why the water goes up if I hold a paper towel up but touching it on a plate? What force is pushing/pulling the water against gravity?
In: Physics
It’s called [capillary action](https://en.wikipedia.org/wiki/Capillary_action), and it happens because water molecules are attracted to each other and to other molecules. This is why water forms a [meniscus](https://en.wikipedia.org/wiki/Meniscus_(liquid)) when poured into a glass. In a sufficiently narrow tube or a porous material, these intermolecular forces are strong enough to draw the water up against gravity. This is also what allows trees to transport water from their roots to their leaves.
It’s called capillary action. It takes advantage of two properties of water– Cohesion: water likes to stick to itself. Adhesion: water likes to stick to other things like paper towel fibers. These two things work together to overcome the force of gravity and bring the water upward through the tiny gaps between the fibers.
Water molecules are actually rather special. They display a slightly polar aspect like a really weak magnet. So water molecules will pull on their neighbors ever so slightly by a method called “hydrogen bonds”. These hydrogen bonds also allow water to cling to surfaces as water makes it into the tiny microscopic cracks and crevices on most surfaces. Which in turn holds up there neighbors.
This “stickiness” for lack of a better term is what give water it’s unique behavior. The one you are describing is known as capillary action. When pulled into a small space like the weave of a paper towel, water will pull a chain of other water molecules behind it. These molecules will push on the one in front of them and keep pulling more in until the force of gravity overcomes the force between the molecules of water.
This is called capillary action.
Have you noticed how water forms drops instead of breaking apart? This is called cohesion.
Have you noticed how water stick to things like the edge of a glass or thr paper towel itself? This is called adhesion.
Both of these working together causes capillary action. The water touches the paper and sticks to it via adhesion. This pulls up the water behind it via cohesion. The surface of the water at the top is able to then touch new paper and repeat the whole process.
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