The physics behind building a durable soap bubble

The physics behind building a durable soap bubble

Bubble blowing, in addition to being a favorite pastime for children, also happens to be 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 various processes at play in these innocuous-looking structures.

“Bubbles are important because they occur in many places, including detergents, cosmetics, building materials and also in nature. For example, sea foam plays a role in atmosphere-sea exchanges,” said Boulogne.

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

Taking the temperature

In a recent study, Boulogne and Rio established the role played by the surface temperature of the bubbles in their stability. “In some cases, bubble aging and bursting was associated with soap film thickness. Recently, researchers have begun to associate the thinness of the soap film with evaporation. However, in our study, we pointed out that heat transfer, which is associated with evaporation, was not taken into account,” said Boulogne.

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

Boulogne said that while there is a link between temperature and bubble aging, the impact of cold temperatures on when bubbles appear remains to be understood — and is likely to remain so for some time. “Until now, we have no model that can make this prediction. “Understanding the stability of bubbles is a challenge that will take several decades,” he said.

He argued that there are several factors to consider when it comes to bubble stability. “This includes temperature, evaporation rate, film thinning, marginal regeneration (the phenomenon of small spots, which are thinner and lighter than the surrounding film, rising to the top) and geometry. Having all these factors in one model is a challenge.”

Building the perfect bubble

While predicting the stability of bubbles in different scenarios can take some time, Rio has identified an optimal combination of ingredients to make bubbles last longer, but at the same time be easy to create.

The key to longevity is glycerol. The other ingredients include a long polymer like natural guar gum and the “optimal proportion” of dishwashing liquid. “If you add more dishwashing liquid, creating bubbles becomes easier. However, their lifespan is shortened. That’s why you have to find the right amount of dishwashing liquid to make sure the bubbles last long enough and are easy to generate,” said Rio.

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

Although the addition of glycerol may make the bubbles more stable, Rio said the impact of other ingredients on bubble stability is still an open question. “Glycerol is a hydroscopic molecule that can help condense water. But we know that surfactant (dishwashing liquid) and polymer also influence evaporation. So the next step in our study is to find out how our recipe influences evaporation,” said Rio.

Rio added that evaporation, which is not yet fully understood, is just one phenomenon that plays a role in the bursting of bubbles. “You also have to consider gravity, which contributes to the thinning of the surface, leading to fluctuation in film thickness. All of this makes it extremely difficult to predict when a bubble will burst,” Rio said.

Dhananjay Khadilkar is a journalist based in Paris.

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