Where Does Water Come From?

When we add a capillary in water the water rises, gaining potential energy. where does this energy come from?

• what is the reason for the rise in level of water in capillary and from where does it get its potential energy which is it gains because of increase in height of water?

• Answer:

  It's a one-shot increase, as you probably know. That is, you can't make a perpetual pump by letting the water flow out from the capillary tube through a hole drilled somewhere below the top of the water column. Like other one-shots that seem to be a free ride, such as tossing rocks into the Grand Canyon, the capillary effect can be viewed as trading one form of energy for another. The input energy in this case is the potential energy of the empty capillary. It takes energy to drive the water back out of the capillary. Surface tension acts like a stretched elastic membrane. It is always trying to minimize its free surface area (that is, surface that is not contacting and wetting some solid). If you have a number of small droplets on a non-wettable surface like waxed paper, and you bring them all together, they form a single drop whose center of mass is higher than when they were all separate. The energy to do this comes from the elastic contraction of the water surface. Note that surface tension that forms drops produces a convex free surface. Water in a capillary wets the walls of the tube above the water level by adhesion, and surface tension tries to minimize the surface area by pulling the water column up to flatten the upper surface (the meniscus). Eventually an equilibrium is reached where the surface tension balances the weight of the water column, which stretches the surface downward making a concave meniscus. See the ref. for a fuller explanation of surface tension.