The physics behind building a permanent soap bubble

Blowing soap bubbles, besides being a favorite pastime for children, is also an art form and a topic of interest for physicists. Emmanuelle Rio, François Boulogne, Marina Pasquet and Frédéric Restagno of the Solid State Physics Laboratory at the University of Paris-Saclay have been studying bubbles for years, trying to understand the different processes played by these seemingly innocuous structures.

“Bubbles are important because they appear in many places, including in laundry products, cosmetics, building materials, and also in nature. For example, sea foam plays a role in terms of exchanges between the atmosphere and the sea.

Now, the team has described a key event in the life of bubbles: when they burst.

temperature measurement

In a recent study, he created Boulogne and Rio The role played by temperature from the surface of the bubbles in their stability. In some cases, the aging and bursting of bubbles was associated with the thickness of the soap layer. Recently, researchers have begun to associate the thinning of the soap layer with evaporation. However, in our study, we indicated that heat transfer, which is related to evaporation, was not taken into account,” Boulogne said.

To explore this aspect, the researchers measured the surface temperature of the bubbles and found a significant difference compared to room temperature. “The surface temperature of the bubbles can drop by as much as 8 degrees Celsius,” Boulogne said.

Boulogne mentioned that while there is a link between temperature and bubble aging, the effect of lower temperatures on the bubbles’ bursting time is still poorly understood — and will likely remain that way for a while. “So far, we don’t have a model that can make this prediction. Understanding the stability of bubbles is a challenge that will take many decades,” he said.

He saw that there were many factors to consider when it came to bubble stability. This includes temperature, evaporation rate, film thinning, fringe regeneration (the phenomenon of tiny spots, which are thinner and lighter than the surrounding layer, rising towards the top), and geometry. Getting all of these factors into one model is a huge challenge.”

Build the perfect bubble

While predicting bubbles to settle in different scenarios may take some time, Rio outlined Perfect combination of ingredients To make the bubbles last longer and at the same time be easy to create.

The key to longevity is glycerin. Other ingredients include a long polymer such as natural guar gum and the “optimal ratio” of dishwashing liquid. “If you add more dishwashing liquid, it becomes easier for bubbles to form. However, their life time is shortened. This is why you need to find the right amount of dishwashing liquid to ensure that the bubbles last long enough and are easy to form.”

Working with French artist Pierre-Yves Vozier, who specializes in bubble art, Rieu and her colleagues developed the recipe, which consists of 40 milliliters of dishwashing liquid, 100 milliliters of glycerin, and 1 gram of a long polymer such as natural guar. Gum is mixed in 1 liter of water. Using this recipe, Rio made bubbles 5 cm in diameter in her lab that lasted an hour.

While adding glycerin may make the bubbles more stable, Rio said the effect of other ingredients on the stability of the bubbles is still an open question. “Glycerin is a water molecule that can help thicken water. But we know that surfactants (dishwashing liquid) and the polymer also affect evaporation. Thus, the next step in our study is to find out how our recipe affects evaporation.”

Evaporation, which is not yet fully understood, is just one phenomenon that plays a role in bubble bursting, Rio added. “You also have to consider gravity, which contributes to surface thinning causing the film thickness to fluctuate. All of this makes it very difficult to predict when a bubble will burst.

Dhananjay Khadilkar is a journalist based in Paris.