- Do thick solutions make thick bubbles?
- Do thick solutions make strong bubbles?
The answer to these questions is: not necessarily. In fact, for many recipes, the answer is a resounding, "No."
This confusing situation is the result of the word "thick" in English being used to describe two different things. "Thick" when used to discuss a solution is being used to mean viscous/slow-flowing. "Thick" applied to a bubble refers to the thickness/depth of the bubble's walls. Viscosity and bubble wall thickness do not have a direct connection. In fact, they sometimes have an inverse relationship. You can have thin, watery solutions that create thick-walled bubbles and very viscous solutions that create thin-walled bubbles.
Viscosity and Bubble Walls
Bubble wall thickness is largely (but not entirely: pH, for instance, has an impact on some solutions) a function of the detergent concentration. Low detergent concentration (high dilution) leads to thick bubble walls. High concentration (low dilution) leads to thin bubble walls. The thickness of bubble walls can be determined by the bubble's colors as explained in Color and Film Thickness. Viscosity, however, does not play a role in the thickness of the soap film.
People often think that because their detergents are thick (as in viscous) adding more detergent will lead to bubbles with thick walls. In fact, the opposite is true. Increasing the amount of detergent actually leads to thinner bubble walls which leads to bubbles with shorter lifespans due to the unavoidable thinning due to gravity and evaporation.
Viscosity is a measure of a liquid's resistance to flow which determines how slowly it flows but that rate of flow does not influence the soap film thickness. The rate at which the bubble juice flows does not influence the thickness of the resulting bubble walls. The viscosity may influence how rapidly the solution can flow from the wand, how rapidly the bubble can expand and how quickly the solution will drain from top to bottom, but it does not relate to the bubble wall thickness.
Example. You can have very viscous solutions that create thin-walled bubbles. For example, Dawn Ultra with no extra water, is extremely "thick" (viscous). It is as thick as many thick syrups. However, if you dip a wand into it and blow bubbles, the bubbles are actually thin-walled which allows you to get a very large number of bubbles from a small volume of solution.
Some polymers both make a solution more viscous and also make the bubble walls thicker. But, this is not true of all polymers. So, in some cases, increasing viscosity by adding more polymer will result in thicker bubble walls, but increasing viscosity by adding more detergent leads to thinner-walled bubbles.
Quite a few polymers make solutions more viscous but have minimal influence on the wall thickness.
For more detailed information on this and the related topics, see these pages:
Viscosity, Wall Thickness and Bubble Strength
There is no fixed relationship between bubble strength and either viscosity or wall thickness. Some thin-walled bubbles are stronger than thicker-walled bubbles, and some watery solutions yield very strong bubbles that are not made stronger by making the solutions more viscous.
There is also no single 'strength' property. Bubbles can be strong or long-lived with respect to some forces but weak in other aspects. For instance, some bubbles may seem strong under certain conditions but fragile in others. A solution might create bubbles that last relatively long in low humidity but break easily when exposed to wind or particulates. Another solution might be resistant to breakage in high humidity and yet be quite fragile in low humidity.
Also, it has been found that bubbles with similar thicknesses may have different strength and longevity properties depending on their ingredients. This situation is no different than with any sort of material. A house with thick unreinforced mud walls is not necessarily stronger than one with thinner but reinforced walls.