Popcorn Physics

After 2 hours searching through the list of movies on Netflix instant watch, you’ve finally settled on one. You dim the lights, grab the popcorn, and put it in the microwave. As you mindlessly watch your prey repeatedly make 360 degree turns in place, you begin to ponder. What exactly makes the popcorn pop? What are the physics involved? Well my friend, we now have the answer.

Researchers in France just published a study in which they captured video footage of a kernel being popped using a high-speed camera. As part of the study, the researchers recorded kernels popping at  several incremental temperatures. Oddly enough, while only 34% of the kernels popped at 338°F, a whopping 96% popped at 356°F. In addition, the scientists measured the strength of the popcorn’s outer “hull” and the radius of the kernel, before and after it popped. They then determined mathematically, the maximum pressure a kernel can withstand before popping. Interestingly, this point was found to be highly dependent upon temperature, and not so much on the geometric shape of the kernel.

So then why does popcorn make that popping sound? The scientists conjectured that this is either due to the hull cracking open, the release of highly pressurized water vapor from inside the kernel, or from the movement of the popcorn. By matching up audio recordings of the popping to their high-speed camera footage, they found out that the pop only occurs after the kernel breaks, when the water vapor is released. They write that “the pressure drop excites cavities inside the popcorn as if it were an acoustic resonator” (basically, like how a guitar or drum works). Finally, it also turns out the reason why the kernels jump is that upon cracking, the kernel grows a sort of “starchy leg” that compresses under the heat, then immediately expands in a spring-like fashion to give it forward motion.

Now, this is normally the part where I tell you what the scientists were actually trying to achieve through their experiments, but this time there was no “other part to the story”. This time the scientists, quite literally, were interested in what the physics behind popping popcorn are. But who cares, clearly these experiments were carried out for the “butterment” of science, which is what really counts at the end of the day!

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