Water Striders Don't Stride, They Row

David Hu, mechanical engineer at Georgia Tech, wanted to understand the basic physics of how water striders glide. By filming them stride on food coloring and building his own robotic strider, he found out that the secret to the stride is in the paddle.

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IRA FLATOW, host: Next up, Flora Lichtman is here, multimedia editor with our Video Pick of the Week.

FLORA LICHTMAN: Hi, Ira.

FLATOW: Hi there.

LICHTMAN: Hi.

FLATOW: And it's...

LICHTMAN: This - I was going to tell you some of the components of this video...

FLATOW: OK.

LICHTMAN: ...because it's mysterious if give them to you: food coloring...

FLATOW: Food coloring.

LICHTMAN: ...robots...

FLATOW: Ooh.

LICHTMAN: ...high speed video of water striders fighting...

FLATOW: See if you can find that anywhere else.

LICHTMAN: ...and water acting like a trampoline.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: I mean, really, like, I don't...

FLATOW: You've got to be...

LICHTMAN: What could this be about?

FLATOW: Probably couldn't buy that if they wanted to.

LICHTMAN: OK. But let me set up the question for you.

FLATOW: Stay with that. Go ahead.

LICHTMAN: David Hu is interested in water striders, you know, the little bugs that you see on top of the water gliding around. And it turns out that they don't really stride on water, they row. They row their middle legs on water. But here's the perplexing thing. Their legs are really spindly, so it would be like paddling with a twig.

FLATOW: Yeah.

LICHTMAN: And so the question is, how do you propel yourself when you have these really skinny legs? And so this is where the food coloring comes in. It turns out, if you put a water strider on water that's filled with food coloring, you can see how the water is moved by the strider's motions. So they saw these - when they did that, that the striders are making these vortexes. What you said, actually, airplanes also...

FLATOW: Airplanes, they have wing tips. They all create vortices when they're flying, so they're moving air or they're moving water.

LICHTMAN: In this case, where jellyfish also make vortices.

FLATOW: Right.

LICHTMAN: So these was a clue to how they're doing it. And what Hu discovered or what he hypothesized was that they're using - they're pushing on the water sort of making this indent, and then they're using that indent to push more water. So the water becomes kind of like a paddle, the end of the paddle, and that's how they're creating the vortex.

FLATOW: So they're using the water to push the water, it's as you say. Yeah.

LICHTMAN: Right. Right, right, right. Like a water oar (unintelligible).

FLATOW: That answers the question how these tiny, little skinny legs could push all that water.

LICHTMAN: Right. So to test this - and here comes the robot - they made their own version, which is - this is the best part...

(SOUNDBITE OF LAUGHTER)

LICHTMAN: ...it was made out of a 7-Up - aluminum can and an athletic sock. Really, they're like - there was an almost - there are, you know, some...

FLATOW: How do you do that? You have...

LICHTMAN: It's amazing.

FLATOW: You have to see it. Go to our website, @sciencefriday.com, up there in the left side is our Video Pick of the Week. What do we have - we have a name for this?

LICHTMAN: Well, the...

FLATOW: The strider?

LICHTMAN: He calls it the robostrider.

FLATOW: The - you can't make this one at home.

LICHTMAN: I asked. He said...

FLATOW: Can you?

(SOUNDBITE OF LAUGHTER)

LICHTMAN: Funny you should ask.

FLATOW: Why?

LICHTMAN: I was thinking what to do this weekend with Hurricane Irene, maybe make robostriders.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: He said, you can make it. It is very difficult to tune the legs so that they spin at the right force, because you don't want the legs to break the water. You just want them to push the water. So getting your wind up part right is a little bit difficult. But he said, making one that just sits on the water is not that difficult. So check out the video and you can see what they look like.

FLATOW: Check out the video. You'll see the video. It's our Video Pick of the Week. It's cool. Watch this robotic little robostriders and see how they move and watch the vortexes. They're really cool looking.

LICHTMAN: Yeah. And the real striders - one of the best parts of this video is just - he calls it www.havefunonthewater. These water striders really going crazy. They're tiny, so surface tension keeps them up, but it's fun to see them jump around.

FLATOW: There you go. When you're watching by candlelight this weekend...

(SOUNDBITE OF LAUGHTER)

LICHTMAN: Yeah.

FLATOW: ...and you want to have something to talk...

LICHTMAN: Or your generator.

FLATOW: ...something fun to talk about as the wind is howling. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: Flora Lichtman with our Video Pick of the Week. That's about all the time we have for today.

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