Soap bubble

The bubble film consists of a thin layer of water sandwiched between two layers of molecules, most often soap. These layers contain molecules, one part of which is hydrophilic and the other hydrophobic . The hydrophilic part is attracted by a thin layer of water, while the hydrophobic part, on the contrary, is pushed out. As a result, layers are formed that protect the water from rapid evaporation, as well as reduce surface tension .

Surface tension and shape

A bubble exists because the surface of any liquid (in this case, water) has some surface tension , which makes the behavior of the surface similar to the behavior of something elastic . However, a bubble made only of water is unstable and quickly collapses. In order to stabilize his condition, some surfactants , such as soap, are dissolved in water. A common misconception is that soap increases the surface tension of water. In fact, it does just the opposite: it reduces the surface tension to about a third of the surface tension of pure water. When a soap film is stretched, the concentration of soap molecules on the surface decreases, while increasing the surface tension . Thus, the soap selectively strengthens the weak parts of the bubble, preventing them from stretching further. In addition to this, the soap protects the water from evaporation, thereby making the bubble life even longer.

The spherical shape of the bubble is also obtained by surface tension . Tension forces form a sphere because the sphere has the smallest surface area for a given volume. This form can be significantly distorted by air flow and the process of inflation of the bubble itself. However, if the bubble is left to float in calm air, its shape will very soon become close to spherical.

Freezing bubbles

There is evidence of freezing of soap bubbles at a temperature of about −10 ° C ^[1] . In order to prevent the destruction of the bubble during freezing, it is recommended to inflate the soap bubble with outdoor air temperature (for example, by rapidly moving the ring), and not with warm air from the mouth.

If a bubble is inflated at a temperature of −15 ° C , then it will freeze when in contact with the surface. The air inside the bubble will gradually leak out and eventually the bubble will collapse under its own weight.

At a temperature of −25 ° C, the bubbles freeze in the air and can break when they hit the ground. If a bubble is inflated with warm air at such a temperature, it will freeze in almost perfect spherical shape, but as the air cools and shrinks in volume, the bubble may partially collapse and its shape will be distorted. Bubbles inflated at this temperature will always be small, as they will quickly freeze, and if you continue to inflate them, they will burst.

Combining Bubbles

When two bubbles connect, they take shape with the smallest possible surface area. Their common wall will bulge out into a larger bubble, since a smaller bubble has a higher average curvature and greater internal pressure. If the bubbles are the same size, their total wall will be flat.

The rules governing the bubbles when combined were experimentally established in the 19th century by the Belgian physicist Joseph Plato and proved mathematically in 1976 by Jean Taylor .

• Soap films are piecewise smooth surfaces, the average curvature of which is constant on each smooth section.
• If there are more than three bubbles, they will be arranged in such a way that only three walls can be joined near one edge, and the angles between them will be 120 °, due to the equality of the surface tension for each contacting surface.
• The lines of intersection of the surfaces intersect at one point in four pieces, and the angle between any two is arccos (-1/3) ≈109.47 °.

Bubbles that do not obey these rules, in principle, can form, but they will be highly unstable and will quickly take the correct form or collapse. Bees that seek to reduce wax consumption connect honeycombs in hives also at an angle of 120 ° , thereby forming regular hexagons .

Interference and reflections

Overflowing "rainbow" colors of soap bubbles are observed due to the interference of light waves and are determined by the thickness of the soap film.

When a beam of light passes through a thin film of a bubble, a part of it is reflected from the outer surface, forming the first beam, while the other part penetrates the film and reflects from the inner surface, forming the second beam. The color of the radiation observed in the reflection is determined by the interference of these two rays. Since each passage of light through the film creates a phase shift proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on two quantities. Reflected, some waves are formed in phase, while others are in antiphase, and as a result, white light colliding with the film is reflected with a tint depending on the thickness of the film.

As the film becomes thinner due to evaporation of water, a change in the color of the bubble can be observed. A thicker film removes the red component from white light, thus making the shade of reflected light blue-green. A thinner film removes yellow (leaving a blue light), then green (leaving a purple), and then blue (leaving a golden-yellow). In the end, the bubble wall becomes thinner than the wavelength of visible light, all the reflected waves of visible light add up in antiphase and we stop seeing the reflection at all (on a dark background this part of the bubble looks like a black spot). When this happens, the wall thickness of a soap bubble is less than 25 nanometers , and the bubble is likely to burst soon.

The effect of interference also depends on the angle with which a ray of light collides with a bubble film. Thus, even if the wall thickness were the same everywhere, we would still observe different colors due to the movement of the bubble. But the thickness of the bubble is constantly changing due to gravity, which pulls the liquid to the bottom so that we can usually observe bands of different colors that move from top to bottom.

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  In this diagram, a beam of light collides with the surface at point X. Part of the light is reflected, and some passes through the outer surface and is reflected from the inside.

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  This diagram shows two red light beams (rays 1 and 2). Both beams are broken into two, but we are interested only in those parts that are shown in solid lines. Consider a beam emerging from point Y. It consists of two rays that overlap one another: part of beam 1, which passes through the wall of the bubble and part of beam 2, which is reflected from the outer surface. The beam passing through the XOY points traveled longer than the beam 2. Suppose it happened that the length of the XOY is proportional to the wavelength of the red light, therefore two beams are added in phase.

