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I've learnt that capillary action is caused by forces acting on molecules near the interface between solids, liquids (and gasses as well). But there are few things I don't understand.

Imagine we have water in a glass container as shown in the picture. Why would molecules near the container rise if the net force acting on them have no more "upwards vector" than let's say molecules in the middle? I can see that the vector of the net force is pointing more towards the container than vectors of the net forces of molecules in the middle, but it is not pushing it upwards!...

capillary action

My second questions is, why does the surface level in a small tube rises the higher the smaller the diameter of the tube is (due to capillary action)?. I think it is because smaller diameter means that a higher percentage of the molecules are being affected by the upward forces and the net force F = F(gravity)-F(capillary) has "more up-direction".

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  • $\begingroup$ Across a meniscus the pressure increases by 2Tcos(Ω)/R so now if the radius or diameter of tube is smaller then we need more liquid in the tube to increase the column height so as to balance this reduction in pressure $\endgroup$ – Aditya Garg Apr 4 '19 at 20:00
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First- the molecules of liquid that form the free surface in contact with air are special: they have only half as many nearest neighbors with which to share attractive forces as do the molecules in the interior of the liquid. This means they interact more strongly with any solid surface nearby- and if that interaction is attractive, then the liquid will tend to climb up the surface against gravity.

Next- the surface tension effect which pulls liquid up a small-diameter tube has to pull more liquid up behind it as it climbs. Gravity pulls back on the mass of that liquid. as shown by Aditya Garg, the pressure difference created at the surface of the meniscus (which is what pulls the liquid up) has the radius of the meniscus in its denominator. small radius = large pressure difference = more pull = higher climb.

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