understand the mechanism of capillarity

Ahmed Abdullah

I can't convince myself that I understand the mechanism of capillarity.
Say the liquid is of that type that rises through a capillary tube. So it's contact angle should be an acute angle.
Liquid molecules on the surface (in contact of gas) exert a force on the wall (of capilary tube) due to surface tension. According to newtons third law the wall also exert an equal and opposite force on the molecules in contact. These molecules are also attracted by a same force by their neibouring molecules. These two force should cancel each other, if so then we should not observe the rise...

the book on the table feels a reaction force upward due to its weight, but it does not rises up. Same idea. The molecules in contact with the wall are constantly under the force, it is exerting on the wall. The wall only does something to make things steady. But why there is a rise?

I am sure I am missing something or i have some fundamental misconceptions about the source of surface tension.........

Bystander

--- and, tow cables push cars, planes, ships; fishing lines push fish --- careful that you don't substitute intuition for Newton's third.

They do; this is what makes capillary rise useful for measuring surface tension.

Mass of fluid lifted above surface level outside the capillary provides the reaction force to the surface tension force lifting it in the capillary (or depressing it --- Hg in glass).

Ahmed Abdullah

Can you simply show how the net force over the liquid ( in a capillary tube) is 2*pi*rcos(theta)*T?

Bystander

If a rhetorical question: yes.

If a question of your understanding: what part of the expression do you not understand?

Ahmed Abdullah

For a rise the liquid must feel some net upward force.
It is said that 2*pi*rcos(theta)*T is the net upward force. But how? The liquid molecules in contact with the wall attract the wall. It is the tension force. The wall also attract the molecules ( it is the reaction force ). So the water molecules are attracted from every direction. So there is no imbalance of force. This part is bothering me.
Hope you understand the problem........

Ahmed Abdullah

Another question: Is there any difference between surface tension and intermolecular(between like and unlike molecules) attraction?
I think the surface tension is due to the extra attraction among surface molecules.

Bystander

At equilibrium there is zero net force; the upward force, from tension, equals the reaction force, mass of liquid displaced upward times g.

No. It is another way of describing the tension force. The reaction force is provided by gravity.

Equilibrium. Zero net force --- you hang your coat on a hook on the wall --- gravity pulls it down and the hook pulls it up --- zero net force, and it hangs there stationary. You hang the liquid on the wall of the capillary --- gravity pulls it down and surface tension pulls it up.