Go pop a balloon. Then, come back.
How did you do it? Did you poke it with a pin? Or did you overinflate it until it popped? Well, you probably did neither, because you don't have any balloons (good for you; that's normal), but if you had, the method you used would have dramatically changed what was actually happening with the balloon. Pop the balloon, and it splits into two pieces. Overinflate it, and it shatters into tiny fragments. Now, French physicists have explained why.
However, first, a demonstration. In this video from the Laboratoire de Physique Statistique, Parisian researchers Sébastien Moulinet and Mokhtar Adda-Bedia set up a simple protocol to film what happens when a balloon pops. In one clip, the balloon is pierced by a blade. In the other, it pops once it fills with too much air. They slowed down the tape to 1/2,000th of the usual speed and captured the rubber actually splitting into either two parts (with the blade) or several parts (with the overinflation).
The physicists then published a paper in Physical Review Letters, in which they explained why these processes differ. In particular, they focused on the bursting model.
"We show that a dynamic instability drives this branching mechanism," reads the study. "Fragmentation occurs when the crack tip speed attains a critical velocity for which tip splitting becomes the sole available mechanism of releasing the stored elastic energy."
In other words, it all comes down to air pressure. By definition, a balloon that is punctured is less-inflated than a balloon that bursts (otherwise, the first balloon would have burst already). When the pin hits the balloon, the air sees a chance to escape and takes the quickest route: by forming a single crack. However, at the higher air pressure, the balloon must fraction into multiple cracks to get the air out.
Interestingly, Moline took his inspiration for the study from art:
"I started to get interested in this question after seeing an artistic photograph of an exploding balloon," he says. "I noticed that the cracks seemed to be separated by the same distance," he explained in a press release.
After the experiment, the researchers concluded that the more air pressure, the more fragments or "branches" were created when the balloon burst.
The entire study is available at the American Physical Society. A free account is required.
