Looking at the way in which water is held inside a rigid, hard container such as a bowl or a cup, it can be baffling to try to figure out how a porous, soft object like a sponge, cloth or disposable paper towel, can absorb and hold water. A paper towel manages to hold water based on a different scientific principle than those that hold water in a cup, one that works because of the towel's supple shape and many tiny holes, which create surface tension.
If you look closely at the surface of a paper towel, you'll see that it's full of tiny pores and holes, somewhat imitating a sponge. In fact, many super-absorbent towels are designed to be more sponge-like in their fibers and construction than cloth weaving, because by imitating the sponge's shape, the towel can have the same absorbent power.
These many tiny holes and gaps between the towel's fibers can hold water due to surface tension, also known as wicking action or capillary action. Capillary action is the small amount of elasticity that naturally occurs between molecules of water, holding them together.
Every little space in the surface of the paper towel has its own tiny "bubble" of surface tension. These bubbles are formed when the towel comes into contact with liquid because the liquid in each tiny space is kept separate from the liquid in other pores and pockets. This allows the bubbles of liquid to be held in place and even sucked upward because each pocket contains so little water that the intermolecular attraction is stronger than the gravitational pull on the liquid.
If you wring out a paper towel, the water will be expelled. This is because the act of squeezing the towel breaks the surface tension of the pockets be compressing the spaces, forcing the liquid molecules to flow together and become weighted down by gravity.
- Mets501, Wikimedia Commons