Soap bubbles… They are a childhood delight. Big and billowy, small and ephemeral… all bubbles are magical.
But the properties of bubbles are more important to life than we can fathom as toddlers chasing magical floating circles around the yard at dusk. Bubbles are a form of self-assembled monolayers… the thin soap-water-soap layer that forms the bubble assembles by itself (given the proper puff of air from our lips into the soap-covered bubble stick, of course).
“I do not suppose that there is any one in this room who has not occasionally blown a common soap-bubble, and while admiring the perfection of its form, and the marvelous brilliancy of its colours, wondered how it is that such a magnificent object can be so easily produced.” – Soap Bubbles and the Forces Which Mould Them, 1959
Soap bubbles self-assemble because of the properties of the soap molecules themselves. Soap molecules are amphiphilic, from the Greek ‘amphis’ meaning both and ‘philia’ meaning love. Each soap molecule has one side that is attracted to water, or water-loving, and one side that is repelled by water, or water-hating. This dual-sided nature of soap molecules causes them to arrange themselves into structures such that all the water-loving ends face water – in other words, they face the water layer inside the thin film that surrounds the air at the center of a bubble. On the other hand, the water-hating ends – also called hydrocarbon chains – face the air outside the bubble or the air inside the bubble… but not the water inside the bubble’s film. Because of their amphiphilic nature, soap molecules automatically form circular structures – a core of air surrounded by a soap-water-soap film in which the water-hating ends of soap molecules crowd to either the inner or outer surface of the bubble – that minimize the energy inherent in the system. Think of lazy bubbles.
This energy-minimization effort on the part of soap molecules and bubbles also gives rise to the typically spherical shape of bubbles. Surface tension tends to mould bubbles into perfectly spherical shapes to minimize the area of the bubble that is surface for a given bubble volume, because energy is highest at the surface of the bubble. Beautifully for our childhood imaginations, the structure with the smallest surface area to volume ratio is the perfect sphere (there is a mathematical proof to show that this is true… trust me).
All the pretty colors…
One of the more beautiful traits of bubbles are the brilliant ‘rainbow’ colors that can be seen at their surfaces. Bubbles shine with bands of color because of the interference of light waves being reflected and refracted (as in a prism) from both the outer soap interface (air-soap-water) and the inner soap interface (water-soap-air). Remember that a bubble has three layers: soap, water, soap. This interference of light waves depends on the thickness of the soap bubble. Thin bubbles that are about to pop may appear dark (with no color) because the light waves reflected from the inner and outer soap interfaces cancel each other out. Thicker bubbles display vibrant colors separated out of white light because the light waves reflected from the inner and outer soap interfaces combine together.
Because different colors of light have different wavelengths (length from the crest of the wave to the trough of the wave, just like in an ocean wave), different colors of light waves cancel out or add together as they are reflected from bubbles of different film thicknesses: bubbles can appear to be different colors dependent on the thickness of their films!
Over time, a bubble loses its inner water layer (due to evaporation of the water), meaning that the bubble’s thickness decreases over time as it floats across your yard. This means (magically indeed) that the color of a bubble can change over time as its thickness decreases until the bubble pops!
I didn’t mean to burst your bubble…
So if a soap bubble forms so easily through of self-assembly, why doesn’t the bubble stay around longer? Why does it eventually have to burst?
The answer to this question is that it’s all about pressure. Starting out, the pressure on the inside of the bubble is greater than the pressure on the outside. As the bubble floats around your kitchen or around your yard, especially if you are blowing bubbles in the sunlight, the water inside the thin film of the bubble will start to evaporate. Eventually the pressure difference from inside to outside the bubble becomes too great for the thinning bubble film to withstand, and thus the bubble bursts!
A bubble can also burst when it comes into contact with a dry surface, because the dry surface sucks the water out from within your bubble’s thin film! So here is a tip: keep your hands wet when trying to catch a bubble!
What is the big deal about bubbles?
So I said that the properties of bubbles are important to life itself. This isn’t just because bubbles make us happy, thus improving our quality of life! This is because the self-assembling soap-water-soap layer of a bubble is very similar to the self-assembling lipid bilayer that forms the membrane around each and every single one of the cells in your body. The cell membrane lipid bilayer is composed of amphiphilic molecules similar to soap molecules. The water-loving ends of these molecules face the outside and inside of your cells, as both the outside and inside environment of the cells in our bodies are composed largely of water! The water-hating ends of these molecules, however, crowd together inside the film that composes a cell membrane. This lipid bilayer structure surrounds the entire cell, and serves as a barrier to intruders (like bacteria) and keeps the cell in a defined shape.
So, what do you see when you look at a bubble now? More than a billowy colorful childhood delight?
1. By Paige Brown. Not for reproduction.