First of all, we must understand what a bubble is and how it is formed.
A bubble forms in the body of a liquid when molecules of the liquid evaporate in a cavity, understanding this cavity as any space within it vacated by molecules of the liquid itself, either empty or occupied by vapor. These cavities are formed under the influence of localized thermal fluctuations and a heat source that causes molecular evaporation is necessary for them to occur.
The bubble formation process can occur at normal pressures when the cooking temperature of a certain liquid is exceeded; It is then that we see the characteristic bubbles that indicate that, for example, the water is boiling.
Bubbles also form at extreme pressures, for example when fluid passes through a narrow space at high speed , such as when we blow into a liquid through a straw. This process also results in cavitation (the hydrodynamic effect described above, which occurs when gas cavities form within a liquid), and is usually accompanied by noise and vibrations.
How are soap bubbles formed?
In the specific case of soap bubbles there is a lot of chemistry at stake; it has to do with the compounds that soap is made of, and their reaction with water (H2O).
Soaps are usually made by the chemical reaction (called saponification) of an alkaline base, such as sodium or potassium hydroxide, along with a lipid, which can be of animal or plant origin. The molecules resulting from this reaction (soap) have two extremes, as Stephanie Chasteen, a physicist, popularizer and researcher at the University of Boulder, in Colorado, explains on her blog. “One end of the molecules ‘likes’ to stick to the water; while the other repels water ”.
Thus, as Chasteen explains: “The surface of a soap bubble is actually a thin layer of water sandwiched between soap molecules.”
When a soap bubble forms, the molecules “surround” the water: some point towards it, and others, away from it. And why in the shape of a circle? By a physical property by which when some volume of water is enclosed in a space, it will have the shape with the smallest possible surface; in other words, save more energy.
The Marangoni effect
Under normal conditions, water molecules attract each other in all directions, but when they are on a surface, their relationship changes: those in contact with air join with more energy, as if they formed a mesh. What happens is that it is a fragile association, and any imbalance in the consistency of the molecular wall ends up puncturing that bubble.
The long life that soap infuses them is due to the fact that it incorporates surface-active substances: its molecules create a film above and below the water layer and reduce its surface tension thanks to a physical phenomenon known as the Marangoni effect. Thus, the bubbles stabilize and last longer.
If you want to make them huge, you can use other ingredients, such as dishwashing detergent and glycerin , which give it an extra consistency.
