How do paper towels soak up water so quickly? asks a reader.
We’ve all seen water rush into a paper towel, as if toward a long-lost friend. And in fact, it is the sweet nature of paper towels that leads to the instant attraction. Paper towels are really perforated sheets of sugar molecules.
But if you’re tempted to take a bite out of your Bounty, stop right there. The sugar molecules are linked up into giant cellulose molecules. The average cow or goat can digest cellulose, releasing the carb-y goodness. But we humans can’t. To us, cellulose is fiber, good for making us feel fuller and moving things through the intestinal tract. (And now you know why goats happily chew random bits of paper. To Bill the Goat, it’s a delightful snack.)
Cellulose is the carbohydrate that most plants are made of, and most paper towels are made from trees. Cellulose fibers are composed of giant molecules called polymers. The individual sugar (glucose) molecules in the polymer chain are called monomers. And it’s the attraction between sugar molecules and water molecules that makes paper towels so thirsty.
We see this attraction every day. When we stir sugar into tea or coffee, it dissolves, forming a sugar-water solution. Why? Water is H2O, two atoms of hydrogen attached to one of oxygen. The hydrogen end of the molecule has a weak positive charge; the oxygen end has a weak negative charge.
Meanwhile, cellulose fibers also have linked-up oxygen and hydrogen atoms on the outer edges of their sugar molecules. Since electrical opposites attract, the positive end of the water molecule is attracted to the negative part of the sugar molecule, and vice versa. So when you lay a paper towel over a puddle of liquid, water quickly seeps into the sheet, its molecules hooking up to the molecules in the cellulose fibers. Presto: a quicker picker-upper.
The way water is pulled into a paper towel is called capillary action. (It’s the same way a tree or other plant absorbs water, wicking it up from the roots.) Water molecules are more attracted to the cellulose molecules than they are to each other. So instead of staying put, they rush in to fill the empty spaces between cellulose fibers.
As the paper towel wicks up water, it becomes heavier and heavier. When the force of the upward pull is balanced out by the weight of the water – the force of gravity — the capillary action stops. And you’ve got a sopping-wet sheet of paper.
In May, MIT researchers announced they’ve created a kind of “paper towel” made from impossibly tiny metal wires. The “nanowires,” only billionths of a meter wide, are woven into a mesh that repels water, but wicks up oil. A membrane made from the mesh can absorb up to 20 times its weight in oil, and is designed to be used to clean up oil spills in the ocean.
For more on capillary action, including a simple experiment you can do at your restaurant table, visit www.physicscentral.com/experiment/physicsathome/mealtime.cfm.
