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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.

A soapbubble

Table of contents
1 Physics
2 How to make soap bubbles
3 Usage
4 External links and further reading


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 pure 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:



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. Another general purpose formula:
  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:

Frozen soap bubbles

Soap bubbles blown into air that is below a temperature of around 0 fahrenheit (-15 celsius) will freeze when they touch a surface. The air inside will gradually diffuse out, causing the bubble to crumple under its own weight. At temperatures below around -15 fahrenheit (-25 celsius), the bubbles will freeze in the air and shatter when they hit the ground.


Soap bubble performances

Soap bubble performances combine entertainment with artistic achievement. They require high skills as well as perfect bubble suds.

Examples of common acts:

Soap bubbles and maths

A soap film forms a natural minimal surface. Minimal surfaces have been an area of intense mathematical and scientific study over the past 15 years.

As an example: In 1884 Schwarz already proved that a spherical soap bubble is the least-area way of enclosing a given volume of air. However, only recently, in the year 2000, it was proved that two merged soap bubbles are the least-area way of enclosing two given volumes of air, called the Double Bubble Theorem.

External links and further reading