and it's caused by the adhesion,

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the attraction of water molecules

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to the container that it's in.

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This adhesive force, this
adhesion force, is important.

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It causes something
called capillary action.

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So, let me get rid of this.

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If you have a container
with liquid, or say water,

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and you took another container.

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You put it in here like a straw.

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If you just stick it in,
what you'll see is that

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because the liquid is
attracted to the walls

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of this inner container, it
doesn't just stay at this level,

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it'll rise above.

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It pulls this up a little bit above

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the surface level of the water.

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And if you took an even
smaller diameter tube

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and put it in there, the smaller the tube

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the greater this effect, and
you'd get this water rising

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to an even higher level within this tube,

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due to the adhesion to the
walls of this container.

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And the name for this
effect is capillary action,

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which is important in
a variety of biological

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and non-biological examples
where fluid is being aided

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in transport partially by
the attraction to the walls

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of the container or the
tube that it's flowing in.