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  This diagram is similar to the previous one, except that the light wavelength is different. This time, the XOY distance is disproportionate to the wavelength, and the rays are in antiphase. As a result, blue light is not reflected from a bubble with such a wall thickness.

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  This computer image shows colors reflected by a thin film of water illuminated by unpolarized white light.

Soap bubbles are also a physical illustration of the problem of a minimal surface , a complex mathematical problem. For example, despite the fact that since 1884 it is known that a soap bubble has a minimum surface area for a given volume, it was only in 2000 that it was proved ^[2] that two combined bubbles also have a minimum surface area for a given combined volume. This problem was called the double bubble theorem. It was also only with the advent of the geometric theory of measure that it was possible to prove that the optimal surface would be piecewise smooth , and not infinitely broken.

The bubble film always seeks to minimize its surface area. This is due to the fact that the free energy of a liquid film is proportional to its surface area and tends to achieve a minimum:

${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\Delta </span></span>}\mathcal{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">F</span></span>}<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">=</span></span>\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\sigma </span></span>}\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">S</span></span>}}${\ displaystyle \ Delta {\ mathcal {F}} = \ sigma S}  

Where${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\sigma </span></span>}}$ {\ displaystyle \ sigma}   - the surface tension of the substance, and${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">S</span></span>}}$ {\ displaystyle S}   - the total surface area of ​​the film. The optimal shape of a single bubble is a sphere, but several bubbles combined together have a much more complex shape.

The easiest way is to use a special liquid for soap bubbles (which is sold as a toy) or simply mix dishwashing liquid with water. But the latter method may not give such good results, which I would like to get, the bubble from the dishwashing liquid will quickly burst. Therefore, below are a few techniques to help improve the result:

Components

• Substances that reduce the surface tension of water, such as liquid soap or baby shampoo. The clearer the soap (without perfume or other additives), the better the result can be.
• Substances that condense water. The most commonly used glycerin (which can be bought at the pharmacy ). You can also use sugar , which is better to dissolve in warm water. However, the density of water may become too large, so it is important to observe moderation.
• Distilled water . Water from the tap contains calcium ions that bind soap. When using distilled water, the effect of this effect on the quality of the soap bubble is much lower.

Procedure

• If you leave the mixture open for several hours, its density will also become higher. But again, if it becomes too high, it will be difficult to blow bubbles.
• It is better to avoid bubbles or foam on the surface of the mixture, carefully removing them or simply waiting for them to disappear.
• How easy it is to make bubbles depends on many different factors. Different soaps, different environmental conditions, for example, it is better to avoid dusty air or wind. Also, the higher the humidity, the better, and therefore it is better to make bubbles on a rainy day. In other words, the best way to find the perfect solution is through trial and error.
• Of great importance is the material and shape of the tube or ring for blowing bubbles. The ring is used to create a variety of relatively small bubbles. A tubule to create one big bubble. If you use a tube made of cardboard, with thick dense walls of 1.5-2 mm and an internal diameter of 10-12 mm, you can get a long-living (up to several minutes), a bubble attached to the tube, with dimensions more than 30 cm in diameter. The use of a large internal diameter allows the air to be blown in sufficient volume and at a minimum speed, reducing the fluctuations of the bubble and the risk of its slipping from the tube. Thick cardboard walls allow you to “stock up” a larger amount of mortar, due to absorption, thereby fueling the bubble in the process. However, an excessive amount of liquid may cause a drop to form in the lower part of the bubble, and its “breakdown” due to the large weight. The length of the tube is chosen individually, since a short tube (8-10 cm) is easier to control and compensate for bubble oscillations, to hold it, and a longer one (15-20 cm or more) allows the air flow generated by inhalation and exhalation, which can "swing" and unhook the bubble. Competition in the size of bubbles - a calm and contemplative exercise, blowing up a lot of small bubbles - a more fun action.

The show of soap bubbles is both entertainment and art. Creating spectacular bubbles requires an artist of a high level of skill, as well as the ability to prepare a perfect quality soap solution. Some artists create giant bubbles, often wrapping objects or even people. Others manage to create bubbles in the shape of a cube , a tetrahedron, and other shapes. Often, to enhance the visual effect, bubbles are filled with smoke or combustible gas, combined with laser illumination or open fire.

On March 2, 2017, Russian Lyudmila Daryina set the Guinness Book of Records record. “The largest number of people inside a bubble” ^[3] is 374 people. On January 30, 2018, this record was entered into the "Book of Records of Russia"] ^[4] as a world one.

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• Antibubble

• "Charles V. Boys" Soap-Bubbles. Their colors. - Dover Publications, New York 1990, ISBN 0-486-20542-8
• Cyriel Isenberg The Science of Soap Films and Soap Bubbles. - Tieto Books, Clevedon North Somerset, 1978, ISBN 0-905028-02-3
• Ya. E. Geguzin "Bubbles"
• Flash allowance for making bubbles at home
• Giant Stinson Beach Bubbles Huge Soap Bubbles (video)