I’ve seen photos of water going up pretty high in rocks and stones from capillary action. Why doesn’t the same happen for dirt/soil/sand?
I’ve seen photos of water going up pretty high in rocks and stones from capillary action. Why doesn’t the same happen for dirt/soil/sand?
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Capillary action depends on the “sticky” force of surface tension exceeding the downward pull of gravity. When the balance is on gravity’s side, it’s just as you expect, and the liquid stays where it is. When the balance is on surface tension’s side, the liquid rises.
And it’s not strictly one or the other either—capillary action won’t keep the liquid rising infinitely. At some point, the weight of the liquid will stop it, as it reaches the point of equilibrium.
As the name implies, capillary action generally works in narrow channels with lots of walls to stick to. The narrow pores in stone fit this description.
But dirt and sand are too loose, and gravity wins.
Capillary action is the movement of a liquid towards solid surfaces. In its mildest form, it just creates a small meniscus.
You can see here that the liquid cannot crawl up forever. In fact, in this example, it barely crawls up at all. So, what stops it?
The dominant force in a glass of water is, of course, gravity. For water to climb the walls, it must somehow gain enough energy to lift itself against gravity, and the higher it climbs the more energy that will be. The attraction of the water molecules to a surface can only provide a limited amount of energy, and this corresponds to the maximum height that water can climb above the overall waterline.
You can use different materials and higher microscopic surface area to raise this energy limit, but it will always exist. Dirt is not exactly an ideal wick, and so water cannot climb it very far above the waterline.