Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the paper. This climbing up is intriguing me. Where is the energy coming from?
Supposedly, a lot of water molecules are moving up and gaining potential energy. This has to be balanced by something. I can think of a few possibilities, but I can't tell which one it is.
- When the water molecules dilute into the paper, they are at a state of lower potential binding energy. Some molecular interaction (van der Waals?) is at lower energy when you have a water+paper solution, compared to a water-only solution. This compensates the gain in gravitational energy.
- The surface between the paper and the water is at lower pressure than the atmosphere. This causes the water to be pushed into the paper by the atmospheric pressure, up until the point that the column of water above the surface is heavy enough to counterbalance. The potential energy would be a result of work done by the atmosphere.
- Some water molecules climb up randomly and loose kinetic energy going up, and somehow get "stuck" up there.
- Something else?
Best Answer
The surface of any fluid has an associated energy-per-unit-area, known as the surface energy, a.k.a. surface tension. This energy is not a property of the fluid alone, but of the fluid and the medium it is in contact with.
In your case you would have associated surface energies for the water-air interface, $e_{wa}$, as well as for the water-paper interface, $e_{wp}$. The total energy of the fluid in a configuration is the sum of the potential energy, plus the product of the corresponding surface energies by their respective surface areas, $S_{wa}e_{wa} +S_{wp}e_{wp}$.
So if you want to look at it in a purely energy balance point of view, the increase in potential energy of the water climbing up the paper is compensated by a reduction of the total surface energy. When capillary action makes things rise, it is because the liquid-solid energy is lower than the liquid-air energy. By wicking into the porous material, the solid-liquid contact area is increased at the expense of the liquid-air one, resulting in an overall reduction of contact energy, which is what drives the rise in potential energy.