A soap bubble is a very thin film of soap water that forms a sphere with an iridescent surface. Soap bubbles usually last for only a few moments and then burst either on their own or on contact with another object. They are often used as a children's plaything, but their usage in artistic performances shows that they can be fascinating for adults too. Soap bubbles can help to solve complex mathematical problems of space, as they will always find the smallest surface area between points or edges.

Structure:
The bubble's 'skin' consists of a thin layer of water trapped between two layers of molecules, often soap. The surfactant possesses hydrophilic heads and hydrophobic tails. The hydrophilic heads are attracted to the thin water layer and keep the bubble intact. When the hydrophobic tails are disturbed, the bubble pops. It may also burst when it hits pointy surfaces.

Physics:

Surface tension and shape:

A bubble can exist because the surface layer of a liquid (usually water) has a certain surface tension, which causes the layer to behave somewhat like an elastic sheet. However, a bubble made with a pure liquid alone is not stable and a dissolved surfactant such as soap is needed to stabilize a bubble. A common misconception is that soap increases the water's surface tension. Actually soap does the exact opposite, decreasing it to approximately one third the surface tension of pure water. Soap does not strengthen bubbles, it stabilizes them, via an action known as the Marangoni effect. As the soap film stretches, the surface concentration of soap decreases, which causes the surface tension to increase. Thus, soap selectively strengthens the weakest parts of the bubble and tends to prevent them from stretching further. In addition, the soap reduces evaporation so the bubbles last longer, although this effect is relatively small.

Their spherical shape is also caused by surface tension. The tension causes the bubble to form a sphere, as a sphere has the smallest possible surface area for a given volume. This shape can be visibly distorted by air currents, and hence by blowing. If a bubble is left to sink in still air, however, it remains very nearly spherical, more so for example than the typical cartoon depiction of a raindrop. When a sinking body has reached its terminal velocity, the drag force acting on it is equal to its weight, and since a bubble's weight is much smaller in relation to its size than a raindrop's, its shape is distorted much less. (The surface tension opposing the distortion is similar in the two cases: The soap reduces the water's surface tension to approximately one third, but it is effectively doubled since the film has an inner and an outer surface.)

Freezing:
Soap bubbles blown into air that is below a temperature of -15C (5F) 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 about -25C (-13F), bubbles will freeze in the air and may shatter when hitting the ground. When, at this low temperature, a bubble is blown with warm breath, the bubble will freeze to an almost perfect sphere at first, but when the warm air cools and thus is reduced in volume there will be a partial collapse of the bubble. A bubble, blown successfully at this low temperature, will always be rather small in size: it will freeze quickly and continuing to blow will shatter the bubble.

Merging:
When two bubbles merge, the same physical principles apply, 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 two or more bubbles meet, they sort themselves out so that only three bubble walls meet along a line. Since the surface tension is the same in each of the three surfaces, the three angles between them must be equal angles of 120 degree. This is the most efficient choice, again, which is also the reason why the cells of a beehive use the same 120 degree angle, thus forming hexagons. Only four bubble walls can meet at a point, with the lines where triplets of bubble walls meet separated by cos-1(-1/3) ~109.47 degrees.

Interference and reflection:
The iridescent colors of soap bubbles are caused by interfering light waves and are determined by the thickness of the film. They are not the same as rainbow colors but are the same as the colors in an oil slick on a wet road.

As light impinges on the film, some of it is reflected off the outer surface while some of it enters the film and reemerges after being reflected back and forth between the two surfaces. The total reflection observed is determined by the interference of all these reflections. Since each traversal of the film incurs a phase shift proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on these two quantities. Thus, at a given thickness, interference is constructive for some wavelengths and destructive for others, so that white light impinging on the film is reflected with a hue that changes with thickness.

A change in color 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 a golden yellow). Finally, when the bubble's wall becomes much thinner than the wavelength of visible light, all the waves in the visible region cancel each other out and no reflection is visible at all. When this state is observed, the wall is thinner than about 25 nanometres, and is probably about to pop. This phenomenon is very useful when making or manipulating bubbles as it gives an indication of the bubble's fragility.

Interference effects also depend upon the angle at which the light strikes the film, an effect called iridescence. So, even if the wall of the bubble were of uniform thickness, one would still see variations of color due to curvature and/or movement. However, the thickness of the wall is continuously changing as gravity pulls the liquid downwards, so bands of colors that move downwards can usually also be observed.

Mathematical properties:
Soap bubbles are also physical illustrations of the problem of minimal surfaces, a complex mathematical problem. For example, while it has been known since 1884 that a spherical soap bubble is the least-area way of enclosing a given volume of air (a theorem of H. A. Schwarz), it was only recently proved in the year 2000 that two merged soap bubbles provide the optimum way of enclosing two given volumes of air with the least surface area. This has been termed the double bubble theorem.

Soap films seek to minimise their surface area, that is, to minimise their surface energy. The optimum shape for an isolated bubble is thus a sphere. Many bubbles packed together in a foam have much more complicated shapes.

Colored bubbles:
Adding colored dye to bubble mixtures fails to produce coloured bubbles, because the dye attaches to the water molecules as opposed to the surfactant. Therefore, a colorless bubble forms with the dye falling to a point at the base. Dye chemist Dr. Ram Sabnis, has developed a lactone dye that sticks to the surfactants, thus enabling brightly colored bubbles to be formed. An example of this dye is crystal violet lactone.

