- 1 About This Page
- 2 Dilution and Film Thickness
- 3 Critical Dilution
- 4 Dilution, Size Potential, Color and Longevity
- 5 Dilution and Concentration
- 6 Recommendations
- 7 See Also
About This Page
This is an important topic, and we are working on an extensive page. In the meantime, we hope that the little bit of information provided below will be helpful as this is an important and often poorly understood topic.
Edward has posted a blog page that is under-construction that has some very helpful information despite being incomplete.
Dilution and Film Thickness
Dilution refers to detergent dilution. It is a critical factor in determining bubble wall thickness which in turn determines the bubbles' color profile. With a little knowledge, you can tune the film thickness and colors of your bubbles by adjusting the dilution.
Dilution has a far greater impact on soap film thickness than any other factor including viscosity (which generally-speaking has little or no direct impact on the film thickness).
The dilution is generally indicated as the water-to-detergent ratio. For example, 16:1 indicates 16 parts water to 1 part detergent.
High dilution solutions result in thick soap films. Low dilution solutions result in thin films. However, when soap concentration increases above a level called the Critical Micelle Concentration (discussed below) adding more detergent has relatively little impact on the surface tension, film thickness, and color.
As you can see in the chart below, the range of colors seen in soap bubbles can vary dramatically with dilution.
Notice that the color profiles of 16:1 and 8:1 dilutions are quite similar even though the 8:1 dilution contains twice as much soap as a 16:1 dilution. On the other hand, there is a dramatic difference in the colors of bubbles at 24:1 and 16:1 despite a much smaller difference in dilution.
This happens because 16:1 is near the Critical Dilution (see below). Once a solution is at the critical dilution, adding more detergent has little impact on the film thickness or color profile.
This video demonstrations the profound affect that dilution has on film thickness and color.
Surface Tension and Film Thickness
Note: This section needs updating. The relationship between dilution and film thickness is well established, but the cause may not be changes in surface tension. The causal nexus between dilution and film thickness is not known for certain. This explanation may be in error. (April 2015, we are hoping to find a lab willing to measure the surface tension of various solutions in the hopes of exploring this more thoroughly.)
The surface tension of a soap solution is determined by the dilution. The surface tension more-or-less determines soap film thickness. The relationship between the film thickness and dilution is the opposite of what you might think. Solutions with a very small amount of detergent have a high surface tension (not much lower than water's) and results in a thick film. Solutions with higher detergent concentrations (which can result in viscous solutions) have lower surface tensions and, consequently, thinner films. So, a thick (as in viscous) solution often corresponds to a thin film!
This relationship between surface tension and film thickness is the result of the strong affinity that water molecules have for each other. They hold tightly to each other which normally allows them to gather in large thick groups. Surfactant molecules interfere with the water bonds, allowing water to spread into thin films.
Water's high surface tension is the reason that water normally beads on a table surface. It is the reason you can fill a glass of water so that the water surface rises slightly above the rim. Add a drop of soap and the surface tension is lowered: the water bead breaks, and the water surface can no longer rise above the rim.
Critical Micelle Concentration
Note: The relationship between Critical Dilution and Critical Micelle Concentration is unknown. Recent discussion on SBF (Winter 2015) has called into question whether there is a connection between CMC and Critical Dilution.
The phrase Critical Micelle Concentration seems to scare bubblers, but it is a simple and important concept. Soap bubbles are made possible by surfactants which you can think of as soap or detergent molecules. They reduce the surface tension of water which makes thin films possible.
Surfactants have a strange property. When you add a small amount of a surfactant (such as detergent) to water, it reduces the surface tension. As you add more and more, it reduces the surface tension more and more until you reach a particular concentration which is called the Critical Micelle Concentration (CMC). Once you reach that concentration, adding more detergent has negligible impact on the surface tension.
Each detergent has its own critical micelle concentration. Detergents with similar critical micelle concentrations can be used at similar dilutions. While it is difficult to directly find the Critical Micelle Concentration, you can easily find your detergent's Critical Dilution which you can use to tune your bubble juice.
