Soap bubble

The structures that soap films make can not just be enclosed as spheres, but virtually any shape, for example in wire frames. Therefore, many different minimal surfaces can be designed. It is actually sometimes easier to physically make them than to compute them by mathematical modelling. This is why the soap films can be considered as analog computers which can outperform conventional computers, depending on the complexity of the system.[3][4][5]

Soap bubbles can easily merge

When two bubbles merge, they adopt a shape which makes the sum of their surface areas as small as possible, compatible with the volume of air each bubble encloses. If the bubbles are of equal size, their common wall is flat. If they are not the same size, their common wall bulges into the larger bubble, since the smaller one has a higher internal pressure than the larger one, as predicted by the Young–Laplace equation.

At a point where three 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 to 120°. 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°. All these rules, known as Plateau's laws, determine how a foam is built from bubbles.

The longevity of a soap bubble is limited by the ease of rupture of the very thin layer of water which constitutes its surface, namely a micrometer-thick soap film. It is thus sensitive to :

• Drainage within the soap film: water falls down due to gravity. This can be slowed by increasing the water viscosity, for instance by adding glycerol. Still, there is an ultimate height limit, which is the capillary length, very high for soap bubbles: around 13 feet (4 meters). In principle, there is no limit in the length it can reach.
• Evaporation: This can be slowed by blowing bubbles in a wet atmosphere, or by adding some sugar to the water.
• Dirt and fat: When the bubble touches the ground, a wall, or our skin, it usually ruptures the soap film. This can be prevented by wetting these surfaces with water (preferably containing some soap).

After experiments, researchers found that a solution containing:

• 85.9 % water
• 10 % glycerol
• 4 % washing-up liquid
• 0.1 % guar gum

gave the longest lasting results as it minimised the Marangoni Effect.[6]

When a soap bubble is in contact with a solid or a liquid surface wetting is observed. On a solid surface, the contact angle of the bubble depends on the surface energy of the solid.,[7][8] A soap bubble has a larger contact angle on a solid surface displaying ultrahydrophobicity than on a hydrophilic surface – see Wetting. On a liquid surface, the contact angle of the soap bubble depends on its size - smaller bubbles have lower contact angles.[9][10]

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  A soap bubble wetting an ultrahydrophobic surface
• 
  A soap bubble wetting a liquid surface

The gas inside a bubble is less dense than air because it is mostly water vapor. Water vapor is a gas that is formed when water molecules evaporate. When water molecules evaporate, they escape from the liquid state and enter the gas state. In the gas state, water molecules are further apart than they are in the liquid state. This is because water molecules are attracted to each other. When they evaporate, they break away from these attractions and move further apart.

The further apart water molecules are, the less dense they are. This is why water vapor is less dense than air. The gas inside a bubble is mostly water vapor, so it is also less dense than air.

The density of a gas can also be affected by its temperature. As the temperature of a gas increases, the molecules of the gas move faster. This causes them to spread out and become less dense. The opposite is also true. As the temperature of a gas decreases, the molecules of the gas move slower. This causes them to bunch together and become more dense.

The temperature of the gas inside a bubble is affected by the temperature of the water around it. The warmer the water, the warmer the gas inside the bubble. This means that the gas inside a bubble will be less dense if the water is warm than if the water is cold.

Soap bubbles have been used as entertainment for at least 400 years, as evidenced by 17th-century Flemish paintings showing children blowing bubbles with clay pipes. The London-based firm A. & F. Pears created a famous advertising campaign for its soaps in 1886 using a painting by John Everett Millais of a child playing with bubbles. The Chicago company Chemtoy began selling bubble solution in the 1940s, and bubble solution has been popular with children ever since. According to one industry estimate, retailers sell around 200 million bottles annually.

• 
  A woman creating bubbles with a long soap bubble wand
• 
  Bubble Blowing at Dolores Park during Dyke March, June 2019
• 
  Adriaen Hanneman, Two Boys Blowing Bubbles (ca. 1630)
• 
  Jean-Baptiste-Siméon Chardin, Soap Bubbles (ca. 1734)

A single light soap bubble photograph taken under macro photography

A bubble is made of transparent water enclosing transparent air. However, the soap film is as thin as the visible light wavelength, resulting in optical interference. This creates iridescence which, together with the bubble's spherical shape and fragility, contributes to its magical effect on children and adults alike. Each colour is the result of varying thicknesses of soap bubble film. Tom Noddy (who featured in the second episode of Marcus du Sautoy's The Code) gave the analogy of looking at a contour map of the bubbles' surface. However, it has become a challenge to produce artificially coloured bubbles.

Byron, Melody & Enoch Swetland invented a patented non-toxic bubble (Tekno Bubbles)[11] that glow under UV lighting. These bubbles look like ordinary high quality "clear" bubbles under normal lighting, but glow when exposed to true UV light. The brighter the UV lighting, the brighter they glow. The family sold them worldwide, but has since sold their company.

A single soap bubble displaying three layers

Adding coloured dye to bubble mixtures fails to produce coloured bubbles, because the dye attaches to the water molecules as opposed to the surfactant. Therefore, a colourless 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, enabling brightly coloured bubbles to be formed. Crystal violet lactone is an example. Another man named Tim Kehoe invented a coloured bubble which loses its colour when exposed to pressure or oxygen, which he is now marketing online as Zubbles, which are non-toxic and non-staining. In 2010, Japanese astronaut Naoko Yamazaki demonstrated that it is possible to create coloured bubbles in microgravity. The reason is that the water molecules are spread evenly around the bubble in the low-gravity environment.

Frozen soap bubble on snow at −7 °C (19 °F)

If soap bubbles are blown into air that is below a temperature of −15 °C (5 °F), they will freeze when they touch a surface. The air inside will gradually diffuse out, causing the bubble to crumble under its own weight. At temperatures below about −25 °C (−13 °F), bubbles will freeze in the air and may shatter when hitting the ground. When a bubble is blown with warm air, the bubble will freeze to an almost perfect sphere at first, but when the warm air cools, and a reduction in volume occurs, there will be a partial collapse of the bubble. A bubble, created successfully at this low temperature, will always be rather small; it will freeze quickly and will shatter if increased further.[12] Freezing of small soap bubbles happens within 2 seconds after setting on snow (at air temperature around –10...–14 °C).[13]