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This chapter is from the book

Racquets and Tennis Balls Made of Soap

Get ready for a new kind of tennis.

Step By Step

Supplies

  • circle, 4 3/4 in (12 cm) in diameter, made of wire or plastic, with a handle (frame)

  • tube from a ballpoint pen (without the ink cartridge) or a piece of drinking straw

  • soap solution from previous experiment

Submerge the wire frame in the solution. When pulling it out of the solution, you will see the film of soap fill up the whole circle. This is your racquet! Hold the pen tube by one end and dip the other into the solution for a few seconds. Pull the tube out and blow into the other end, forming a soap bubble. This is your tennis ball! Now you can practice your serve.

Fun Facts

You can play tennis in this new way thanks to the properties of certain molecules that are found in detergent, called surfactants. These molecules can be drawn as little tadpoles, each with a "head" and a "tail." If the head of the molecule loves water (for this reason, it is called hydrophilic), the tail hates it (called hydrophobic). From the illustrations, you can see that in the soap ball, as much as in the racquet, the tails of these detergent molecules are on the outside of the film of soap. The term surfactant comes from the fact that when a detergent molecule is put in water, as many as possible will crowd to the surface so that the heads can stay in the water while the tails stick out into the air. Surfactants therefore mostly affect the surface of water. It is these molecules that make soap bubbles stable. Without them, the bubbles would spontaneously break apart into tiny water droplets. The ball and racquet don't stick to each other because the "tails" of one want to avoid touching the water that is on the film of the other! In reality, the surfactant molecules are much smaller than as illustrated here, about 1,000 times smaller than the thickness of the bubble wall. Bubble walls are typically a few micrometers thick (the diameter of a human hair is around 50 to 100 microns, 1 micron = 10-6 m), while the molecules are just a few tens of nanometers long (1 nm = 10-9 m).

As you can see in the picture, there are also detergent molecules inside the water of the bubble wall, some of them by themselves and others in little balls known as micelles. In the micelles, many molecules ball up with their tails bunched up inside the ball and their heads on the ball surface. The tails thus avoid water, while the heads get to touch it. This is a handy way of storing soap inside the bubble wall. If the bubble is stretched, the molecules on the surface spread apart as the surface is enlarged. The additional molecules stored in micelles replenish the surface, so it remains nice and stable. When you use detergent with water to remove dirt from clothes and dishes, for example, the tails of the detergent molecules stick to the dirt, which tends to be greasy or oily, while their heads prefer the water. So, when you rinse the dish you are washing, the water takes the dirt and grease along with it, and the dish becomes clean.

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