MadSci Network: Physics Query:

### Re: Why does water stay in a straw when you cover one end.

Date: Sun May 28 15:30:50 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 956940134.Ph
Message:

Straws and Water.

Start with a glass of water, put a straw in it, and take a close look at it. Here is a diagram:

```                        air         air          air
|           |            |
|           |            |
v           v            v
| |
| | straw
| |
| |
|             | |               |
|             | |               |
glass |             | |               |
|    water    | |               |
|-_ _ _ _ _ _-|-|-_ _ _ _ _ _ _-|
|             | |               |
|             | |               |
|             | |               |
|             | |               |
|             | |               |
|             | |               |
```
Air has weight, so gravity tries to pull it down to the ground. The air that is already down on the ground is in the way of the rest of the air, so the rest of the air stays above our heads. The important thing is that gravity pulls air down equally everywhere you look. That means that the of weight of air on the water outside the straw is the same as the weight of air on the water inside the straw. The level of the water in the glass is almost the same everywhere, because the weight of the air is the same everywhere.

The exception is inside the straw, and at the edges of the glass. The water level is slightly higher there. This is something that water can do by itself; the air has nothing to do with it. There is a small amount of attraction between the water and the glass, and between the water and the straw. That attraction lets the water climb up the side of the glass (and the straw) a little bit. Here is another diagram, so we can see it more closely:

```                 |                         |   | straw
glass |                         |   |
|                         |   |
|\                       /|\_/|\
| \_ _ _ water level _ _/ |   | \_ _ _ _
|                         |   |
|                         |   |
```
The thing to think about is the amount of water that is lifted near the side of the glass. Water not only attracts the glass, it also attracts itself. Only gravity keeps it from climbing all the way up the side of the glass. But notice how the water inside the straw has climbed higher than the water level inside the glass. This will only happen if the straw is narrow, and the narrower the straw, the higher the water can climb inside it. The closer together the walls around the water, the easier the uppermost water can attract the straw. And there is less total weight of water to be lifted, as the uppermost water attracts the water underneath it. The very narrowest straws are usually called "capillaries"; capillaries inside of trees let water climb hundreds of feet upwards. This climbing process is called "capillary action".

The attraction that water has, as just described, is mostly noticed only at the surface of water. The usual name for it is "surface tension". Surface tension causes small drops of water to become completely round, like a tiny ball. This is because the ball-shape, otherwise known as the "sphere", can hold the largest amount of water with the smallest amount of surface. Remember that we are talking about attraction at the moment. Attraction causes things to move closer together, and they take up less total space as a result. Since the surface of a sphere is smaller than the surface of any other shape, surface tension always causes water to form round droplets.

We often do not see a droplet of water after it touches something; it 'splats' into a flat film. This is just another consequence of the fact that water is attracted to other things. Whenever a drop of water flows into a flat film, the process is called "wetting". Anything that can truly get wet is something that water attracts. Other things, like wax paper, repel water, so if a drop is placed onto wax paper, the drop remains drop-shaped, thanks to surface tension.

Now back to the straw in the glass of water. If you put your finger over the top of the straw, and pull the straw part way out of the glass, you will see something like this:

```                                      _______
/         fingertip
\_______
|   |
|   |
|   |
|\_/| water in straw
|   |
|                         |   |
glass |                         |   |
|                         |   |
|\                       /|   |\
| \_ _ _ water level _ _/ |   | \_ _ _ _
|                         |   |
|                         |   |
```
Remeber that first sketch showing air pressure everywhere? When you lift the straw with your finger on it, you prevent air from getting in. The air outside the straw, pushing down on the water in the glass, weighs enough to hold the water up inside the straw. The water will stay there as long as no air can get into the top of the straw, where its weight can push the water back down to the level in the glass.

Now, what about when you pull the straw all the way out of the glass?

```                                 _______
/         fingertip
\_______
|   |
|   |
|   |
|\_/| water in straw
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
|   |
\_/
weight of water forms a droplet at bottom of straw
```
Remember that a droplet has surface tension, and attracts the sides of the straw. The only way that air can get into the straw is by BREAKING the droplet's surface tension. If the straw is narrow, air is not strong enough! Sometimes, for a medium-diameter straw, if you tilt the straw, the weight of the droplet will pull itself away from part of the straw. This will help the air break the surface tension, so it can get in. The water will drain out, drop by drop.

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