Designing The Pied Piper Of Fish

Mechanical engineer Maurizio Porfiri, of the Polytechnic Institute of New York University, designs robot fish. A few years ago, he found that real fish would mill about his aquatic robot, and now he's trying to understand why. His research suggests that it has less to do with how the robot looks, than how it makes fish feel.

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IRA FLATOW, HOST:

And now it's Flora Lichtman with our video pick of the week. Hi, Flora.

FLORA LICHTMAN, BYLINE: Hi, Ira.

FLATOW: What have you got for us today?

LICHTMAN: We're checking back in on fan favorite this week. It's the robo-fish part duex.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: You may...

FLATOW: When we last left.

LICHTMAN: Yeah, when we last left. This is an amazing story. So we visited a lab at Polytechnic Institute of NYU, and it's mechanical engineer Maurizio Porfiri's lab. And a couple of years ago, he shared with us this amazing video of his robotic fish swimming around a tank with little - giant danios, danios following it around, milling about the robot as it swam around. And this was, you know, this sort of blew my mind. It combines two of my favorite things: robots and fish. So I immediately loved it.

(SOUNDBITE OF LAUGHTER)

FLATOW: It's a Pied Piper fish, right?

LICHTMAN: Absolutely. And when we did that video, which you can see on our website, he was sort of still trying to figure out just why the real fish followed this robotic leader. So here we are a few years later and he has some insights into what's going on here. And what he found is that it has to do with the tail flapping. So basically - and he designed this - this is not like a coincidence. I mean, he designed the tail so - of the robot so that it moves water like a real fish would.

And what he did is set up an experiment in his lab in this long water tunnel, and you have water kind of cycling through, and you put the robot in and then you put a real fish in, and you see if you change the tail flapping, what does the real fish do? And it turns out if you flap the tail at certain rates, at certain water flows, the fish always positions itself, or most of the fish position themselves in this particular spot right behind and to the side of the robot. And anyone who's seen like Tour de France or, you know, race car driving...

FLATOW: NASCAR.

LICHTMAN: Yeah, exactly, know what's going on here. The fish is drafting.

FLATOW: In the water, the currents are set up to, like, pull you along. See, the other fish position themselves behind the robot fish. They draft the fish.

LICHTMAN: Yeah, they draft it. It's a hydrodynamic advantage, and the question is here...

FLATOW: (Unintelligible)

LICHTMAN: Yeah, and he actually has like a laser set-up, and you can watch the flow in slow motion of the particles. You can see these little swirls of water being sent in the fish's direction, and they think that this is helping propel it. So the interesting thing is, you know, you see that in the water tunnel, but what does that mean for fish schooling and for this robotic fish? And he thinks that, you know, maybe some of this behavior has to do with this hydrodynamic advantage, which I thought was really interesting, you know.

FLATOW: Yeah, yeah. So by schooling, they're actually helping themselves swim...

LICHTMAN: Right.

FLATOW: ...in the most efficient way.

LICHTMAN: Right. So the ones in the back may get an advantage by swimming behind the fish, but they, you know, pay the cost of not getting to food first. They also are safer because they don't get eaten first if a predator is coming right at them. So it's a complicated issue, and he's just looking at this one little piece, but...

FLATOW: It's fascinating. The video is - I call it the Pied Piper fish.

LICHTMAN: Yeah, absolutely.

FLATOW: Robo-fish and friends.

(SOUNDBITE OF LAUGHTER)

FLATOW: It's a great little video up there on our website at sciencefriday.com. It's our video pick of the week and...

LICHTMAN: And you can see this following, the milling about of the robots. One thing that he said that was really interesting to me is, you know, not every fish will follow. And so his next installment - so stay tuned, this could be number three in a few years - is to figure out how personality drives sort of the differences between the fish. It seems that the bolder fish are less apt to follow the robotic fish.

FLATOW: They go their own way.

LICHTMAN: They go their own way, exactly.

(SOUNDBITE OF LAUGHTER)

FLATOW: Got their own draft.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: Exactly.

FLATOW: And, you know, they go in circles, just like NASCAR. No...

(SOUNDBITE OF LAUGHTER)

FLATOW: It is up on our video pick of the week. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: And it's up there at sciencefriday.com, and also you can download it on your iTunes and also on your iPod to go with you.

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