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 arou
nd 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?
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