History of bubbles as playthings:

17th century Flemish paintings show children blowing with clay pipes. This means that bubbles as playthings are at least 400 years old. The London based firm of A. & F. Pears created a famous advertisement campaign for its soaps in 1886 using a painting by Millais of a child playing with bubbles. A Chicago company called Chemtoy began selling bubble solution in the 1940s, and they have captivated children ever since. According to one industry estimate, retailers sell around 200 million bottles annually, perhaps more than any other toy.

Bubble Blowers:
The easiest way is to use either a normal straw or one of the plastic blowers (bubble wands) 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. Bubble wands have been around since the 1920's.

Most closed-ring structures will work. A blower can be made by bending a wire into a loop with a handle, where the wire should be thick enough so the ring remains stiff. It can be improved by wrapping a thread or bandages around the wire so the soap water can stick better to the ring.

Klutz Press popularized a "giant bubble" blower, invented by a man named David Stein, which used a cloth loop attached to a plastic wand, with a slide permitting the loop to be gently opened or closed. Klutz sells bubble books which offer how-tos and fun ideas, usually with a ready-to-use bubble loop.

Bubbles can be blown by using a bubble pipe, which is made of plastic and usually takes the shape of a smoking pipe, sometimes containing multiple bowls. The bubble solution is poured into the bowl of the pipe; when someone blows into the mouthpiece, bubbles rise from the bowl.

Performance Art:
Soap bubble performances combine entertainment with artistic achievement. They require a high degree of skill as well as perfect bubble suds. Some artists create giant bubbles or tubes, often enveloping objects or even humans. Others manage to create bubbles forming cubes, tetrahedra and other shapes or sculptures. Bubbles are often handled with bare hands. One such performer is Tom Noddy. To add to the visual experience, they are sometimes filled with smoke or helium and combined with laser lights or fire. Soap bubbles can be filled with a flammable gas such as natural gas and then ignited.

Antibubbles
What are antibubbles? "Skins of air which float around underwater, and vanish when touched."
http://www.antibubble.org/

http://www.eskimo.com/~billb/amateur/antibub/antibub1.html

http://www.hot-streamer.com/antibubbles/

Art and Science of Bubbles
This site is written for kids from the Soap and Detergent Association.
http://www.sdahq.org/new1198/kids/bubbles/Welcome.html

Bubble Activities
This site has several things that you can do with bubbles such as saving them or freezing them.
http://www.monroe2boces.org/shared/esp/bubbles.htm

Bubble Geometry
Have you ever seen a square bubble?
http://www.sci.mus.mn.us/sln/tf/b/bubblegeometry/bubblegeometry.html

Bubble Graphics
Free Bubble Graphics.
http://www.geocities.com/xmas_baby75/Bubbles.html

Bubbles Inside Bubbles
Madras College in Scotland
http://www.madras.fife.sch.uk/maths/Sheet301/Sheet301a.html

Bubble Mania
Some Amazing photos of bubbles.
http://www.bubblemania.com/photos/

Bubbles and Math Olympiads
Here you will find an article on bubbles from Science News.
http://www.sciencenews.org/20010616/mathtrek.asp

Bubblesmith
Easy bubble instructions and a trouble-shooting link, but don't miss the Gallery.
http://www.handblownbubbles.com/index.html

Bubblesphere
Everything you ever wanted to know about bubbles can be found here. It is designed with students in mind.
http://www.bubbles.org

Bubble Town
This site has a huge guide to the art of blowing bubbles.
It explains the science of bubbles, discusses what bubble solution is made of and shows how to make different kinds of bubbles.
http://www.zurqui.com/crinfocus/bubble/bubble.html

Double Bubbles
Article from The American Scientist on Bubbles and Double Bubbles.
http://www.amsci.org/articles/96articles/Hass.html

Here you will find some great pictures of double Bubbles.
http://www.math.berkeley.edu/~hutching/pub/db2ann/dbpictures.html

Electric Bubbles
Article written about electric fields and bubbles.
http://www.beyond2000.com/news/Nov_99/story_310.html

Enquiring into Bubbles
Explore a range of ideas about bubbles and surface tension, and how to make bubbles of unusual shapes.
http://www.members.ozemail.com.au/~macinnis/scifun/bubbles.htm

Exploratorium
This is the San Francisco Science and Discovery Center site that is all about the science of bubbles.
http://www.exploratorium.edu/ronh/bubbles/bubbles.html

Create geometric art with soap films.
http://www.exploratorium.edu/snacks/soap_bubbles.html

Create giant bubbles
http://www.exploratorium.edu/snacks/bubble_tray.html

Bubble Suspension
Soap bubbles float on a cushion of carbon dioxide gas.
http://www.exploratorium.edu/snacks/bubble_suspension.html

Make Your Own Bubble Maker
http://www.billybear4kids.com/graduation/summer/
bubbles/youmake.html

Maarten Rutgers Bubble Web Site
Great work with bubbles for fun and science.
http://www.home.earthlink.net/~marutgers/index.html

Papers on the Double Bubble Theorem
http://www.ugr.es/~ritore/bubble/bubble.htm

The Science of Bubbles
This is the web site of The Science Museum of Minnesota.
http://www.bubbleart.com

Wholesale Events Solutions
Experiment with bubbles~Big Beautiul Bubbles.
http://www.eventswholesale.com/article/experiment-with-bubbles.htm