For more information, see Critical Micelle Concentration
Critical Dilution is a concept we developed at Soapbubble Wiki to give mixologists a practical way of profiling and understanding their detergents and how dilution will impact their bubble juice.
The Critical Dilution is the dilution where adding more detergent suddenly has a decreasing impact on the film thickness (and its resulting color profile). Knowing your detergent's approximate critical dilution is key to being able to tune your solutions.
Different detergents -- sometimes even those in the same family -- can have different critical dilutions. For example, Dawn Pro, Dawn Ultra and Dawn PowerClean all have a critical dilution somewhere right around 16:1. Non-concentrated Dawn has a much lower (but not yet determined) critical dilution. Sun Sunsational Scents has a critical dilution that is somewhere around 5:1 or 6:1 (based on preliminary analysis).
Comparison of the color profile to that of Dawn Pro's can help you find your detergent's critical dilution.
Critical Dilution may be influenced by a solution's pH and in some (rare) circumstances the polymer amount or some other ingredients. For example, the excellent Japanese detergent, Charmy, has a critical dilution that can be shifted by a factor of 2 or more by adjusting its pH.
Dilution, Size Potential, Color and Longevity
MARCH 2014. There have been many requests recently for the long-promised article about the relationship between dilution and size potential. Unfortunately, my time has been very limited lately. I will add to this bit-by-bit. I suspect that it deserves an article all to itself. Here are some brief thoughts that I will expand as I am able.
(DILUTION EQUIVALENCE NOTE: In this article, the dilution values given are for Dawn Pro. If you use a different detergent, the actual dilution numbers would have to be adjusted accordingly.)
There is not a simple relationship between dilution and size potential as many factors interact together to influence the final result: dilution, wick, juice properties (such as flow rate and surface tension), environmental factors (temperature, humidity, wind speed, particulates and other as yet unidentified factors) and a bubbler's technique all influence the size potential.
In good, but not perfect conditions, with a particular loop, there may be a pretty well-defined relationship between size potential and dilution. But, if you use the same wick material at a different size, you may get quite different results as the size-limiting factors may change with the loop size (and material). For example, with large loops with reasonable capacity, and good, but not ideal conditions, the soap film's longevity may have a bigger influence on the size potential than the capacity or flow rate from the loop. The reason for this is that the time that a large bubble takes to form is long enough that it cannot be ignored whereas with small bubbles the formation time is relatively small compared to the film's longevity. In such case, a 20:1 or 25:1 dilution (especially if pH-adjusted) may make bubbles that are as large or larger than can be made with 13:1 or 16:1 dilution due to the increased lifespan. A smaller loop of the same material might be able to make much longer tubes with a 13:1 or 16:1 dilution due to the fact that the same volume of fluid can make a larger film (because they create thinner films than more dilute solutions).
With a low capacity loop, such as a single strand of twine, a solution at or slightly above the critical micelle concentration, may be able to create dramatically larger bubbles and more of them from a single dip than a more dilute solution because the juice volume is the limiting factor.
On a dry day, however, the medium-sized loop that made larger bubbles with a 16:1 dilution may make larger bubbles with a 25:1 or 30:1 dilution due to the increased longevity.
In many cases, especially for large loops with good capacity, the difference in size potential for closed bubbles may not be dramatically different in the range from about 16:1 - 24:1. For unclosed tubes, there may be a more noticeable difference.
The differences may change with wind speed. A steady wind may challenge a particular loop's flow rate. If that is the case, the increased effective flow rate (bubble-feet-per-second) you get with a more concentrated solution may give it an advantage because the rate at which the film can grow may be the limiting factor.
Color. People often ignore the impact that the dilution has on colors. As you increase the detergent concentration (and reduce the film thickness), the colors can change dramatically. With 16:1 and lower dilutions, you will often get colorless bubbles towards the end of multi-bubble dips or tubes with large colorless sections. People often comment to me that they can never get the same saturated colors that they see in some of the pictures on the wiki. There are two primary reasons, lighting and dilution. A lot of people use much more concentrated solutions than I typically use. Large bubbles made with those more concentrated mixes often result in large areas of the bubble that are very thin and have little color. I also tend to photograph/video my bubble tests in an environment that leaves the bubbles lit by indirect light and against a dark background that shows the colors well. (See Evaluating Bubble Color.)
