Physicists Tie Water Into Knots

Reporting in the journal Nature Physics, William Irvine and Dustin Kleckner, physicists at the University of Chicago, have created a knotted fluid vortex in the lab ? a scientific first, they say. The knots resemble smoke rings ? except these are made of water, and they're shaped like pretzels, not donuts. Understanding knottiness has extra-large applications, like understanding dynamics of the sun.

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IRA FLATOW, HOST:

Flora Lichtman, our correspondent and managing editor for video is here with our...

FLORA LICHTMAN, BYLINE: Video Pick of the Week.

FLATOW: You've got it.

LICHTMAN: I got it.

FLATOW: Let me just remind everybody that I'm Ira Flatow with Flora. This is SCIENCE FRIDAY from NPR. And our Video Pick...

LICHTMAN: This week, Ira, it's out there. This one's loopy. That's a joke, and you're going to find out why.

FLATOW: It's literally loopy.

LICHTMAN: It's literally loopy.

FLATOW: You're not - you were looking at me saying loopy. So...

(LAUGHTER)

LICHTMAN: No.

FLATOW: OK.

LICHTMAN: The Video Pick. The Video Pick this week is a study that appeared in Nature Physics from William Irvine and Dustin Kleckner at the University of Chicago. They're physicists, and they did a scientific first, they believe, which is they tied water into a knot.

FLATOW: They tied water into a knot.

LICHTMAN: What could that even mean, you might be thinking.

(LAUGHTER)

FLATOW: You have to see it.

LICHTMAN: You have to see it.

FLATOW: You have to see it. It's on our website up there at sciencefriday.com. But...

LICHTMAN: It's really neat. OK. Here's how you can understand or here's how I did. If you take a smoke ring, people are familiar with these, these vortex rings, right?

FLATOW: Right.

LICHTMAN: So the kind of donuts of smoke. And then you were able to twist that flow, the fluid flow into a pretzel, you would have what these researchers have created.

FLATOW: Like a knot, a pretzel - tied up into a pretzel.

LICHTMAN: A pretzel of bubbles and water.

FLATOW: So you can see the bubbles - so it starts out like a smoke ring and then gets twisted into a pretzel of bubbles?

LICHTMAN: No. But...

(LAUGHTER)

LICHTMAN: ...here's out here's how it works. They - and this is part of the innovation of this study.

FLATOW: Yeah.

LICHTMAN: They printed these three - using a 3-D printer. 3-D printer saves the day again, right?

FLATOW: Of course.

LICHTMAN: These wings, they look sort of like twisted - it looks a lot like a pretzel, but in a wing shape and it traces the outline of the knot. And then they put that in water and little bubbles attached to it and then they accelerate it really fast and the bubbles fly off and the water flows in this knotted fluid flow. You have to see it. It's...

FLATOW: You have to see it. You know, and one of the fun parts about the video, which is up there on our Video Pick of the Week at sciencefriday.com, is also watching them blow smoke rings out of a cannon. When you talk about...

LICHTMAN: There's an added bonus. That was from Dan (Unintelligible) in University of Maryland, only semi-related. But, you know, the same idea. The idea is to understand how fluids flow, and this is an idea, this idea of knottiness of fluid, so air or water, had been suggested over a hundred a years ago by Lord Kelvin. So it's an old idea, but no one had managed to actually make one of these in the lab until now. And the reason, you know, even - it's kind of out there and interesting on its own, but there are also applications. For instance, the sun's corona, which you may have seen from NASA, these beautiful images of the sun...

FLATOW: Oh, yeah.

LICHTMAN: ...with these projectiles of plasma, those are thought to be knotty too. And so part of this is trying to understand what happens to these knotted liquids. Do the knots untie? If they untie, how do they untie? Do you conserve knottiness? That's the idea. Maybe knottiness doesn't quite go away. Maybe it has to be translated into twisting. So there's really some interesting questions raised by this - interesting physical questions.

FLATOW: You can't make this at home, though, right? You can't make these bubble knots on your own at home, I don't think yet.

LICHTMAN: You've got to have a very - I was trying to think if you could do like a smoke ring with your tongue - I don't think so.

(LAUGHTER)

FLATOW: No, no. Blow a smoke ring...

LICHTMAN: It's way too complicated.

FLATOW: By the way, what's also gorgeous about the video up there is that there are these three dimensional - you go around them, right?

LICHTMAN: Yeah. Thanks for bringing that up, because I think one of the most amazing parts of this study is how they image to this. OK. So first of all we're talking very high-speed video. But then to see the knot of liquid - they use bubbles to see it - but they have to do it very fast and with a laser. So the laser scans across the bubble knot, you know, many times over the course of a second. I think it's like a hundred times, making these sheets of images and then they are stacked together digitally so that you have a 3D picture of what the knot looks like. And then you can fly around the knot.

FLATOW: It's like a CAT scan of - a CAT scan without the X-rays.

LICHTMAN: Exactly.

FLATOW: It's like a CAT scan of a 3D - and you fly around there and it's all up there on our Video Pick of the Week. If you want to - beside the cannon that shoots these great smokes.

LICHTMAN: Ira, you really love the cannon.

FLATOW: I love the cannon part. The three dimensional bubble images. How you take bubbles and they twist them into a pretzel is quite fascinating, Flora. Thank you.

LICHTMAN: Thanks, Ira.

FLATOW: So our Video Pick of the Week up there on our website at sciencefriday.com and also you can get them downloaded onto your app. We have a video app that you can get up there. We have a new SCIENCE FRIDAY app up there on iTunes, brand new. It's got great new features on it. I want to make sure you download it.

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Source: http://www.npr.org/2013/03/15/174409388/physicists-tie-water-into-knots?ft=1&f=1007

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