Justin Burton’s fascination with soap bubbles began in Barcelona, Spain, six summers ago. “It was hot and humid, and there were guys on the street making these huge bubbles,” he says. “I thought it was the coolest thing.” When he returned home, his sister gave him a giant bubble wand. It included a recipe for a bubble-making solution. Burton began to blow. Now he’s improved the recipe and is ready to share it.
By the way, Burton is hardly your typical bubble-blower. As a physicist at Emory University in Atlanta, Ga., he studies forces in nature. Those forces include effects on fluids that flow. The more he thought about bubbles, the more he wanted to know just how they were made.
“What is it about bubble solutions that makes them [stretch] without ripping — and for such a long time?” he asks. Now, he’s found out. And he and his team reported the results of their scientific investigation on August 1 at arXiv.org.
Big bubbles exist at just the right balance of natural forces, they found. The bubble has to be stretchy, but not so much that it rips. And the wet soapy mixture has to flow evenly to form the bubble, resisting a tendency to pop. It also has to stay together, even as gravity pulls it down and some of the water evaporates.
In their new study, Burton and his coworkers identified the secret ingredient behind these traits: a polymer. It’s what holds the bubble together but lets it flow, too.
Polymers are long chains of identical molecules. In Burton’s case, he recommends one called guar. It’s made from natural fibers. Those fibers help form long-chain polymers when dissolved in water. That’s why guar is used as a thickener in many foods, including ice cream and puddings.
Adding polymers like guar to bubble recipes can produce colossal results. The official largest free-floating bubble was blown in 2015 in Cleveland, Ohio. It had a volume of 96.27 cubic meters (almost 3,400 cubic feet). That’s about the same volume as about 13,545 basketballs. It’s also about the size as a large classroom (although not in the same shape, of course.)
Perhaps most surprisingly, that record-setting bubble probably included only about 300 milliliters, or just over a cup, of bubble solution. Burton says that in such big bubbles, the soap film thins across the ballooning structure to just about one micrometer (also called a micron) thick. That’s a millionth of a meter (0.00004 inch).
“It’s just amazing that you can have a micron-thick film that extends to these dimensions,” Burton says.
The polymer advantage
“Polymer molecules added in soapy water not only increase the viscosity [thickness] of the liquid, but also change the specific way it flows,” says Laurent Courbin. He’s a physicist at the University of Rennes, in France. He didn’t work on this study with Burton, but he does study bubbles. A few years ago, he reported on how air flow can affect the formation of bubbles.
The Emory scientists used a number of different experiments to test bubbles. To measure the thickness of soap films, they built a device that could produce a soap film and hold it in place. Then, they shone a light through it. They couldn’t use visible light, which would pass through without changing. (That’s why you can see right through a bubble.) Instead, they used infrared (In-fruh-RED) light, which has longer wavelengths than ordinary light.
Water absorbs infrared light. (This is one reason why the air gets hot: Water vapor absorbs energy from the sun, warming the air molecules.) So when the infrared light passed through the soap film, some wavelengths were absorbed by water. By measuring the infrared light that did pass through, they could see how much had been absorbed. They used that information to calculate the thickness of the film.
They also rigged up devices to let the soap film form droplets on the end of small nozzles. As the droplets grew larger, gravity pulled them down. That created a thin thread of soap between the droplet and nozzle. Measuring that thread told them how far the soap could be stretched without snapping.
Big bubble-makers often share their recipes with others online. Many have reported on different concentrations of water, detergent and polymers. But the study by Burton’s team is likely the first to seriously analyze the chemistry of bubbles.
“I’ve never been aware of a study where people really tried to make a quantitive analysis of the components of the solution,” says Pascal Panizza. He didn’t work on the study, but he has worked with Courbin on bubble studies at the University of Rennes in France.
What’s funny, says Courbin, is that scientists are just now asking questions about the physics and chemistry of bubbles. “When you think about it, it should have been in the scientific literature 200 years ago,” he says. “Everybody blows bubbles.”
Note: This story was updated on October 17, 2019 to fix a conversion from meters to inches.