Size and Longevity. Sometimes there is a tradeoff between size potential and longevity. A thin film (from a less-dilute solution) may provide maximum bubble size (since a larger film can be produced from a given amount of solution) whereas a thicker film (from a more-dilute solution) may provide greater bubble longevity. In some conditions (such as when conditions are ideal or terrible), size may not come at the expensive of longevity. In other conditions, the thinner film that results in a maximum size may come at the expense of bubble lifetime. In some conditions, the lifetime of bubbles at 16:1 are quite noticeably shorter than with higher dilutions.
Recommendations. The tradeoffs are so intimately tied to the materials you use and your typical conditions that we recommend doing trials using solutions at 16:1, 20:1 and 25:1 (assuming a similar detergent strength to Dawn Pro -- adjust these dilutions accordingly). 16:1 is somewhere around what we call the critical dilution. Adding more soap may somewhat improve size potential but may have a big impact on bubble lifetime. Perform these trials with a couple of different size loops as you may find that you prefer one dilution for one size loop and another for a different one. Repeat the tests on a few occasions. If you can, use a different loop for each dilution -- it can take up to 10 dips (even if you wring out the loop between solutions) to fully clear the old dilution when moving between solutions. Make sure to perform 10 or more dips with each solution. Repeat the test on a different day. You may find that your preferences change over time. So, you may want to revisit the test as your skill changes.
Why does dilution influence size potential?
Dilution sometimes influences the "bubble-feet" (or "bubble-meters") that can be created with a single wand dip. Many factors come into play to determine actual size potential and many involve tradeoffs. The influence of dilution is due to dilution's influence on surface tension which in turn influences the soap film thickness. (See Color and Film Thickness.)
When solutions are very dilute, the surface tension is high which results in a thick soap film (because the water molecules hold tightly to each other creating a thick film). If the surface tension is lowered, the film will become thinner. So, the same volume of bubble juice will be able to cover a larger volume.
(Edward: add in a specific example demonstrating the bubble-feet of 1 tsp of bubble juice at 50:1 and25:1 and 16:1.)
When the detergent concentration is below the concentration called the Critical Micelle Concentration (CMC) the surface tension is in a direct inverse relation to the detergent concentration. More detergent (until the Critical Micelle Concentration is reached) results in a lower surface tension which results in a thinner film. Once the Critical Micelle Concentration has been reached, adding more detergent has little or no effect on the film thickness.
At the wiki, we have been using the phrase Critical Dilution to refer to the dilution at which changes to the film thickness become relatively small in relation to the amount of added detergent. The critical dilution is related to the CMC. For typical Dawn Pro mixes, the Critical Dilution seems to be about 15:1 or 16:1. The influence of the added detergent to the film thickness becomes noticeably less at this dilution. (See Dawn Pro Dilution Evaluation.)
Dilution and Concentration
Here is a handy listing of common dilutions expressed as ratios and their equivalent concentrations expressed as percent.
12:1 = 7.69%
14:1 = 6.67%
16:1 = 5.88%
18:1 = 5.26%
19:1 = 5%
20:1 = 4.76%
24:1 = 4%
25:1 = 3.85%
30:1 = 3.22%
32:1 = 3.03%
35:1 = 2.78%
40:1 = 2.44%
Dilution recommendations depend on many factors: the detergent being used, the equipment, the conditions, the type of bubbles, etc. You will find some recommendations for some recipes. In many cases, you will want to experiment yourself with a range of dilutions. When comparing solutions, keep a few things in mind:
- comparisons should be made in the same session with a different wick being used for each solution if at all possible. It really is worth making a few identical wicks for testing solutions.
- keep in mind that for very dilute solutions, you may need to increase the amount of polymer that you use in the solution
Dilution Q&A - common questions related to dilution