Why Does a Straw Retain Liquid? - Gordon

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In summary, the conversation discusses the phenomenon of a straw being able to retain liquid while an upside down bottle will release it. The reason for this is due to the cohesion of the liquid in the small space of the straw preventing air from passing through and replacing the liquid. The individual plans to experiment with different straw diameters and water with varying surface tensions to further understand this concept. The size of the straw's mouth and the surface tension of the liquid are important factors to consider. The individual also shares their experiment using an airtight apparatus and salty water to demonstrate this concept. Overall, the conversation supports the theory and thanks the other person for their help in understanding it.
  • #1
escapistgoat
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Why is it that a straw with it's top blocked off retains a liquid while an upside down bottle will release it?

Is it because the cohesion of the liquid in an area so small will prevent air from passing up through the liquid and replacing it within the straw?

If this is the case I intend to experiment with straws of various diameters and water with different surface tensions (by adding soap, salt or altering it's temperature.)

Thanks very much for any time you can spend helping me understand this.

-Gordon (is part of a complete breakfast)
 
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  • #2
Yes. The size of the mouth and the surface tension of the fluid are determining factors.

If you increase the surface tension (say, but subbing corn syrup for water) you'll need a correspondingly larger mouth before it will be able to bubble.
 
  • #3
Well I assembled an airtight and almost completely vibrationless apparatus to test this theory (small brass nozzle mounted into the lid of a large upturned glass jar which I sat atop a drinking glass on a concrete slab.)
Then I added salty water to my jar-'straw', which didn't seem to have any effect. So I slowly warmed the water and it began to blurp-blup out.
In conclusion; theory supported, thanks very much Dave!
 

Related to Why Does a Straw Retain Liquid? - Gordon

1. Why does a straw retain liquid?

A straw retains liquid because of the principle of atmospheric pressure. When you suck on the straw, you create a lower pressure inside the straw compared to the atmospheric pressure outside. This difference in pressure causes the liquid to rise up and fill the straw.

2. What is the role of surface tension in a straw retaining liquid?

Surface tension is the cohesive force between liquid molecules that causes them to stick together. In a straw, the surface tension of the liquid helps to keep the liquid inside the straw, resisting the force of gravity.

3. Can any type of straw retain liquid?

Yes, any type of straw can retain liquid as long as it is airtight and has a small enough diameter to create a significant difference in pressure when sucking on it.

4. What happens if the straw is bent or kinked?

If the straw is bent or kinked, it can disrupt the flow of liquid and prevent it from traveling through the straw. This is because the air pressure inside the straw is not able to equalize with the atmospheric pressure, causing the liquid to stop moving.

5. How does the height of the liquid in a cup affect the flow through a straw?

The height of the liquid in a cup does not affect the flow through a straw. As long as the liquid is above the end of the straw, the pressure difference created by sucking on the straw will cause the liquid to travel up the straw and into your mouth.

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