why does a liquid rise in a capillary tube

Capillarity, rise or depression of a
in a small passage such as a tube of small cross-sectional area, like the spaces between the fibres of a towel or the openings in a porous material. Capillarity is not limited to the vertical direction. is drawn into the fibres of a towel, no matter how the towel is oriented. Liquids that rise in small-bore tubes inserted into the liquid are said to wet the tube, whereas liquids that are depressed within thin tubes below the surface of the surrounding liquid do not wet the tube.


Water is a liquid that wets glass capillary tubes; is one that does not. When wetting does not occur, capillarity does not occur. Capillarity is the result of surface, or interfacial, forces. The rise of water in a thin tube inserted in water is caused by forces of attraction between the molecules of water and the glass walls and among the molecules of water themselves. These attractive forces just balance the of gravity of the column of water that has risen to a characteristic height.


The narrower the bore of the capillary tube, the higher the water rises. Mercury, conversely, is depressed to a greater degree, the narrower the bore. A liquid drop near the surface of a solid experiences a force from the liquid interior molecules as well as due to solid molecules near the liquid-solid interface. Forces due to gravity and air are neglected. In figure (a), the two vertical lines denote a solid and the right part to it is the liquid. The adhesive force between the solid and the liquid is greater than the cohesive force. $F_mathrm{S}$ is the force due to the attraction of solid molecules. $F_mathrm{I}$ is the force by the liquid molecules. $W$ is the weight of the small part of the liquid considered.


My book says that the resultant force $F$ is as shown and the liquid surface should be perpendicular to it. But I think that if the liquid has to rise upwards, there must a component of force in the upward direction initially to pull the liquid upwards and I don't see any.

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