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Capillary Action (Capillary Depression)

  1. Dec 8, 2007 #1

    I have questions regarding capillary action and capillary depression for practical purposes. I'm fairly educated in physics and chemistry, but have some "what would happen if..." questions. Please help, and add any information you'd think helpful!

    1) Imagine you have your typical capillary pressure illustration: a glass cup with water in it and a hydrophilic tube sticking out of it. Capillary pressure drags the water up into the tube and above the water line of the cup. What are the main forces acting on the water in the tube?

    a) My answer: gravity pushing down, while the capillary pressure from the adhesion of the water to the walls of the tube pulling up.

    Then why, when you take away the base cup, does the water fall out the bottom? I understand the water in the cup pushes up, but it also pulls down from cohesion. I thought these forces would cancel.

    Now, imagine a hydrophilic capillary of changing radius, like "V" but turned upside down like ^. Now, put a drop of water in the middle of the capillary. We know the smaller the radius, the more pulling power. Is the drop just going to fall out without a base cup of water? Will it move up the walls towards the smaller radius?

    Imagine the same scenario, but the capillary is a regular V (not turned upside down) but is hydrophobic, so water needs pressure to get in. Drop a drop of water in the middle of the capillary. What happens, especially without a "base" of water anywhere?

    Can there be capillary action/depression without a "base cup" of water on the outside of the capillary?

    These questions are actually for fabric analysis of wicking and capillary depression.

    Thanks for any help.
  2. jcsd
  3. Dec 9, 2007 #2
    The capillary force has nothing to do with base cup. Only weight and adhesion involve.

    So: if you take out the cup, the water inside the capillary stilll holds.
    I'll come back the V expreriments later. Could you please draw it out?
  4. Dec 9, 2007 #3

    Sure, here are the illustrations, with a 3rd situation added. I think you may have answered my question when you said nothing happens when the cup falls out.


    In the first situation, the water is drawn all the way to the top.

    In the second situation, the water is pushed to the top and out.

    In the third situation, despite the water being so close to the bottom, because the pressure on the north edge is less than the pressure on the south edge, the drop is pushed all the way to the top and out.
    Last edited by a moderator: Apr 23, 2017
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