Why water balloons popped underwater seem to explode

A YouTube video, shot at 36,000 frames per second, shows a balloon popping underwater. What’s going on there?

|
YouTube
A water balloon pops underwater.

Why are slow-motion YouTube videos so popular? Maybe because they allow us to view the world in a new way. We’ve all seen someone sneeze, so we know the action looks (and feels) like. But slow it down and you get a whole new perspective. Brows furrow. Eyelids flutter. Snot issues forth in alarming quantities. An everyday action becomes fascinating because of the thousands of hitherto-unseen details that make it up.

It’s the same story with a balloon popping underwater. You can imagine the process in your head: As the balloon is pricked, the tension in the rubber surface is release and it contracts rapidly, allowing the water inside and outside the balloon to mix. But when you watch the balloon popping in super slow-motion, you notice something surprising: the water inside the balloon seems to boil, erupting into the surrounding liquid with speed and ferocity.

What’s going on there? Presumably the water temperatures inside and outside the balloon are about equal – so why the eruption when they suddenly meet?

There are actually two kinds of instabilities at work here, causing the violent-looking reaction between the balloon water and the surrounding water. The first is Rayleigh-Taylor instability, which describes the interaction between two fluids of differing densities. John William Strutt, also known as Lord Rayleigh, discovered that the more-dense fluid will disturb the less-dense one, causing “perturbations” along the boundary between the two and leading to further disturbance. G. I. Taylor, a British physicist, later realized that the same thing can happen when two fluids are accelerated – such as when the balloon itself disappears and the water inside accelerates outward into its surroundings. That’s why, after the initial ripple caused by the contraction of the balloon’s surface, the water inside begins to bubble.

Rayleigh-Taylor instability explains the rippling seen as the two bodies of water initially collide. But it gives an incomplete picture, since it describes fluids with different densities, such as oil and water. Why does the balloon water expand outward so violently? That behavior is described by Richtmeyer-Meshkov instability, which explains how small perturbations in fluids can grow linearly over time to form highly chaotic interactions. You can see that chaos in the video, as the balloon water rolls and roils, eventually floating upward to the surface of the surrounding water.

The underwater balloon-popping video is undeniably cool because it shows a surprising interaction that’s normally invisible to our eyes. The video was filmed with a special camera at 36,000 frames per second, and the whole interaction takes only about 0.16 seconds in real time. Without a high-speed camera, you might blink and miss all of these complex interactions.

If you’re interested in seeing other surprising interactions only observable with high-speed cameras, check out paint exploding (at 15,000 frames per second) and a man being slapped in the face (at 4,000 frames per second). Watching paint dry is the standard for boredom, but with a high-speed camera, watching paint explode is fascinating.

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to Why water balloons popped underwater seem to explode
Read this article in
https://www.csmonitor.com/Science/Science-Notebook/2015/0313/Why-water-balloons-popped-underwater-seem-to-explode
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe