Soap bubble

A soap bubble is a thin film of soap water that forms a hollow spherical shape with an iridescent surface. They usually last only for a few moments until they burst either on their own or when coming in contact with another object. Due to their fragile nature they also became a synonym for something that is attractive, yet very insubstantial.
They are mostly used as a toy for children, but their usage in artistic performances show how fascinating they can be even for adults.
Soap bubbles can also help to solve complex mathematical problems of space, as they will always find the smallest surface area between points or edges.

Table of contents

1 Physics 1.1 Surface tension 1.2 Spherical shape 1.3 Merging bubbles 1.4 Interference and reflection 2 How to make soap bubbles 2.5 Additives: 2.6 Procedure: 2.7 Bubble blowers: 2.8 Here are some sample formulas: 3 Usage 3.9 Soap bubble performances 3.10 Soap bubbles and maths 4 External links and further reading

Physics

Surface tension

Soap bubbles can exist because the surface layer of a liquid - in this case water - has a certain surface tension, which causes the layer to behave as an elastic sheet. A common misconception is that soap increases the water's surface tension. Actually soap does the exact opposite of decreasing it to approximately one third the surface tension of water. It is so hard to make bubbles with clear water because the surface tension of water is actually too high, causing the bubble to pop instantly. Additionally, the soap reduces evaporation so the bubbles last longer.

Spherical shape

Their spherical shape is also caused by surface tension. The tension forces the bubble to form a sphere, as a sphere has the smallest possible surface area for a given volume. In the absence of gravity, all bubbles, like water drops as an example, would form a sphere, but subjected to gravity they are usually more conically shaped. For soap bubbles, however, gravity is negligible as their weight is minimal, so that they form a - nearly - perfect sphere.

Merging bubbles

When two bubbles merge, the same physical principles take place, and the bubbles will adopt the shape with the smallest possible surface area. Their common wall will bulge into the larger bubble, as smaller bubbles have a higher internal pressure. If the bubbles are of equal size, the wall will be flat.
At a point where multiple bubbles meet, they do so at equal angles. For example, if three bubbles meet, they have an angle of 120 degrees with respect to each other This is the most efficient choice, again, which is also the reason why the cells of a beehive use the same angle, thus forming hexagons.

Interference and reflection

The iridescent colours are caused by interfering light waves. As the wall of a soap bubble has a certain thickness, light waves are reflected twice, once on each side. The ray of light reflected off the inner side of the wall travels slightly longer, so that, when the two waves become slightly out of sync, thus causing interference. Different thicknesses cause different hues, so that a change in colour can be observed while the bubble is thinning due to evaporation. Thicker walls cancel out red (longer) wavelengths, thus causing a blue-green reflection. Later, thinner walls will cancel out yellow (leaving blue light), then green (leaving magenta), then blue (leaving yellow). Finally, when the bubble's wall becomes thinner than the wavelength of visible light, all the waves cancel each other out and no reflection is visible at all. When this state is observed, the wall is thinner than about one million of an inch - and is probably about to pop.
If the wall of a soap bubble had an evenly thick wall, the bubble would have only one colour. However, the thickness of the wall is continiously changing as gravity pulls the liquid downwards, thus usually bands of colours that move downwards can be observed.

How to make soap bubbles

The easiest ways are to use commercially produced soap bubble fluid (marketed as a toy) or to simply put some dish washing soap in water. However, this latter might not work as well as expected, and there are several tricks to improve the soap sud formula:

Additives:

• Something to reduce the water's surface tension: Dish washing soap, liquid soap or baby shampoo. It may work better the more pure (devoid of perfume or other additives) the soap is, or perhaps with more expensive soaps.
• Something to thicken the water: Most commonly used is glycerin (available at the pharmacy), which makes the bubbles more colourful, too. Sugar, icing sugar or corn syrup have similar effects. It may be advantageous to dissolve the sugar in hot water. However, the soap sud can also be too thick and heavy, so it is important not to add too much of these thickening substances.
• distilled water: As tap water contains lime (calcium oxide), and as the lime binds the soap, distilled water works better.

Procedure:

• Leaving the soap sud in an open container overnight makes it thicker, too. But again, if the solution becomes too heavy it will be harder too make soap bubbles.
• Bubbles or foam on the surface of the soap sud should be avoided by stirring gently, skimming them away or simply waiting until they are gone.
• How easy it is to make soap bubbles depends on a vast number of factors. Every soap is different, and environmental conditions influence performance, too. For example, dusty air is unfavourable, and so is wind. Also, the more humid the air is, the better, which means making soap bubbles is easier on rainy days. Altogether, the best procedure for finding the perfect solution is the trial and error method.

Bubble blowers:

The easiest way is to use one of the plastic blowers that are sold with most commercial soap bubble solutions. However, as the blower's diameter determines the size of the soap bubble it might be necessary to build a blower oneself. Generally, any closed ring structure works. A blower can be made by bending wire into loop with a handle, where wire should be thick enough so the ring remains stiff. It can be improved by wrapping thread or bandages around the wire so the soap water can stick better to the ring.

A "giant bubble" blower, using a cloth loop attached to a plastic wand, with a slide permitting the loop to be gently opened or closed, was popularized by Klutz Press Publishing, which published a bubble-blowing book with the blower attached.

Here are some sample formulas:

1. General purpose formula:
   • 2/3 cup dish washing soap
   • 1 gallon water
   • 2 to 3 tablespoons of glycerin
2. Another general purpose formula:
   • 100 g sugar
   • 2 - 3 tablespoons salt
   • 1.4 l water, better distilled water
   • 150 ml dish washing soap
   • 12 ml glycerin
3. For long living bubbles:
   • 1/3 cup commercial bubble solution
   • 1/3 cup water
   • 1/3 cup glycerine
4. For No-tears soap bubbles:
5. 60 ml baby shampoo
6. 200 ml water
7. 3 tablespoons corn syrup