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I was doing a physics activity (found online) which aimed to move water from one container to the other using a string. It involved the following steps:

  1. Wetting the string
  2. Attaching the string to the 2 containers with the help of a tape
  3. Filling one of the containers with water
  4. Making the string taut and tilting the container with water in such a way that it tries to flow through the string to the other container which is below the filled container container

OBSERVATION: Water traveled from filled container to the empty one through the string. I understand that due to cohesive force, water molecules present on the string attract the other water molecules which are escaping the container, and hence flow along the string.


MY QUESTION:

I observed a strange thing: Some water molecules tried to form a small curve (see image)Drawn using Paint - Approximate Figure and attempted to move along the string instead of falling down. Can I conclude that the cohesive force of water molecules is large enough to overcome the gravitational pull on them?

PS

Created the image of the water using Paint and editing a stock image.

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  • $\begingroup$ Here, I think the surface tension of water holds it from falling down. $\endgroup$
    – AlphaLife
    Commented Oct 8, 2017 at 16:31
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    $\begingroup$ You can see water molecules? I am quite sure there are water molecules all over the place in the air. IMO it is more helpful to view the water body as a continuum. But yes, it is the cohesion (surface tension) and adhesion to the surface, capilary action. $\endgroup$ Commented Oct 8, 2017 at 17:09

2 Answers 2

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You can indeed conclude that the cohesive force of water is large enough in your experiment to overcome the gravitational force on them.

This is how, even had you started the experiment with the string completely dry and not bothered to tilt the full container towards it, first of all the capillary action would have begun wetting an assumed dry string, and then as it became wet to the extent that the wet bit ended below the level of the water in the full container, only then would the gravity assistance create a siphon of the wet string and the full container would eventually be completely emptied.

I should declare a criterion which legitimises my confirming of your question: I did virtually the same experiment on two occasions years apart and achieved the same result. One time I wetted the string before starting the experiment; the other time I deliberately left it dry. Same result: one progresses to a completely wet string and then the gravity/siphon effect overcomes the capillary tendency to keep the water absorbed in the string.

And that last action I describe especially, as it does relate obliquely to your observation in your own experiment, confirms the veracity of your question, because gravity is demonstrated to be weaker than the cohesive force which is holding together (keeping homogenous?) a continuous column of water in the string, whether stretched tight and (apparently, according to the drawing) not so far from the horizontal in your case or, as in my experiments, let dangle loosely, vertically into the empty container.

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  • $\begingroup$ Thanks a lot for the answer.Is it possible to give a mathematical equation to the curve formed in the question given all the necessary constants and data or not? Just another question that came up to my mind. I tried to get an equation using basic fluid mechanics (using expressions similar to those in cohesive and adhesive forces) but without any success ? $\endgroup$
    – user36160
    Commented Oct 8, 2017 at 18:58
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    $\begingroup$ If I seemed a little "precious" by trying to validate my right to pose the answer as I did, above, it is because, in another stackexchange forum, I was just chided because I simply stated my answer, forgetting, as a "newby" on stackexchange, that perhaps I should have "presented my credentials" as it were, especially as in that other case they did not know that I wrote that answer from a knowledge base of four years of college study in the, relevant for them, fields of physics, engineering & metallurgy, but did not go for any degree at that time (half a century ago). Bit rusty now I suppose $\endgroup$
    – user171258
    Commented Oct 8, 2017 at 19:05
  • $\begingroup$ User36160 - Just saw the question in yr comment: it was half a century ago when I was a scholastic star in physics for some years. This is why my posts in this forum will be based on scholastic knowledge without much math, or my empirical research undertaken in intervening years. however I suggest that any relevant equation would be a complex one because of the many considerations in the specific case: cohesion, adhesion, gravity, the angular consideration of what the curve hangs off, the mass of water, and unsure if there's a further "capillary" variable specific to the material used. Sorry. $\endgroup$
    – user171258
    Commented Oct 8, 2017 at 19:21
  • $\begingroup$ You are right. If I get any expression after approximations I will share. Thanks again. $\endgroup$
    – user36160
    Commented Oct 8, 2017 at 19:23
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Yes, this is the same as capillary action, such as water being sucked up a wooden post.

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  • $\begingroup$ Isn't capillary action defined only when water is moving against force of gravitation or without gravity's help? In this case it is moving with support from gravitation since it is moving down $\endgroup$
    – user36160
    Commented Oct 8, 2017 at 16:53
  • $\begingroup$ @user36160 But it is not free-falling in the gravity, is it? $\endgroup$ Commented Oct 8, 2017 at 17:12
  • $\begingroup$ Not as I understand it, capillary action implies adhesive forces and surface action combine in limited spaces to induce a flow, irrespective of gravity. In this case the resultant movement is the result of 2 vectors, 1 from capillary action and the other from gravity. $\endgroup$ Commented Oct 8, 2017 at 17:13
  • $\begingroup$ @VladimirF Aren't the water molecules which are appearing to bend attempting to free fall due to gravity ?Not a gravity free space. $\endgroup$
    – user36160
    Commented Oct 8, 2017 at 19:01
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    $\begingroup$ @user36160 Do not think of molecules. It is not helpful at this scale, water is better described as a continuum. And yes, it is the point that they are trying to fall, but they don't fall. So there must be a force which prevents them from falling. That is the surface tension. $\endgroup$ Commented Oct 8, 2017 at 19:16

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