When Water Flows Uphill

In the Leidenfrost Effect, a water droplet will float on a layer of its own vapor if heated to a certain temperature. This common cooking phenomenon takes center stage in a series of playful experiments by physicists at England's University of Bath, who discovered new and fun means to manipulate the movement of water.

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IRA FLATOW, HOST:

Up now, where Luke - hey, Luke Groskin is sitting right next me. As I turn around and look at him. And our Video Pick of the Week. Hi, Luke.

LUKE GROSKIN, BYLINE: Hi, Ira.

FLATOW: What have you got today cooking, so to speak?

GROSKIN: So funny you would say cooking. So today we're going to explore a phenomenon known as the Leidenfrost Effect. So you've probably seen this while you've been cooking. So you take a pan, not a Teflon pan but a cast iron pan, and you put a little water on it and you start to boil it.

FLATOW: That's how you test whether it's hot enough.

GROSKIN: Yes.

FLATOW: If the pan is hot enough, you take some water and you throw it on the there and all these water droplets...

GROSKIN: Exactly. Exactly.

FLATOW: And that's the effect.

GROSKIN: No. No.

FLATOW: It's not.

GROSKIN: You have to keep heating it up. You keep heating it and heating it and heating it. And once you reach a certain point, all of sudden, the droplets will - instead of just immediately evaporating...

FLATOW: Right.

GROSKIN: ...they'll just suddenly start to float there.

FLATOW: They're dancing.

GROSKIN: Yeah. Exactly. So, I mean, think of it like an air hockey table. You think of the puck that was on there, like it's kind of floating around on the air.

FLATOW: Yeah.

GROSKIN: So what's actually going on when this occurs is that the water droplet is actually floating on a thin layer of its own vapor. So the...

FLATOW: Steam.

GROSKIN: Yeah. So the pan is so hot it's creating steam underneath the droplet and the droplet is kind of spherizing and it's just kind of floating there. And it's really, really cool to look at and it's really beautiful to look at. And so this researcher from the University of Oregon, this fellow named Heiner Linke, he studies microfluidics, he discovered that when you take one of these droplets that's heated up to a certain degree that it starts to float like that.

FLATOW: Right.

GROSKIN: And you put it on a surface that looks kind of like a really, really gradual staircase, the droplets all of a sudden start to propel themselves up the staircase.

FLATOW: They walk themselves up?

GROSKIN: Yes. They...

FLATOW: They climb up the staircase.

GROSKIN: They climb the stairs. Yeah. It's really, really cool. And especially when you watch it, like, in slow motion it's really beautiful. So these other set of researchers at the University of Bath in the Department of Physics, they started playing around with this notion of climbing and self-propelled droplets of water. So they first started testing by changing the stairs into, like, ridges. And they made them sharper and sharper until the droplets started to be able to climb those stairs at a steeper incline. Then they started noticing that they could get them to climb even better if they dropped the temperature just below that so-called Leidenfrost point.

FLATOW: Mm-hmm.

GROSKIN: So before they're floating but they're sort of floating but they're not exactly floating. So they still have a little wetness so they're gripping the teeth.

And while they started playing with the temperature, they noticed that they could control the direction of the droplets. So they go under the Leidenfrost temperature and the droplets curve to the left. They go over the Leidenfrost temperature, then the droplets curve to the right. So when you start putting all these pieces together and you put a bunch of undergraduates together, they get a little creative.

FLATOW: Yeah?

GROSKIN: And they create a little art public awareness project.

FLATOW: Right.

GROSKIN: They created a maze.

FLATOW: They could get the droplets to run through a maze?

GROSKIN: Yes. The droplets and run through the maze completely on their own.

FLATOW: So you drop the water in and it takes off through the maze on its own?

GROSKIN: Exactly. And it's absolutely spectacular to watch.

FLATOW: It's like a video game. And speaking of watching it, so that's our Video Pick of the Week. It's Luke's video droplet maze up there.

GROSKIN: Yes. So go to sciencefriday.com, check it out. If you like it, share it.

FLATOW: Share it. And this is something you can try at home, at least, making the droplets.

GROSKIN: Yes.

FLATOW: It looks like a video game.

GROSKIN: Yes, it does.

FLATOW: I mean, you look at it, it's like...

GROSKIN: It does.

FLATOW: It's almost like it's a combination of ping pong, bouncing off the sides of walls, going around a maze.

GROSKIN: Yes. And if you're a kid, do this with some supervision. You know, you'll get into hot water if you do this without supervision.

FLATOW: If you're lucky.

GROSKIN: Yes. Yes.

(LAUGHTER)

GROSKIN: If you're lucky.

FLATOW: I don't know. I guess you could not really build the maze yourself.

GROSKIN: No.

FLATOW: But if you're - you need a little bit of equipment to build that.

GROSKIN: Yeah. You need a dremel and you need all sorts of gear. And if you look at the video, you could. If you have that sort of gear you can create it. It's pretty - it just runs itself.

FLATOW: Yes. If you build one, we will see it. So take a picture of it, if you build this maze. It's our Video Pick of the Week. It's up there on our website. It's really amazing to watch these droplets of water ping-ponging around, all bouncing off walls, going up those stairs. And as if they're, you know, and they escape.

GROSKIN: On a mind of their own. Yes.

FLATOW: On the mind of their own. Thank you, Luke.

GROSKIN: No problem.

FLATOW: That's about all the time we have for today.

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