Bubble-ology

Now that you've had some fun playing with bubbles, let's talk a little about the science behind them (you knew that Dr. B would get to the science part, didn't you?)

Most of the bubbles we come across are made from soapy water. In "Floating Paper Clip" (August 28) and "Break the Tension" (August 31), we talked about surface tension. Just to remind you, we can think of water as having a "skin" because the molecules on the surface are more crowded together than the rest of the water molecules.

So, you might think that a bubble could form from pure water, with the surface of the bubble held together by surface tension. But you know that doesn't happen. In order to get bubbles, you also need something called a surfactant (short for surface-active agent), like soap. Surfactants are long molecules with particular properties. The chemical structure on one end makes it hydrophilic (water-loving). The other end is hydrophobic (water-fearing). You already know some other hydrophilic and hydrophobic substances. What happens if you put sugar in water? It dissolves - sugar loves water (it's hydrophilic). But if you put vegetable oil in water, they separate into two layers because the oil doesn't want to mix with the water (the oil is hydrophobic). Back to the surfactant - here's a molecule that doesn't know what to do in water! Should it mix or separate?? What's a poor surfactant to do?

Actually, surfactants are pretty clever. Since the molecules in a liquid can move around easily, they just rearrange themselves so that the hydrophilic parts (purple circles) and the water (blue ovals) are close together, with the hydrophobic parts (green lines) sticking out. The picture on the right gives you a simple picture of the structure of a bubble. Imagine lots and lots of surfactant and water molecules lined up like this. Then curve them around into a sphere, and you have a bubble. The purple circle - blue oval - purple circle part is the actual bubble. The green lines are stick out into the air inside and outside of the bubble. Bet you didn't think that bubbles were so complicated, did you?

Break the Tension!

In the last experiment, you learned about surface tension, which allows lightweight objects to float on the surface of water. Can we break that tension? Yes! And by using something you're very familiar with!

Soaps and detergents can actually break apart water's surface skin. Here's an easy experiment to try... First put a drinking glass somewhere that is safe if some water spills. You might want to place the glass on a few sheets of newspaper or some paper towels. Fill the glass of water very, very full with water (an eye dropper will help). When you look at it from the side, you should be able to see that the water surface is actually a little above the top of the glass. The top of the water is curved because of surface tension.

Now take a toothpick and dip one end into some dishwashing soap. Carefully touch the end of the toothpick to the water surface. What happens? Dr. B warned you that there would be a mess! The soap breaks up the close-packed molecules in the water's "skin", so they can no longer support the curved surface.

This is actually a good lead-in to September's topic, which is BUBBLES! You'll be experimenting with soap again!

Floating Paper Clip

The results of the "Ship Shape" experiment might be confusing to you. Clay is denser than water, and sinks when it's shaped into a ball. But if you make a boat shape, it floats! In this experiment, you'll see that even things that are the same shape sometimes float, and sometimes don't.

Drop a paper clip into a glass of water. The clip sinks. But now bend the end of the clip upwards so that you can use it as a handle. Carefully place the clip on the surface of the water, so that it floats (you may have to try a few times). Take a careful look at the water around the floating paper clip. Here's the puzzle: the density of the metal in the paper clip didn't change, and neither did the density of the water. So why does the clip sink in the first case and float in the second?

The answer is that the very top (surface) of water is different. The surface molecules crowd together, creating a "skin" that is actually denser than the rest of the water. This effect is called "surface tension". If you look at the water around the floating paper clip, you'll see that it doesn't look flat - the paper clip has caused the skin to stretch. Surface tension isn't a very strong effect, so it only helps lightweight things to float, like water bugs, leaves, or pieces of paper.