| abstract
| - 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. The color differences between the bubbles above is solely due to the amount of detergent. See this blog post. 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 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. See the article Dawn Pro Dilution Evaluation to see the color profiles that result from different dilutions of Dawn Pro dishwashing liquid.
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