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Water not flow out of capillary

  1. Dec 1, 2012 #1
    Why does water, in a small glass tube,open at both sides(for example) not flow out if we put it in vertical position and put our finger on the top?(we close it)?
     
  2. jcsd
  3. Dec 1, 2012 #2

    HallsofIvy

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    Surface tension. The force binding the surface molecules is greater than the weight of the thin coumn of liquid.
     
  4. Dec 1, 2012 #3
    Ok, but why does than have our finger on the top any influence on that?

    And why does water not flow out of the pipette, is there the same reason?
     
  5. Dec 1, 2012 #4
    Are you thinking more of a drinking straw? The reason why water doesn't come out when you place your finger over the top of the filled straw is that the air pressure acting on the bottom surface of the water results in an equal (and opposite) force to the weight of the water.

    Since you essentially have a vacuum over the top of the column (no air pressure, or very little, acting on the top of the column) the water column has no net forces acting on it.

    edit: friction plays a part, too, and is partly contributing to the water column staying still.
     
    Last edited: Dec 1, 2012
  6. Dec 1, 2012 #5

    K^2

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    If a capillary is really thin and walls are hydrophilic, the water will stay inside even if you don't have your finger on it.

    If the capillary is a bit thicker and/or the walls are hydrophobic instead, then surface tension won't actually hold the fluid to the capillary. But it will prevent fluid from breaking up into droplets or allowing bubbles to form. That means, entire column of liquid must move as one. That means, additional air can't get in between the closed end and the liquid. Pressure at the top drops, and atmospheric pressure from bellow holds the liquid in. If you remove your finger, pressure equalizes, and liquid is released.

    And yes, pipette works the same way. It relies on ambient pressure forcing or holding liquid in the pipette.

    (Some of this is redundant to what DocZaius has said, but it's important to point out that surface tension plays a role either way.)
     
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