¡La gravedad no da miedo! Los “microescaladores” que hacen que la esponja absorba agua
Soy Ken Kuwako, Science Trainer. Every day is an experiment!
When you quickly wipe up spilled water in the kitchen with a cloth, or use a sponge to soak up water after a bath, you’re doing something so ordinary that you probably never give it a second thought. But surprisingly, something rather strange is happening: the water is moving against gravity. For example, try dipping a piece of kitchen paper into a cup of tea and you’ll see the liquid smoothly climb upward. Why does this happen?
Let’s take a closer look at the amazing mechanism that allows sponges and cloths to soak up water.
A Porous Structure That Draws Water In
Take a close look at a sponge and you’ll notice that it’s full of tiny holes. This kind of structure is called “porous.” Inside a sponge is a jungle-like maze of tiny tubes and narrow gaps, all tangled together. These countless “tiny spaces” are the key to pulling water upward.
Capillary Action
The main reason kitchen paper, cloths, and even sponges can suck up water is a phenomenon called “capillary action.” It occurs when a liquid rises through a very narrow tube or space, seemingly defying gravity.
Two forces play a major role here.
- Adhesion: The force that makes water molecules stick to the walls of the sponge.
- Surface tension (cohesion): The force that makes water molecules hold tightly onto one another and stay together as a group.
When these two forces work together, water can make its way deep into narrow spaces all by itself. In fact, the narrower the gap, the stronger the capillary action becomes.
Adhesion and Surface Tension
So how can water actually “climb” up the walls of a sponge or a glass tube? The secret lies in a structure found on the surface of molecules called the “-OH (hydroxyl group).” Cellulose, the main component of cloth and natural sponges, contains countless -OH groups. These -OH groups have a strong electrical imbalance, or polarity, and they attract polar water molecules as if saying, “Come on over!”
When people hear “capillary action,” they may picture water climbing up a thin glass tube rather than being sucked into a paper towel. Why does that happen? Surprisingly, even the seemingly smooth surface of glass contains structures called “silanol groups (Si-OH),” which form when the glass surface reacts with moisture in the air. When a glass tube is placed in water, the water level rises because the following steps happen over and over at high speed:
- Climbing the wall (adhesion): The -OH groups on the wall attract water molecules, causing the water to climb upward.
- Surface tension pulls more water upward (cohesion): Water molecules stuck to the wall pull their neighbors upward, as if saying, “Come along with me!”
The water keeps climbing until the upward force is balanced by the weight of the water itself.
The “Hidden Force” That Makes Oil Climb Up Paper
But doesn’t this make you wonder: “Why can oil be absorbed by kitchen paper even though it doesn’t have the kind of polarity that water has? Can oil really be wiped up in the same way as water?”
Oil doesn’t have the kind of hydrogen bonding (strong intermolecular attraction) found in water, but there is another, gentler attractive force at work in the molecular world: van der Waals forces. These are the “subtle forces that make neighboring molecules snuggle up together” and act between all kinds of molecules. Cellulose, the main component of kitchen paper, is made largely of carbon and hydrogen, and oil molecules are also largely made of carbon and hydrogen. Thanks to van der Waals forces, the two can actually stick to each other reasonably well.
In the case of oil, its surface tension is much weaker than that of water. As a result, oil has a strong tendency to spread out rather than stay clumped together with its fellow molecules. Thanks to this “I’ll spread wherever there’s space!” tendency, oil can slip into the tiny gaps between the fibers of kitchen paper.
However, as you may have noticed when comparing the two, oil is much harder to wipe up than water when you spill it. Without the strong polarity that water has, oil tends to remain greasy and sticky, and its ability to be drawn upward is much weaker.
Once an oil molecule takes its first step into the tiny spaces between the fibers, “capillary action” takes over. Kitchen paper is essentially a super-microscopic jungle made of countless fine fibers tangled together. Because the gaps are incredibly narrow, even a relatively weak adhesive force such as van der Waals force can be enough to pull liquid into these confined spaces.
Capillary action in water can even create frost columns! Check out this article as well.
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