Understanding the mechanism of capillarity

1. Nov 14, 2007

Ahmed Abdullah

I can't convince myself that I understand the mechanism of capillarity- the rise and the drop of liquid level.
For conveniene I am just talking about liquids that rises up liquids that form acute contact angle.
Now 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 some are exactly behind. For every pull forward there is a molecule behind to counterbalance. I mean 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?

Another example; suppose a car is decending from an inclined plane(friction free) with an inclination of (90-θ). It is connected with a wall standing upright with a rope. The car is decending and the wall is on the top. Suppose the angle between the wall and rope is an acute angle θ and the pull of the car is F(including mgsinθ). So accordingly the car feels an upward pull Fcosθ due to newton's third law. But we don't observe the car rising up or even a sign of it, to be more precise the car don't even feel a net upward force for a single moment. And this result isn't surpring. But the result of tension force goes on suprising me!

Now my question is what is the difference between the pull of the car on the wall (due to gravitation and car engine) and the pull on the wall by liquids due to surface tension.

The first case estabilishes an equilibrium the latter gives rise to a rise.

Last edited: Nov 14, 2007