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Scoop the liquid with a straw

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What type of experiment is this?

Experimental procedure and explanation:

  • Scoop coffee with a straw.
  • You can scoop up the coffee by putting the straw in the coffee, holding the straw's mouth, and lifting it up.
  • At this time, the surface of the liquid is at atmospheric pressure at the lower end. Conversely, the air pressure at the top is lower than the atmospheric pressure because of the height of the liquid. The force caused by the pressure difference between the top and bottom and the weight of the liquid is balanced to support the weight.
    (Difference in vertical pressure) = (Density of water) × (Gravitational acceleration) × (Difference in height)
  • Droppers and pipettes used in science experiments also use this principle.
  • Thick straws do not scoop liquid well. In general, the smaller the effect of surface tension is, the stronger the effect will be. In this experiment, the effect of surface tension is strong with a thin straw, and the water surface can be maintained; with a thick straw, the water surface cannot be maintained.
  • It should be noted here that the force caused by surface tension is too small to support the weight of the liquid and only keeps the water surface from breaking. What supports the weight is the force caused by the vertical pressure difference.
  • You may not be able to scoop well with even with objects other than a thick straw. For example, dissolving dishwasher detergent (the surface tension becomes weaker), applying a waterproof spray to the inside of the straw (repels water), and using an extremely long straw (at around 10 m, the pressure approaches that of a vacuum and water vaporizes) can make this not work.
[Keywords] Depth and pressure, surface tension
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Plastic bottle with holes open, Water that Won't Overflow

[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P38-41.
“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P18-19.

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Last Update:4.13.2021