The Anatomy of a Soap Bubble

Few children can resist the charm of soap bubbles. With a piece of wire and a little soapy water, you can spend an afternoon blowing and creating iridescent, multicolored bubbles that seem to float in the air. But behind this apparent simplicity, these soap bubbles hide an extraordinary complexity.

A soap bubble is no more than a very thin film of soapy water, whose spherical surface encloses a volume of air inside and displays an iridescent appearance. Despite their ephemeral existence — usually lasting only a few seconds and then bursting by themselves or when touching an object — their appearance and simplicity make few children resist using them as part of their games.

However, behind these colorful bubbles hides a great complexity that physics and mathematics try to unveil. In fact, these objects, which are even used in artistic performances, are absolutely fascinating to scientists, because they can help solve complex mathematical problems related to spatial geometry.

A Very Thin Sandwich

The surface of a bubble is nothing more than a (very) thin sandwich, where the two outer layers are formed by surfactant molecules (yes, soap) and the central one by pure water. These surfactant molecules have an end with hydrophilic properties (attracted to water) and another with hydrophobic characteristics that, logically, are repelled by the water of the central layer. The film that acts as the “skin” of a soap bubble is extremely thin. In fact, it may be one of the thinnest objects you can build in your home: its thickness is about five thousand times smaller than a hair.

A Problem of Surface Tension

The secret of the existence of soap bubbles is, of course, the so-called “surface tension.” This phenomenon, present to a greater or lesser degree on the surface of all liquids, is responsible for some very strange things, such as allowing some insects to “walk” on water or filling a glass above its rim. To explain the concept of surface tension in simple words, we can say that water molecules are in constant pulling with their neighboring molecules.

Inside the liquid, molecules find themselves in an equilibrium of forces because for every molecule that pulls downward there is one that pulls upward (and the same happens in any direction you can imagine). On a given molecule there is no net force, but on the surface things are different. There are no upward forces able to balance those exerted by the molecules below, since there is no liquid on the surface. The result is that surface molecules tend to be attracted toward the interior of the liquid.

However, if we used pure water, we could not create important or durable bubbles. This is because water has a high surface tension, which causes the rapid destruction of the film. But things change if we add some kind of soap. Soap reduces the natural surface tension of water by one third, preventing bubbles from bursting “spontaneously.” At the other end of the equation, if we used pure soap we also could not create durable bubbles, since the low surface tension would prevent it. The other big problem with pure-water bubbles is evaporation on their surface, which makes them thin quickly, causing the inevitable burst. But soap also helps delay this process.

So, as any child knows, we must use a mixture of water and soap to create bubbles that last at least a few seconds. The “ideal formula” for creating a bubble includes 50% water, 40% liquid soap, and 10% glycerin. This last component is a kind of alcohol that helps solve the evaporation problem while also giving it greater resistance. You can buy it in pharmacies without problems, but we must be careful if there are children nearby, since glycerin can be toxic if ingested.

Let’s Blow!

When we start blowing, the soap film and the surface of the soap bubble begin to stretch because the pressure inside is slightly higher than the outside pressure. This pressure (simplifying a lot) is what transmits energy to the soap film, which stores it as “elastic energy” on the surface. Roughly like what happens when we press a spring. As the diameter of the bubble increases, along with its surface, the bubble becomes more elastic.

Thanks to the surface tension acting on the molecules on the surface of the bubble, there is a “resistance” to the evaporation of the superficial liquid layer. This is because the resultant force on each surface molecule is directed toward the interior of the liquid. In some way, the intensity degree of this surface tension is responsible for the size and shape of the bubble.

The tension makes the bubble form a sphere, because the sphere is, of all possible figures, the one with the smallest surface area for a given interior volume. Of course, this shape can be visibly distorted by air currents (or by a blow), but if a bubble is dropped in still air, it remains almost spherical.

Curiosities

It’s quite likely you’re already thinking about mixing a little soap and water to play with some bubbles. In fact, you can use a little sibling as an excuse not to feel bad when you start blowing your bubbles. In case you haven’t decided yet, here are some curiosities about these structures.

First, they have a great ability to change size with ambient temperature. If a bubble moves from a warm place to a colder one, it becomes smaller as the air inside it compresses slightly. It can also be demonstrated that soap bubbles explode due to the force of gravity. In the mixture that gives them life, soap weighs more than water, so as it “migrates” toward the lower part of the sphere, the upper part begins to evaporate and eventually the bubble breaks.

Another experiment you can do is to create bubbles in very cold environments. Soap bubbles created in air at a temperature lower than 15°C below zero freeze when they touch a surface. Then the air inside is gradually lost, causing the bubble to wrinkle under its own weight. And if you make them at about 25°C below zero, the bubbles will freeze in the air, breaking when they fall to the ground. At such low temperatures, bubbles fill with the warm air from your lungs, but they quickly freeze into perfect spheres. If the temperature is very low, they will freeze before you can “inflate” them too much, and if you keep blowing, you will only succeed in breaking them.

As you can see, behind something as seemingly simple as a soap bubble hides an important complexity. Fortunately, you don’t need to know any of this for a child to enjoy these floating, multicolored spheres.

We were inspired by reading the Wikipedia article on soap bubbles.