Why and how do towels absorb liquids?
In: Chemistry
Some materials like to be wet. These materials are called “hydrophillic” (water loving). When the dry surface of a hydrophilic material is wetted with water, the surface of the material actually pulls on the water so that the water spreads out over the surface of the material. Your skin, for example, is mildly hydrophilic. You’ll notice that drops of water stick to your skin, and some drops hang on even when gravity is pulling them down. Other materials are hydrophobic (water hating). Water doesn’t stick very well to hydrophobic materials: it tends to bead up and roll off the surface. Towels are made from a cloth called terrycloth, which is usually woven from cotton threads. Cotton consists mostly of cellulose, which is basically a plastic, except it’s natural. Cellulose is a little more hydrophilic than your skin, but a towel can absorb a lot more water than the surface of your skin can hold. Why? It’s because of the structure. Your skin is mostly smooth, but cotton thread consist of hundreds of super thin fibers, all lined up. Having so many thin fibers so close together means that there’s a lot of surface area in a small volume. Remember that the surface of a hydrophilic material pulls on the water when it gets wet, spreading the water over the surface. Having so much dry surface area close together in a small volume means the threads have a strong pull on water, and they pull the water along the length of the thread from the wet part to the dry part. A dry thread can lift water against the pull of gravity, and it can pull water away from your wet skin. Also, terrycloth has lots of thread, which means it can hold a lot of water. There are little loops of thread sticking out both sides of terrycloth, making it thicker and heavier than regular woven cloth. Once a thread is wet, it loses its attraction for water. That’s why a wet towel won’t dry you off.
Incidentally, having a lot of surface area in a small volume is the same reason that a glass capillary tube or a glass fiber wick can pull water up higher than a glass sheet does. It’s the reason paper towel (which is made from wood fibers) holds more water than the smoothed surface of a piece of wood. It’s why sand (which is tiny rocks) holds more water than the surfaces of big rocks. It’s why microfiber cloth is more absorbent than other kinds of polyester cloth.
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