Street performers making giant soap bubbles have inspired a new discovery in fluid mechanics.
Researchers at Emory University found that mixing different molecular sizes of polymers within a solution increases the ability of a thin film to stretch without breaking.
In layman’s terms: Your backyard bubble-blowing is about to get a lot more impressive.
“This study definitely puts the fun into fundamental science,” senior paper author Justin Burton, an associate professor of physics at Emory, said in a statement.
The findings, published in the unfortunately named journal Physical Review Fluids, could also help improve the flow of oils through industrial pipes and clear polluting foams from streams and rivers.
Fluid dynamics—the branch of applied science concerned with movement of liquid and gases—is a main focus of Burton’s lab.
“The processes … are visually beautiful and they are everywhere on our planet,” he explained. “From the formation and breakup of droplets and bubbles to the aerodynamics of airplanes and the deep-sea overturning of the world’s oceans.”
While in Barcelona for a conference, Burton watched street performers create massive bubbles by dipping a thick cotton string into soap solution.
“These bubbles were about the diameter of a hula hoop and as much as a car-length long,” he recalled. “They were also beautiful, with color changes from red to green to bluish tones on their surface.”
(I’ve seen them, too. But was only moved to take photographs, not make a scientific breakthrough.)
For Burton, the stunning sight sparked a physics question: How could such a microscopically thin film maintain its integrity over such a large distance without breaking up?
Practice, I always assumed.
A physicist’s recipe
for giant soap bubbles (via Emory University)
During extensive online research, Burton learned the secret ingredient to any good giant bubble: a polymer additive, which enable the suds to flow smoothly and stretch further without popping.
With help from then-students and study co-authors Stephen Frazier and Xinyi Jiang, he began experimenting with different recipes—making and bursting bubbles and recording their speed and dynamics.
“The polymer strands become entangled, something like a hairball, forming longer strands that don’t want to break apart,” according to Burton. “In the right combination, a polymer allows a soap film to reach a ‘sweet spot’ that’s viscous but also stretchy—just not so stretchy that it rips apart.”
Varying their molecular sizes, meanwhile, helps strengthen soap film—a “fundamental” physics observation the team made entirely by accident (after abandoning containers of polyethylene glycol for six months).
“Sometimes your bubble gets burst, but in this case, we discovered something interesting,” Burton said.
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