This Week's Experiment - #467 Towels


The idea for this week's experiment came from a water leak. We woke up to a lot of water and wet floors. The clean up gave me plenty of time to think about how a towel works, and why it soaks up water. To explore this, you will need:

a glass of water or your favorite drink
2 soda straws
Thin straw or coffee stirrer
a paper towel

Fill the glass with some tasty liquid and put one of the straws into it. Before you take a drink, look closely at the straw. Even if the straw is not clear, you should be able to see that the liquid has risen slightly up into the straw.

This is called capillary action. Water is very sticky stuff. When it comes in contact with the straw, the water molecules stick to the plastic. They are attracted so strongly that the water climbs slightly up the surface. How high the water will climb is controlled by several things.

First, it is controlled by the material. There are some substances, such as oil and wax, that water does not stick to, so the water will not climb up their surface. A towel made of wax would not work very well.

Second, to do much climbing, the water needs a small space. Notice that the water climbs the inside of the straw, but does not go nearly as high on the outside. Is the outside of the straw made of something different? No. Put the second straw into the glass, beside the first straw. Place the straws side by side, and then move them very slightly apart. You should see that the water has now climbed up between the two straws. Why?

On the outside of the straw, the water is only sticking on one side, the side towards the straw. All the water on the side away from the straw is being held up by the attraction on the other side. The water will climb until the attraction of the water to the plastic is balanced by the downwards pull of gravity. With the two straws, you have water sticking to the plastic on two sides. Twice as much support holds up twice as much weight, which lets the water climb higher. Inside the straw, the water is surrounded, sticking on all sides, so it climbs even higher.

Is there a way to make the water climb even higher? Yes. You could make the straw smaller. If you can find a very thin straw, or one of the straw-like coffee stirrers that they give at fast food places, compare a thin straw with a thick one. You will see that the thinner the straw is, the higher the water will rise. Why does the water climb higher in a thin straw? The water in the center is being supported by sticking to the water that is sticking to the plastic. A smaller straw means that there is less water in the middle to be supported, so the water can climb higher.

So for a towel to work well, it needs to be made from a substance that water sticks to very well. It also needs to have lots of tiny spaces for the water to climb into. The fibers in the threads of the towel serve that purpose.

Now, think back to the straws. Which do you think would support more water, one large straw or two small ones? Right. Two small straws would have more surface area for the water to stick to, so they would support more water. A towel that has many, very thin threads would work better than one with fewer, thicker threads. That is the idea behind the new, microfiber towels. Lots of very small fibers work very well at soaking up lots of water.

Now, you can finish your drink and think about how towels work, without the need for a major water leak to get you started.

Have a wonder filled week.