How and Why of Soap Bubbles

Bubbles are beautiful, fun, and fascinating, there is a  lot of science behind bubbles.

We can see bubbles in plain water. But these bubbles will be in water, or floating on the surface of water. But they do not float through air. We can see bubbles when soda pops. These bubbles are carbon dioxide gas in the liquid form. We can find bubbles in many other liquids also.

When soap and water are mixed together and air is blown into the mixture, the soap forms a thin skin or wall and traps the air, creating a bubble. The special thing about soap bubbles is that they can float freely in the air. They will not be clinging or adhering to water.

On the surface of water, the molecules of water exert an attractive force on each other to keep water molecules intact. This force is called Inter Molecular Force. The attractive force is exerted upon the surface molecules of water by the molecules beneath the water. This tends to draw the surface molecules into the bulk of the liquid and makes the liquid assume the shape of the vessel having the least surface area. This is called is Surface Tension.  Surface Tension is the force that causes the molecules on the surface of a liquid to be pushed together to form a layer.

Soap helps us to make bubbles out of water. Soap makes the surface tension of water weaker than the normal. It also forms a very thin skin that is more flexible than water. When air gets trapped under the surface of the mixture of soap and water, the flexible skin stretches into a sphere shape (round like a ball), making a bubble! We can see the flexible skin that forms a bubble by dipping our hand into bubble solution. When we pull our hand the hole will be filled with a stretchable skin of liquid. If we blow gently on the flexible skin, then we will blow a bubble!

A soap bubble has three layers. A thin layer of water is sandwiched between two layers of soap molecules. Each soap molecule is oriented so that its polar (hydrophilic) head faces the water. The tail end of the bulb extends away from water.

Bubbles of soap are made from soap and water. They can only last as long as the water lasts. In dry air, water evaporates. When   the soap bubbles come in contact with dry air the skin of the bubble gets thinner and thinner until it finally pops! Evaporation isn't the only thing that pops bubbles. Anything dry can pop them. When a bubble floats through the air it can land anywhere.

 It can land on a blade of dry grass, on the wall of the house, or on our dress. When something sharp and dry touches the bubble, it pokes a hole in the bubble's skin, all the air goes out of it, and the bubble disappears!

A soap bubble may have any shape when it starts. But it becomes a sphere or round. Spherical shape minimises the surface area of the structure to hold lot of air.

When two bubbles meet, they will merge walls to minimize their surface area. If bubbles that are the same size meet, then the wall that separates them will become  flat. If bubbles that are different sizes meet, then the smaller bubble will bulge into the larger bubble. Bubbles meet to form walls at an angle of 120°. If enough bubbles meet, the cells will form hexagons.

Most bubble solution is made from soap. Soap bubbles can also be made from dissolving detergents in water. But detergents will form bubbles even in tap water, which contains ions that could prevent soap bubble formation. Soap contains Carboxylate group while detergents lacks. Addition of little glycerine extends the life of a bubble by forming weaker hydrogen bonds with water, slowing down its evaporation.

  

The colours we see in bubbles are caused by light that is reflected off the walls of the bubble. When light is reflected, light touches the outside of the bubble and bounces. When the light reaches our eyes we see colours. As the surface of a bubble gets thinner, we will see different colours.

When we first blow a bubble, we will probably see green and blue, then magenta or purple, and then when the bubble pops, it will be a dark golden yellow colour, or almost black.

Sometimes the surface of the bubble is very thin. When that happens, we will see lots of different colours on the bubble at once making it look like a rainbow!

How? It is because of constructive and destructive interference of light rays.

Toss two pebbles at the same time in a very calm lake. We see ripples spreading out in circles. The two pebbles will hit the water at the same time, a few feet apart. We will get two sets of ripples rippling toward each other. When the ripples meet they overlap. Some of the ripples add together, some cancel out. This is called interference. When the ripples add, it's constructive interference; when they cancel out (or subtract), it's destructive interference. The result of interference is a brand new set of ripples quite different from the ones that both the stones make.

Interference can also happen with light wave when they hit a bubble. Some of the light rays bounce straight back of the outer part of the soap film. Others carry on and then bounce on the inner part of the film. So one set of light rays shine into a soap bubble, but two sets of rays come back again. When they emerge, the waves that bounce off the inner film have travelled a bit further than the waves that bounced off the outer film. So we have two sets of light waves that are now slightly out of step. Like two sets of ripples on a pond, these waves start merging. Just like on a pond, some add together and some cancel out. The overall effect is that some of the colours in the original white light disappear altogether, leaving other colours behind. These are the colours you see in soap bubbles.

If we look at any one soap bubble and we will notice that the colours vary across its surface from place to place and they also gradually change with time until the bubble bursts. The soap film is not quite the same thickness all over. When the soap film is thick, red light is cancelled out leaving the bubble looking blue or green. When the film is thinner, green is cancelled, leaving the film magenta. If you blow on the film, the soap solution starts to evaporate and the bubble gets thinner. If you blow gently enough, you can make the colours change slowly from blue or green to yellow and violet, in the exact order you see them in a rainbow: red-orange-yellow-green-blue-indigo-violet.