(This article is in progress. This article is based on the pre-2015 Dawn Pro. An update is planned, but the information here should still be useful in terms of getting a feel for how dilution differences influence film thickness.)
The image below illustrates the color profiles and film thickness of bubbles created with Dawn Pro at various dilutions. The photos were taken in the same location on different days (with varying degrees of cloud cover). Photos taken on many occasions with these dilutions were examined. The color profiles are quite consistent across sessions even when the wick materials and loop sizes differed.
This page is provided as a resource for people evaluating the detergents that they use.
See Color and Film Thickness for more information about this topic.
The Critical Dilution (the dilution at which there is rapidly decreasing influence when adding more soap) seems to be somewhere around 16:1 to 18:1. Adding more detergent has increasingly little effect. You'll notice in the images above that the color profile at 8:1 is not very different from 16:1 despite containing twice as much detergent. Some additives may shift the critical dilution slightly.
It must be mentioned that all the bubbles shown on this page were made with pH-adusted solutions using either baking powder (for the guar mixes) or baking soda+citric acid for the HEC mixes. pH-adjusted solutions (with a target pH between 7.2 and 7.8) created thinner films than solutions with pHs above 8.0 (which are typical of solutions made with tap or most distilled water). Also, guar gum, thickens the film slightly. So, you may see slightly different color profiles at these dilutions.
This article needs to be expanded with additional information about the behavior of the soap solutions at dilutions illustrated. 16:1 and lower. Somewhere around 16:1 (possibly slightly higher), the last bubbles of multi-bubble dips (or the last bubbles from a garland) will be very thin on top resulting in bubbles that are almost colorless (or with a little bit of amber). With long tubes, the last portion of the tube will tend to be colorless. As the bubbles thin, they may quickly thin above the mid-line to result in large almost colorless sections.
32:1: similar to 40:1 but shifted somewhat thinner. Dark blue is prominent just below the top of the bubble. On multi-bubble dips, the last bubbles have color. Film Thickness: 400 - 770(?). The blue that appears at the top of the bubble seems to be the one that is at about 400 nm on the color key; however, the color key shows some turquoise and yellow between the green (the one below the below) and magenta which we don't see. Perhaps, the key is not quite correct here?
50:1: thicker film than with 40:1. Colors look similar to 40:1 but 'muddier' or less saturated. Easy to make bubbles if polymer levels are adjusted. The size potential (especially for tubes) much less than at lower dilutions but on low humidity days, there are distinct gains in longevity due to the thick film -- which changes color beautifully as it evaporates Above 50:1. With dilutions more than a little higher than 50:1, the bubbles are nearly colorless, and it is hard (regardless of polymer level) to close anything but small bubbles.
More Examples Edit
The following pair of photographs demonstrates the influence of pH on the film thickness. These tubes were the same length and taken moments apart (to control for lighting as best as possible). The solution whose pH was 7.4 has a noticeably thinner film than the solution whose pH was adjusted to 8.3. The solution at 8.3 has a similar profile to the 25:1 solution whose pH was adjusted to 7.6. Notice the prominence of the amber/gold color in the upper picture and the magenta and green in the lower picture.
As a bubble grows and starts to push the wick's limit to feed the bubble, the film may undergo additional thinning as seen below. The solution is the pH 7.4 20:1 mix taken within minutes of the other two photos above. Notice the appearance of the turquoise and blue due to the film thinning due to stretching as the wick's ability to feed the bubble is challenged.
At 25:1, before the bubble is "stretched", blue, green, and magenta dominate with a hint of gold at the top which will become more prominent as the film either stretches or ages. As any bubble ages, its film thins due to evaporation and gravity. As s result, the colors will shift over time to the profile of a thinner film.