For the past two decades, giant bubble enthusiasts have been creating soap film bubbles of ever-increasing volumes. As of 2020, the world record for a free-floating soap bubble stands at 96.27 cubic meters, a volume equal to about 25,000 U.S. gallons. For a spherical bubble, this corresponds to a diameter of more than 18 feet and a surface area of over 1,000 square feet—larger than some apartments!
A simple glance at the multitude of colors reflected by the bubble film suggests a film thickness on the order of a few microns, approximately the diameter of a human red blood cell. In other words, the film's size is nearly one million times its thickness, which is quite staggering considering that a single hole can lead to the film’s demise. How are such large films created, and how do they remain stable?
The secret, as they say, is in the sauce. The key to creating giant bubbles is controlling the viscoelasticity of the soap films. Viscoelasticity combines two properties of the soap solution: its viscosity, which describes a liquid's resistance to flow, and its elasticity, which describes how easily a material springs back into shape when deformed.
We know that the addition of long-chain polymer molecules creates optimum viscoelasticity for producing stable soap films. Two of the most common polymer types are guar gum and polyethylene oxide (PEO). When dissolved in water, guar forms long starch chains of varying sizes. PEO is more well-controlled, producing chains of more uniform size. In both cases, the polymer strands become entangled, something like a hairball, forming longer strands that don’t want to break apart. In the right combination, a polymer allows a soap film to reach a "sweet spot" that is viscous but also stretchy—just not so stretchy that it rips apart.
Most bubbles are held together by surface tension, but the physics of making giant bubbles with polymer solutions involves additional forces that are still poorly understood. The behavior of a soap bubble polymer solution depends on the properties of individual chains as well as the interactions between these chains. Polymer properties such as molecular weight, concentration, and chain length are exceedingly important. It's no wonder there exists a complete online wiki devoted to the fine-tuning of giant bubble recipes!
We have identified some of the underlying physical mechanisms by which PEO or guar give rise to giant bubbles. The best solutions for making bubbles have intermediate concentrations and a mixture of polymers of various molecular weights and chain lengths, allowing a large volume of liquid to be continuously drawn into the soap film without breaking. By measuring the thickness of films over time, we also found that a film's durability increases at higher concentrations than those often used in bubble solutions. This suggests that polymers may enhance film longevity even after some of the liquid has evaporated or drained out of the soap film.
Understanding the physics of polymer solutions and soap films is not just important to giant soap bubble enthusiasts. Foams with various additives contribute to long-lasting pollution in rivers and waterways contaminated with industrial runoff. Thus, the formation, mechanics, and stability of these solutions are also important environmental problems.
For more information and a physicist's recipe for making giant bubbles, check out the links below!