Questions we ask ourselves - What is a soap bubble, and why is it so colorful?
It’s a question that many of us likely have asked as children - why are soap bubbles so resilient, compared to water bubbles, and why are they so colorful on their surfaces? The answer is as fascinating as the question.
What is a soap bubble?
First, a soap bubble is actually a bubble of water between two layers of soap molecules. Soap is what is chemically called a surfactant - its molecules have hydrophilic, or “water-loving,” heads, and hydrophobic, or “water-hating,” tails. These parts are -philic/-phobic based on their solubility in water (polarity). This quality thus enables a surfactant to allow oil and water to mix, which is useful in cleaning products like soap.
So in a soap bubble, the hydrophilic heads are attracted to the sandwiched water, while the tails extend away from the water (Figure 2). This creates surface tension, and the material strives to achieve the smallest area based on simple physics, thus forming a round surface (and if the bubble is in the air, a sphere).
So what’s with the colors?
The beautiful colors seen on soap bubbles are a result of what’s called interference, an effect observed from butterfly wings to crystals of calcite (Figure 1). When light reaches the bubble, some of the waves reflect off of the outer soap film, but others travel farther to reflect off of the inner film. Because there’s a distance between these two layers, there is an extra distance the second wave has to travel to catch up with the first wave. These waves are transmitted parallel to each other (Figure 3).
There are two possible outcomes of this effect, which is called interference (overall presented in Figure 4). The first is if the waves are “out of phase.” This means that the extra distance the second wave travels is equal to half a specific wavelength. As a result, the second wave will partially “cancel out” the first wave, a process called destructive interference. This leads to a reduction in the intensity of the color.
The second outcome is constructive interference. If the extra distance is equal to a specific wavelength of light, the waves are “in phase” and will enhance each other.
Different colors thus form on the surface of a bubble based on the angles of incident light waves. At a direct, right angle, the extra distance is much shorter than if the light enters from a wider angle (think of a pencil on a lined sheet of paper - if oriented up and down, the total length of pencil one row intersects is less than if the pencil is at an angle).
By the same principle, the colors are dependent on thickness, which would, again, affect those extra distances of the second waves. A bubble becomes thinner over time due to evaporation - so as time passes, the range of colors a bubble presents should change. As a bubble changes from thick to thin, the ranges are cancelled in the order of red, yellow, green, and, last, blue, until the bubble is (visibly) colorless. Usually bubbles with thick films present blue and green colors, while thinner bubbles will be yellow to colorless.
And that’s that!
(Image & supplementary info source)