Capillary Action

dEdt

I'm not too sure if this is the right board, but it seems the most apt, so here it goes:

Question 1: If a column of water rises due to capillary action, its centre of mass must rise too, so where does the energy come from?

Question 2: Regarding the origin of capillary action; most diagrams I have seen have the surface tension pointing tangential to the surface and upwards, hence driving the column of water up. But shouldn't the surface tension point inward and tangential? If feel like it should because, appealing to the balloon analogy, the surface of the water should try and keep itself closer together, if that makes sense.

Thanks a lot.

Dr Lots-o'watts

Initially, electrical potential energy forces is greater than potential energy from gravity. The electrical potential energy comes from the attraction between the water molecules and between the water and the capillary medium.

Not sure what figures, what surface, and tangential to what? Needs clarification.

dEdt

Hopefully this clarifies what I mean. The diagram on the capillary action part shows what I mean. http://www3.interscience.wiley.com:8...88/ste/ste.pdf

Dr Lots-o'watts

Referring to figure 5, the vertical component of that F is electric based and it's what competes with gravity. The forces making the water stick to itself makes the meniscus, but it's that F from figure 5 that makes the water rise against the walls.

The meniscus happens because water attract water (inward and tangential).

The capillary action happens because water attracts the walls (tangential to the surface and upwards).

Ultimately, both are due to the dipole nature of water molecules which may originally point in any random direction, and that rearrange themselves according to the context (being stuck in a vertical tube).

dEdt

Wait, capillary action happens because water attracts glass (adhesivity)? I thought it was because of surface tension (as the article states)? Or am I misunderstanding what you wrote?

This is my understanding of what's happening: Adhesive forces between the water and glass is greater than the cohesive forces between the water molecules. This results in an upward miniscus. But doesn't surface tension consist of the forces between the water molecules on the surface? So shouldn't this force be downwards rather than upwards? Sorry, I'm having trouble visualizing this.

Dr Lots-o'watts

The article states:

"In this case, the adhesive forces are stronger than the cohesive forces, so that the water molecules are attracted to the glass more strongly than to each other. The result is that the water surface curves upward against the glass. It is said that the water “wets” the glass."

That is what causes the concave meniscus. Then :

"The surface tension leads to a force F acting on the circular boundary between the water and the glass. This force is oriented at an angle φ, which is determined by the competition between the cohesive and adhesive forces."

The top water molecules that are stuck against the glass and creeping upwards pull the inner ones through surface tension, this causes the inner water molecules to rise as well - capillary action.

In this analogy, the axe and boots are the adhesion force, and the rope is the surface tension :

http://www.generalcomics.com/funny-s...ng-cartoon.jpg

dEdt

Gotcha, thanks.