In this lab, popping balloons is serious stuff

What happens in the fraction of a second when a balloon goes POP!?

It may seem like a trivial question, but the answer could provide a better understanding of how aneurysms burst and why airplanes occasionally fall apart in midair.

Yet scientists have spent relatively little time trying to understand exactly how a balloon comes apart.

Researchers knew that balloons could pop in one of two ways.

If you stick a pin into a moderately inflated balloon, the result will be a crack that races around its circumference, splitting it into two large fragments. However, if you keep blowing air into a balloon until it bursts, it will divide into a multitude of fragments in what looks like a flower pattern.

(You can see this for yourself if you’ve got a couple of balloons and a high tolerance for loud noises.)

A pair of researchers decided they wanted to know more. They were inspired to explore the physics of balloon fragmentation after encountering a remarkable photograph of a balloon in the process of bursting.

The photo was taken by Jacques Honvault, a French engineer and scientist who uses high-speed photography to observe phenomena that cannot be seen with the naked eye.

“I had never thought much about it before, but in the photo I noticed the regularity of the distances between the cracks of the exploding balloon,” said Sébastien Moulinet, a researcher at the Laboratory of Statistical Physics at the École Normale Supérieure in Paris. “This image triggered my curiosity.”

Moulinet teamed up with his colleague Mokhtar Adda-Bedia, a specialist in fracture science, to try to figure out what determined the distance between the cracks they observed in the photo. In particular, they wanted to understand the tipping point where a ballon goes from opening up along a single line to bursting into multiple strands.

To do this, they created an contraption that consisted of a metal frame with a two-inch hole in the center, a flat piece of latex, a pressure sensor, an air inlet tube, two high-speed cameras and an X-Acto blade. Moulinet and Adda-Bedia also secured two pairs of heavy-duty earmuffs — protection for their ear drums.

“You really need it when you make a 1-millimeter-thick balloon explode,” Moulinet said.

To run the experiment, the researchers attached the latex behind the frame and turned on the air. Within a few seconds, the latex would inflate just like a party balloon. They repeated their experiment about 150 times.

By changing the position of the blade and the thickness of the latex, Moulinet and Adda-Bedia could see what happens when a balloon is popped in various states of inflation.

The apparatus may sound unnecessarily complex, but scientific rigor demanded it.

“We started with a classic rubber balloon, but it was difficult to have reproducible results,” Moulinet said. “A rubber balloon has a complicated shape, and its thickness is not homogenous. Using a latex sheet, we have a more controlled system.”

Each test was quick, lasting less than 15 seconds. Still, it was plenty of time for the high-speed cameras to observe the explosion in great detail, snapping pictures at rates of up to 60,000 frames per second.

The scientists positioned one camera just above where the blade struck the latex and the other camera off to the side. Together with readings from the pressure sensor, the researchers were able to determine the overall tension of each “balloon” when it popped.

The scientists came up with a grand unified theory of sorts to explain the way balloons pop. If a balloon is burst when it is just slightly inflated, the crack moves relatively slowly across the latex, splitting it into just a few pieces. As the tension of the balloon increases, the crack speeds up.

At a certain point, however, the cracks can’t go any faster.

“Instead, they are destabilized and split into two,” Moulinet said. “Then a tree-like network grows.”

This network — which resembles a series of Ys, like the branches of a tree — is what shreds a balloon that is stretched so thin it can no longer contain the air inside it. (And shred it does: In one of their tests, the researchers counted 64 fragments from a single piece of latex.)

The pair’s explosive results were recently published in the journal Physical Review Letters.

Krishnaswamy Ravi-Chandar, a professor of aerospace engineering at the University of Texas at Austin, said the results would have implications beyond birthday parties.

“The same principles of high-speed fracture are at work when you pop a balloon in a laboratory and other failure events,” said Ravi-Chandar, who was not involved in the study. “That’s what makes this work so exciting.

“Anyone can break a balloon, but to understand what happens well enough to use that information in other problems is complicated,” he added

Moulinet said his balloon-bursting days are not over yet.

“The fragments you get from an exploding balloon display a rich variety of shapes, like oscillating cracks or 90-degree turns,” he said. “We want to understand how such shapes occur.”

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