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Here’s How to Blow the Perfect Giant Soap Bubble, According to Physics

Sometimes, science really blows

Now that's a big bubble. (Public Domain)
smithsonianmag.com

Even the teeniest of soap bubbles can be fragile little things. When blown up to the size of a person or a car, these sudsy wonders get downright delicate, lasting only seconds before bursting in a tragic wet pop.

In a paper published last week in Physical Review Fluids, a team of fluid dynamics experts at Emory University in Atlanta, Georgia, has mixed up the ideal recipe to generate bubbles up to 100,000 liters in size—big enough to swallow a Volkswagen Beetle.

Key to their soapy solution is the addition of polymers, or long chains of repeating molecules, of different lengths, writes Lissie Connors for Physics Buzz. Like a Gordian knot of sorts, these stringy chains become entangled when mixed, generating a mesh that keeps bubbles intact—even when they’re blown up to gargantuan size.

The Emory team isn’t the first to tackle the physics of soap bubbles. As Jennifer Ouellette reports for Ars Technica, scientists have been enchanted by the amorphous blobs and their thin, light-reflecting surfaces since at least the 1800s, when a Belgian physicist formally described the physical laws governing the structure of the thin, soapy films. Now, some two centuries later, researchers have seriously expanded the bubble research repertoire. We now know a lot more about how the sizes and shapes of different bubble wands affect their final products, and the optimal speeds at which to inflate bubbles of different sizes.

Rather than focusing on bubble-blowing equipment, the latest addition gets at the soapy stuff itself.

Inspired by the giant bubbles produced by Spanish street performers, physicist Justin Burton decided to investigate the ingredients that best hold sudsy solutions together. He and his team first turned to Soap Bubble Wiki, a delightfully titled open-access database for bubble-blowing enthusiasts, or “bubblers.” After perusing a dizzying array of recipes and bubble-blowing tips, the team tested several of the best candidate concoctions in the lab.

Recorded on a high speed camera, different bubble concoctions blew up and burst at different speeds and with different textures, cluing the team into which ingredients conferred the most structural integrity. “Focusing on a fluid at its most violent moments can tell you a lot about its underlying physics,” Burton says in a statement.

All standard recipes included two crucial ingredients: water and dishwashing liquid. On their own, this dynamic duo is stuff enough for bubbles, which hold together thanks to a touch of surface tension. To counteract water’s tight pull, soaps and detergents thin out the solution, reducing surface tension just enough to allow bubbles to expand while still maintaining an energy-efficient spherical shape.

The addition of polymers can then push the limits of this delicate interplay even further. Like seams stitched into fabric, they hold a structure together, giving it the strength to stretch without breaking apart. The most common polymers included in the recipes, the team observed, were guar, a common food additive, and a lubricant called polyethylene glycol. Both did the trick, giving the researchers’ biggest bubbles more longevity.

But the best addition turned out to be polyethylene glycol that had, in a sense, expired. Left in a storage container for six months, some of the polymers had degraded into shorter chains and created a mixed bag of differently-sized strings of molecules. When mixed into soapy water, this irregular jumble did an even better job of tangling up and strengthening the bubbles produced, Burton explains in the statement.

Bubblers eager to hit the streets with Volkswagen-Beetle-sized suds can find a detailed recipe here. As Andrew Liszewski reports for Gizmodo, the ideal wands will be those made of thick strings or ropes, which can absorb a bit of the soap mixture, creating an unbroken connection between burgeoning bubble and wand while easing the fluid’s transition into the air.

You might want to wait until summer when humity and temperatures level rise, according to Physics Buzz. Warmth and humidity are apparently friends to suds, delaying the inevitable evaporation that will thin and ultimately rupture the soapy cocktail’s thin film. Sorry to burst your bubble, but even with the perfect ingredients, these frothy concoctions can’t last forever.

About Katherine J. Wu
Katherine J. Wu

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.

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