Scientists Use Party Gag to Explain NFL Brain Injury

Broken bottles share a lot with broken brains.

Mark Rober

Several YouTube videos show cool bros smacking the mouth of a liquid-filled bottle to make its bottom shatter into wet shards. It’s a useful trick for getting a few “whoa, dudes” at parties, but scientists recently found that it’s also a great way to visualize what happens when a brain gets injured.

In an article published in the Proceedings of the National Academy of Sciences on Thursday, an international team of engineering professors explained how the bottle-shattering process, technically known as cavitation, is linked to brain trauma. Preventing injuries to the brain is a major concern in the NFL these days, especially after a study published in July showed that nearly 100 percent of deceased football players showed evidence of chronic traumatic encephalopathy, a severe, degenerative brain disease resulting from repeated brain trauma.

“The brain is surrounded by fluid, and when you have impact, it’s possible you are experiencing cavitation within that fluid,” said study co-author Scott Thomson, associate professor of mechanical engineering at Brigham Young University, in a statement.

The scientists took a series of high-speed videos of the cavitation process to show what exactly happens when you hit a closed container filled with non-carbonated liquid (more on that later), like a bottle of juice or a football player’s skull. Watching it in slow motion, it’s clear that it isn’t the sudden strike at the top of the container that breaks the bottle. Instead, it’s the bubbles that do it — specifically, their sudden collapse.

When you hit the top of the bottle, its acceleration increases downward, creating pressure near the bottom. That sudden pressure spike forces gas out of the liquid, forming bubbles. All these bubbles rise through the liquid and suddenly burst in a phenomenon called cavitation collapse, and the burst sends a shockwave through the bottom of the bottle, causing it to shatter.

The sudden collapse of bubbles sends a shockwave through the glass, causing it to shatter.

Pan et al./PNAS

Somewhat counterintuitively, the same doesn’t happen to bottles filled with carbonated drinks, so don’t expect smashing results with a bottle of beer. With fizzy drinks, the researchers explain, the bubbles form through cavitation fill with dissolved carbon dioxide, preventing them from collapsing.

While cavitation science is nothing new — engineers have long known it’s responsible for damaging pipes and marine propellers — the paper outlines a new formula for predicting when, exactly, cavitation will happen after a strike.

The researchers are hoping that their new formula will help predict when injury to the brain occurs after the skull slams into something hard. In football, repeated strikes to the head frequently lead to concussions, which are injuries caused by the brain shaking around the skull in its surrounding brain juices. Experiencing repeated concussions and other traumatic brain injuries has been linked to the development of CTE, which has serious psychological symptoms and is currently untreatable.

When it comes to dealing with football-induced brain damage, prevention is key. Using the cavitation formula to predict when brain injury will occur will allow researchers to “better design safety devices to help prevent serious brain damage,” Thomson says.

Like this party trick, football is entertaining in its violence, but in the end, the costs aren’t exactly worth it. Whether it consists of needlessly wasted drinks or damaged brains, the price we pay is arguably already too steep.

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