Gravity? No Problem! Meet the “Micro Mountaineers” Inside Your Sponge
I’m Ken Kuwako, your Science Trainer. Every day is an experiment.
Wiping up spilled water in the kitchen with a cloth, or soaking up water with a sponge after taking a bath—these everyday actions actually involve something rather surprising: water moving against gravity. Try dipping a piece of kitchen paper into a cup of tea, for example, and you’ll see the liquid mysteriously creeping 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 in Water
Take a close look at a sponge and you’ll see that it’s filled with countless tiny holes. A structure like this is called “porous.” Inside a sponge is a tangled maze of tiny channels and gaps, almost like a miniature jungle. These countless “tiny spaces” are the key to its ability to draw water upward.
Capillary Action
The main reason kitchen paper, cloths, and even sponges can pull water upward is a phenomenon called “capillary action.” When a narrow tube or passage comes into contact with a liquid, the liquid can creep upward, seemingly defying gravity.
Two forces play a major role here.
- Adhesion: The force that causes water molecules to 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 naturally makes its way deeper and deeper into narrow spaces. In fact, the narrower the gap, the stronger the capillary force becomes.
Adhesion and Surface Tension
So how can water actually “climb” the walls of a sponge or a glass tube? The secret lies in a structure called the “-OH (hydroxyl group)” found on the surface of molecules. Cellulose, the main component of cloth and natural sponges, contains countless -OH groups. These -OH groups have a strong electrical polarity, which means they effectively beckon to polar water molecules, saying, “Come on over!”
When people hear “capillary action,” they often picture water rising through a narrow glass tube rather than 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 interacts with moisture in the air. When a glass tube is placed in water, the water level rises because the following steps are repeated incredibly quickly:
- Climbing the wall (adhesion): The -OH groups on the wall attract water molecules, causing the water to climb upward along the surface.
- Surface tension pulls the water upward (cohesion): Water molecules attached 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.
The “Hidden Force” That Makes Oil Climb Paper
But here’s an interesting question: If oil doesn’t have the same kind of polarity as water, why can it still be absorbed by kitchen paper? How can we wipe up oil in much the same way as we wipe up water?
Oil doesn’t have water’s powerful hydrogen bonds, but the molecular world has another, much more subtle attractive force: van der Waals forces. These are essentially a “gentle tendency for neighboring molecules to snuggle up together” that acts between all kinds of molecules. Cellulose, the main component of kitchen paper, is made largely of carbon and hydrogen, while oil molecules are also largely made of carbon and hydrogen. Thanks to van der Waals forces, the two can actually stick together 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 stick together with its fellow molecules. This eagerness to spread means that when oil encounters the fibers of kitchen paper, it readily slips into the tiny spaces between them.
Still, if you compare oil with water, you’ll notice that oil is harder to wipe up after a spill. Because it lacks water’s strong polarity, it tends to feel greasy and sticky, and its ability to be drawn upward by capillary action is much weaker.
Once an oil molecule gets its foot into the tiny spaces between the fibers, however, the “capillary action” we’ve been learning about takes over. Kitchen paper is an “ultra-fine jungle” made of extremely thin fibers tangled together in a complex network. Because the gaps are so incredibly narrow, even relatively weak adhesive forces such as van der Waals forces can provide enough pull to draw liquid into these confined spaces.
Capillary action in water can even create frost columns! Check out this article as well.
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