Why does soap make bubbles?

Published December 7, 2013 by teacher dahl


Two-sided soap molecules create soap bubbles:

Try an experiment: Pour some cooking oil into a bottle and then add some water. Notice how the liquids won’t mix. Even if you close the bottle and shake it, as soon as you stop shaking the oil floats to the top of the water. In other words, they don’t like to mix.
Why don’t the oil and water mix? Oil and Water are made of different types of molecules that are far too small to be seen. When you poured the molecules of water into the bottle, they settled to the bottom as a liquid. The molecules didn’t fly apart and fill the entire bottle because water molecules stick together. Likewise, the oil molecules also stick together. Different molecules have different tendencies to stick together. The oil and water did not mix. This shows that oil molecules do not like to stick to water molecules.

Soap will mix with both water and with oil. Why? The soap molecule has two different ends: One end likes to stick to water molecules, while the other end likes to stick to oil molecules. The oily end also likes to stick to air molecules.

If you mix water, soap, and air, the soap molecules line up side-by-side to make a layer. One side of this layer has the oily, air-liking ends sticking out and the other side is covered with the water-liking ends. The water molecules stick to the water side and the air to the other side. If you have a lot of air and a little water, which is what happens if you blow a lot of air into a soap-water mixture, then you wind up with a sandwich of three layers: Soap with the oily ends facing the air on either side and water in between. This is just the wall of a soap bubble. So the reason soap, water and air makes bubbles is because air and water molecules don’t like to stick together but will stick to different ends of soap molecules.

Why are soap bubbles round?

Let’s put it this way: You’d be pretty surprised if they were square, wouldn’t you?

Bubbles are round — spherical — because there is an attractive force called surface tension that pulls molecules of water into the tightest possible groupings. And the tightest possible grouping that any collection of particles can achieve is to pack together into a sphere. Of all possible shapes — cubes, pyramids, irregular chunks — a sphere has the smallest amount of outside area.

As soon as you release a bubble from your bubble pipe or from one of those more modern gadgets, surface tension makes the thin film of soapy water assume the smallest surface area that it can. It becomes a sphere. If you hadn’t deliberately trapped some air within it, the soapy water would continue to shrink down to a solid spherical droplet, as rain drops do.

But the air on the inside is pushing outward against the water film. All gases exert a pressure on their captors because they consist of freely flying molecules that are banging up against anything in their way. In a bubble, the inward surface-tension forces of the water film are exactly balanced by the outward-pushing pressure of the air inside. If they were any different, the bubble would either shrink or expand until they were equal.

Try to blow more air in to make a bigger bubble? That makes more air pressure inside. All that the water film can do to counterbalance the increased outward pressure is to expand its surface, making more inward-directed surface-tension forces. So it very cooperatively grows in size. But it must get thinner in the process, because there is only so much water to go around. If you keep blowing more air in, the film eventually won’t have enough reserve water to spread out into a bigger surface, and the ultimate catastrophe occurs: Your bubble bursts.

Exactly the same thing happens with bubble gum, except that instead of surface tension as the inward, contracting force, it’s the elasticity of the rubber in the gum. (Yes, rubber.) Elasticity, like surface tension, “means let’s always try to assume the smallest possible shape.”

source: whyzzz.com/ robert wolke.com

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