There’s a science behind the art of blowing soap bubbles.
It’s not the thickness of the soapy film but rather the speed of the blowing gust of air that determines whether bubbles will emerge, scientists in France report in the Feb. 19 Physical Review Letters.
“We have all blown soap bubbles,” says study coauthor Laurent Courbin, a physicist at the University of Rennes in France. “It’s nice to be able to explain simple experiments that we have all experienced in our lives.”
Courbin and his colleagues looked through centuries of studies, but they couldn’t find any explanation of the physics behind bubble blowing. So the team built a device that pumps a controlled, uniform layer of bubble liquid over a roughly 1-meter-high opening of adjustable width. The scientists peppered this large, precise cousin of a bubble wand with pressurized jets of air, light helium or heavy sulfur hexafluoride gas. Quick gusts pushed out bubbles, but slow-moving jets only dimpled the film.
The scientists came up with a set of equations that predict this minimum speed cutoff under different conditions — for example, when a gas-pumping nozzle is placed right against the bubble layer or when a gust is wider than the film itself (testing that required the use of smaller circular bubble wands).
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Factors including the density of the gas, the width of the soapy film and how far a gas-blowing nozzle was from the film determined how fast a gas jet had to travel to make a bubble, the team found. So did nozzle size: The researchers blew bubbles of increasing size starting with tubes smaller than a millimeter across and going up to wind tunnels as big as 20 centimeters wide. For most experiments, the minimum speed that it took to blow a bubble ranged from 10 meters per second to 100 meters per second.
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NEED FOR SPEED Slow-moving jets of gas make an indentation in a layer of soapy fluid. The jets must reach a minimum speed, which depends on factors including the size and position of the gas-squirting nozzle, before they make bubbles. L. Salkin et al/Physical Review Letters 2016
The study provides a nice illustration of physics principles, says Howard Stone, a fluid mechanics researcher at Princeton University. The researchers explored the science of physical interfaces, “but they took the motivation from something you have in a science museum, or even a children’s museum,” he says.
Now that the bubble basics have been established, future studies could test different fluids, Courbin says. The results might help inform other research, such as making foams, which use bubbles as building blocks. And bubbles have inherent value as study subjects. “They are useful for education, for practical reasons,” Courbin says, “and for fun, too.”