How does water stay in a straw when you place your finger over the opposite opening?

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My son just asked me at dinner and I do not have an exact answer.

In: 7

It’s because the straw is narrow enough that water’s inherent “thickness” (I assume a child doesn’t know what viscosity means) stops it from just sliding out if it can’t be replaced by air. If you block one end of the straw, an air bubble cannot travel up through the water, and cannot enter through the top. So it just sits there.

But it’s important that it’s a straw. If you start increasing the diamater of the tube the water is in (to, say, bottle neck size), at some point the water isn’t “thick” (viscous) enough to overcome gravity anymore, and will buckle on one side, allowing a bubble of air to rise and replace it on the other. That’s why if you tip a bottle upside down, it doesn’t just all flow out at once either, but gurgles. It still has enough viscosity to kind of stick together. But it will flow of you spin it, because then the air enters through the centre of the whirlpool.

To get the water in the straw, the suction force you apply had to overcome the gravitational force pulling the water down (the obvious bit). When the suction stops (the one end of the straw is opened), gravity draws the water down again. The downward movement of the water in the straw creates a slight low pressure as the water flows down which sucks in air in the wake of the dropping water (the air replaces the water in the tube, almost like what happens when you drain a big bottle of liquid). However, when you block the top of the straw, you’re removing the flow of air from the equation. Now, the gravitational force is still pulling down on the water in the straw, but since the low pressure that forms as the water is pulled down cant be equalised by the air through the top, a vacuum forms between the top of the water and the blocked straw opening, which is equal to the gravitational force downwards (which is why the water in the straw stays at a constant level after adjusting).

How does your son drink the water through the straw?

He SUCKS on the straw, creating a vacuum (suction), and the water comes up because it’s being suctioned in. So if he puts his finger there, the finger won’t let the air in, maintaining the suction.

What is suction? Well, all of the air that’s above our heads is also pulled down by Earth’s gravity like everything else. So it creates pressure, like the pressure in a balloon. It’s about 14 pounds of pressure per each square inch of surface. It’s about half of what car tires have in them (36 PSI).

We live inside a balloon of air. We don’t feel it because we’re used to it, but if you go up in space you have to have a space suit to keep yourself “inside a pressurized balloon” like you’re used to on Earth.

Anyway, suction is less pressure than this “normal” air pressure.

So basically, the air actually pushes the water INTO the straw, if you create suction (less pressure) at the other end.

Air pressure. All air likes to be at the same pressure. It takes a lot of energy to force a pocket of air to be at a different pressure than the other air around it. Think about how hard it would be to blow up a basketball with your breath and you’ll understand.

When you pick up a straw with one end covered, you have a straw that is filled with water at the bottom and air at the top. When water flows out the bottom of the straw, it leaves empty space behind, and that side has to be filled with something. Normally, that side would be filled by air that would get sucked in the top of the straw, but it can’t because your finger is covering it. The only other way the water could flow out is if the air in the straw agreed to lower its pressure so that the same amount of air filled a larger space. But, there’s no way in hell you’re gonna find a pocket of air that would agree to do that. So, the water just stays in the straw until it finds something that can take its place if it were to flow out.

Three things. Capillary forces, vacuum, and surface tension.
Capillary forces are the interaction between the water and the straw. The water is “holding onto” the sides of the straw.
Vacuum is created by gravity pulling down on the liquid in the straw when it is sealed at the other end with your finger.
Surface tension is the water holding onto itself in the bottom of the straw. It is the same thing that makes a drop of water sit in a half sphere when spilled on the counter.
If any of these things break down the water will not stay in the straw.