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Strawfuls of water

  1. Feb 16, 2008 #1
    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)
  2. jcsd
  3. Feb 16, 2008 #2


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    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.
  4. Feb 16, 2008 #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!
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