Soap bubbles have very thin walls. The range can be anywhere from 10 nanometers at the top of a thin-walled bubble to over 1000 nanometers. By contrast, human hair's thickness range is on the order of 40,000 to 60,000 nanometers. According to a post on SBF soap films may be as few as a few soap molecules thick.
A bubble's colors are caused by a phenomenon known as Thin-film Interference. When light bounces off of a soap film, there are reflections from both the front and back of the film that reach the eye. Interaction between these reflections cause the colors that we see. The colors are directly related to the film thickness (as well as light color and lighting angle). You can accurately gauge the thickness of a bubble's walls by examining its colors.
Due to the influence of gravity, a soap bubble's walls are thinnest at the top (as the solution is drawn downward by gravity) and thickest at the bottom which results in the variety of colors that we see. As a bubble is tossed and turned by the wind, there will be a disruption of the gradual progression from thin film to thick film.
As a bubble ages, evaporation will cause the film to become thinner all around causing the colors to shift. There also may be some draining from the top to bottom due to the effects of gravity. Watch the video below to see this in action. The bubble will become so thin that it becomes colorless and transparent.
The chart below was prepared by a scientist to map bubble colors to film thickness :
Notice that as the film becomes the thicker it becomes dominated by increasingly dull-looking shades of green and pink. Yellows blues only appear in the thinner range of the film.
The film's colors provide information that has practical applications. Careful observation of the colors as a bubble forms can help a bubbler determine when the film is thinning out, indicating that it is time to close a bubble. A bubble juice chef can observe the color profile bubbles and adjust the dilution to achieve a desired film thickness (since dilution influences soap film thickness).
The colors are useful in determing the effective dilution range for a particular detergent. When there is relatively little surfactant, the soap film will be very thick because the surface tension will be high. Such films will be dominated by pink and green. With increasing detergent concentration, the soap film becomes thinner producing the colors seen on the left portion of the chart. Once you reach a particular concentration called the critical micelle concentration adding more detergent has relatively little impact on the film thickness and surface tension. (JAN 2015 NOTE: There has been a question raised on SBF as to whether or not the changes in film thickness are the result of surface tension changes. There seems no question that film thickness varies with dilution but the cause may not be a change in surface tension as it has been proposed that the Critical Micelle Concentration and Critical Dilution are very different.)
The color information can be used to tune your bubble juice in the field to a particular film thickness. Thin films can be useful for maximizing size potential (in some situations), and thick films can be useful for extending bubble life.
Dilution and ColorEdit
Dilution can have a big impact on bubble colors because of the way that dilution influences the soap film thickness. These color variations are especially noticeable with medium and big bubbles. They are harder to see with small bubbles. Soap dilution has some unusual properties. Read more about it in the Dilution article.
DemonstrationEditThis video demonstrations the profound affect that dilution has on film thickness and color.
In the pictures below, pay close attention to the colors at the tops of the bubbles. These color profiles are consistent across bubble sessions as long as the lighting is similar. The difference between the 24:1 and 19:1 is fairly subtle. The difference between the 19:1 and 16:1 bubble are quite pronounced despite the much smaller difference in concentration. Changes in concentration have relatively small impacts when the concentration rises above the Critical Dilution. For Dawn Pro, the Critical Dilution seems to be somewhere between 16:1 and 18:1. The turquoise and yellow that appear at the top of the 24:1 and 19:12 bubbles are found at about the mid-line of the 16:1 bubble indicating a much thinner film for the 16:1 bubbles.
Very very dilute solutions create very thick films may even be colorless and glossy -- as opposed to very very thin film that are colorless, glossless, and almost invisible. Very dilute solutions (where the surfactant concentration is low) have thick films that result in films dominated by shades of green and red. As the dilution becomes less extreme, the greens, reds, and pinks become more saturated, and more purples, yellows, and blues start to appear. Once the Critical Micelle Concentration (CMC), has been reached, further increases in concentration have decreasing impact on the surface tension, film thickness and color. (EDITOR: clean up confusion about CMC as CMC and critical dilution may be different).
These color changes are related to changes in the solution surface tension. Due to the way that surfactants work, these changes occur primarily in the neighborhood of the CMC. The dilution ratios where this occurs varies (sometimes considerably) from detergent to detergent. The CMC (or at least the surface tension at a given dilution) seems to be influenced by the pH for some detergents (such as Charmy Dish Detergent ).
The images below are of similar-sized bubbles created with PEO-based bubble juice whose only difference was the soap dilution. Note how each dilution has a somewhat different color profile:
For a detailed analysis of Dawn Pro by dilution, see the article See Dawn Pro Dilution Evaluation.
See also: blog entry that compares HEC giants with dilutions from 16:1 to 24:1.
Thank you to Bjorn of Seifenblasenmann.defor the use of his chart.
Index of RefractionEdit
Some question has been raised as to whether the assumptions about the relationship between color and film thickness are influenced by the components of the bubble juice. A physicist specializing in optics gave some thought to the problem and expressed confidence that the difference in the index of refraction of mixes would not vary enough to have significant impact on the relationship.
Soap Films And Molecules Edit
As explained in The Secret Molecular Life of Bubbles, it is explained how the colors of a soap film were used as evidence in establishing the that matter is made up of discrete particles. It is a fascinating article that we highly recommend.
How many molecules thick is a soap film? Since molecules are not either cubes or spheres, the question can be precisely answered. There seems to be some consensus that you can treat soap molecules as being on the order of 4.5 nanometers thick and water water molecules as being contained by a cube roughly 0.278 nanometers per side. With these estimates, we find that the amber color at the top of the color chart indicatesa film about 150 nanometers thick. This would be 33 soap molecules thick or about 540 water molecules thick. The "black film" (a thickness less than 25 nanometers) is less than 6 soap molecules or 90 water molecules thick. The turquoise typical of the top of a newly-formed bubble made with a 24:1 Dawn Pro dilution is about 270 nanometers thick or almost 1000 water molecules thick. You can see why on a warm dry day, it can be useful to have a thicker film.
The surfactant molecules in modern detergents may be larger than this.
A note about the color chart. If you do research, you will find a few charts which map color to thickness. There is some small disagreements between the different available charts due to slightly different assumptions about the parameters such as index of refraction, viewing angle, etc.
Dawn Pro Color and Dilution Evaluation. Click here to see an evaluation of Dawn Pro and the color profiles at different dilutions.Vertical color chart. Vertically-oriented color chart with some helpful markings.