Help! I’m Trapped in a Drop of Water

I am looking at a blob of water. Not pond water. Not pool water. Just ordinary H2O floating about in what appears to be zero gravity. And inside its wetness there’s a passenger, a goldfish, a very alive goldfish …

… that is trying desperately to escape—or so it seems. It flings itself at the blob’s edge, pushing it outward.

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Then it tries to get out the back. That’s its head peeping through.

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Then it charges and stretches the skin of the bubble almost to the breaking point.

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But try as it might, it can’t get free. Poor little fish, I thought. It’s a prisoner.

I found it, the fish and its floating prison, on my favorite fluid dynamics blog, FYFD, which is curated by aerospace engineer Nicole Sharp. She had posted the video version produced by Professor Mark Weislogel and his students at Portland State University in Oregon. They used a “drop tower”— basically, a free-falling elevator—to create near-weightless conditions. So they’re the ones who packed the goldfish into its droplet-y cage. Here it is in full “get me out of here” motion.

What a struggle! As Nicole put it on her blog:

For years, I have wondered what a fish swimming in microgravity would look like. Finally, my curiosity has been rewarded. Here is a sphere of water in microgravity, complete with a fish. Personally, I am impressed that, despite the fish’s best efforts, the surface tension of the water is strong enough to keep it confined. This may not bode well for microgravity swimming pools at space hotels.

Got it. Fifty years from now, when I book my room at the Hotel Galactica a hundred miles from Earth, I’m not bringing a swimsuit. And anyway, who flies to the edge of space to go for a swim? Not me. Not you. We’d go for the view—obviously. So this video shouldn’t have bothered me. But after I saw it once, then again, then yet again, something about it didn’t seem … um … quite right.

EXCUSE ME, but …

What, exactly, is holding that fish inside the blob? Could it be the water? Here on Earth, fish have no trouble breaking through a pond surface to snatch a fly or a bug. Yes, the pond surface is a little resistant due to molecular or surface tension, but a goldfish is stronger than water. It whips its tail, propels itself up, and grabs lunch. No problem.

Is there something about zero gravity that changes that? I called my friend Henry Reich, author/illustrator and physics explainer over at Minute Physics. I showed him the video and asked, Is this fish trying to escape? And if it is, why can’t it get free?

“This fish is thrashing, yes,” he told me on the phone, “but I have a hunch it isn’t trying to escape. I think it’s testing its surroundings …”

Me: What do you mean “testing”?
Henry Reich: Well, I can’t think of any good reason why the fish couldn’t break through, even though it’s at zero gravity.
Me: So the surface isn’t holding it in?
HR: No, I don’t think so, and if you look at its fins and tail, you can see what’s really going on. Up close, you’ll see it’s paddling up to the edge, pushing forward and then, just when it could escape, its peddling back.
Me: You can see that?
HR: Yeah, look back at the video, and watch the fins. And the curled back tail. That’s what you’ll see …

I did. And I’m not sure if I saw what Reich saw. He sent me a video of goldfish swimming backward, and I couldn’t quite tell if my fish was doing what that fish did.

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Drawing by Robert Krulwich

But just to go along with his notion, I asked Reich: If our goldfish was able to burst out of the blob—if physics didn’t prevent it—what’s making it stay? Henry said, OK, remember that a) I am not a goldfish, and b) This is just a wild guess, but …

HR: I think it’s scared.
Me: Scared?
HR: Yes.
Me: Of what? It’s a goldfish.
HR: Of the strangeness of being in a bubble of water.

For millennia, he went on to say, fish have evolved in rivers, lakes, ponds, seas—places where the surface was always “up,” or above them. Fish have no experience with surfaces that are underneath them or to the left or right.

So here’s this poor fish that finds edges in all the wrong places. It’s encountering a world that’s totally strange, and it’s poking about, testing, and discovering—uh oh—an edge here, and, oh my, an edge here too?

Pardon the unpardonable anthropomorphism, but this fish is freaking out. “It’s thinking, This is weird,” says Reich.

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Drawing by Robert Krulwich

So maybe the bubble is not caging our fish? I have another physicist pal, Aatish Bhatia (whom I play with over at Noticing.co, where we solve puzzles together). He suggested that it’s possible the blob of water is pushing back on our fish—at least a little. Water at microgravity likes to be sphere-shaped. I might resist being splatted and stretched because, says Aatish, “The ideal shape of a water drop would be round … It’s the most compact shape possible.” So the water might be pressing back at the fish’s thrashes, but, like Reich, Aatish says do not pity this fish.

It’s no prisoner. It can break free. And then, lo and behold, Aatish proved it!

“Born Free, Free as the Wind Blows”

On October 23, 2014, Weislogel published a YouTube video from Portland State University. It was a lecture he gave, and I don’t know how Aatish found this, but 27 minutes in, up there on the big screen, is our entombed fish, the very one I saw—but with a different ending!

Apparently, the video on the fluid dynamics site was chopped, and in real life, our fish escapes! It flings itself out of the water blob, and breaks through! Here’s the moment:

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But Wait …

My heart leapt at the sight, until I thought, Wait a second, where did the fish go? It’s in an elevator dropping six floors at 55 miles an hour in a near weightless state heading to the ground floor of the engineering building. When the elevator lands this fish is going to, um, land with it. Did they retrieve it? Give it a medal? A martyr’s burial?

I called Weislogel.

Not to worry, he told me. The fish (not a goldfish, actually, but a neon tetra) belonged to his son’s friend. This was a high school experiment done on campus, and there were a number of fish involved, all of them on loan “from somebody’s aquarium.”

The one in that video, he said, was the “most curious” of the bunch. Many stayed stock still during weightlessness. Some curled. This one poked, probed, and, because it was so lively, it probably made several trips. It was his favorite.

OK, but—what about the landing?

“No fish was harmed in the making of this video,” Weislogel said in an announcer’s voice. When gravity kicked back in, both the blob of water and the fish settled softly (“like a baby in the womb kind of thing”) into a receiving bowl placed at the base of the chamber. Nobody died. All were, gently, returned to the aquarium.

Of course. I completely forgot this experiment was conducted in Portland, Oregon, the town that’s made unpleasantness illegal. If this story ever becomes a plotline on the TV show Portlandia, each fish will have its own monogrammed landing pillow.

I should never have worried.

Special thanks to Henry Reich, who was coming off a plane from Cambodia when he got my “Help, what’s this fish doing?” message and, before getting on his next plane, called to tell me his nutty—but, as it turned out, totally accurate – opinion of what was going on. And thanks also to Aatish Bhatia, who’s my partner over at the other blog, and the guy I go to to when I can’t figure someting out, because he always can (and would have found the Higgs boson in two minutes somewhere on the Internet, no need for an atom smasher, if he’d only been asked). He can find anything.