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A Tough Little Droplet Fights To Stick Around

You drop into the world. You are beautiful, full-bodied, ready for anything, but the world is bigger than you — to your surprise, you are small. You try to make your way, but the big world spits you out, so you fight your way back.

And because life is like that, you get hit again ...

And again.

And again.

Each time, you try to rebound ...

But each time, you've got a little less to offer.

A little less bounce.

A little less of yourself.

But still you don't quit ...

You keep bouncing back.

I should warn you: The people who shot this video didn't think they were telling a story, a parable for life. They were studying fluid mechanics. Life just crept in and turned their science experiment into a poem ...

Just so you know, we are looking at an effect called the coalescence cascade, shot with a high-speed camera. It's a very slowed-down version of a drop of liquid falling gently onto a surface of the same liquid.

As Nicole Sharp explains on her saucily named blog about fluid dynamics (FYFD):

"When a droplet impacts a pool at low speed, a layer of air trapped beneath the droplet can often prevent it from immediately coalescing into the pool. As that air layer drains away, surface tension pulls some of the droplet's mass into the pool while a smaller droplet is ejected. When it bounces off the surface of the water, the process is repeated, and the droplet grows smaller and smaller until surface tension is able to completely absorb it into the pool."

And yet, I root for that little droplet; I do a happy dance every time it reappears, flinging its smaller and smaller self from that giant pool, trying, it seems to me, to keep itself going.

The Toughest Little Droplet Ever?

I'm not alone. Robert T. Gonzales, an editor at io9, the daily blog from Gawker Media, was so taken by the many returns of the droplet, that he searched around, looking for an even better bouncer. In the video we've just seen, he writes, "the effect manages to repeat itself four times (in what scientists who study fluid mechanics call 'events') before the viscous properties of the resting pool become too strong for the smallest drops to withstand coalescing completely." Is four (which includes the first drop) the limit?

No. Gonzales found a better one! Here's a five-bouncer ...

OK, five, then. How about six? What's the outer limit on droplet toughness? The great boxer Jake LaMotta, (played by Robert De Niro in Raging Bull), took a million blows from Sugar Ray Robinson and never fell down. Is there a LaMotta droplet out there somewhere? Gonzales looked, and believes the highest number of bounces ever seen was seven. Gonzales says that John Bush, a professor of applied mathematics at the Massachusetts Institute of Technology who has studied coalescence cascades, once saw that many; but no corroborating video has ever been published. Too bad. It could have been released as "Raging Droplet."

Thanks to Aatish Bhatia for pointing me to these videos. The first one was shot by Steven Trainoff and Noah Philip, using their own high-speed camera. The second one comes from theLaboratory of Porous Media and Thermophysical Properties at Brazil's Universidade Federal in Santa Catarina. The droplets, I realize, don't really fling themselves from the fluid below; they are resting on a thin blanket of air, which, when it dissolves, captures part of the drop, but leaves a remainder.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Robert Krulwich works on radio, podcasts, video, the blogosphere. He has been called "the most inventive network reporter in television" by TV Guide.
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