Chapter 3: Figure and Ground
49 - Liquid Logic
When a fish swings its tail, it leaves behind a swirl of spinning water. As the tail swings back, it stirs another eddy. Each stroke sends a liquid swirl spinning in the opposite direction. Though that seems commonplace, research now shows that fish use their tails to arrange those swirls in precise patterns, and that this behavior is a key to their exceptional speed and mobility. Each swirl reduces drag from the wake even as it provides something for the fish to press its tail against on the return stroke, giving it more power.
One of the new biologists responsible for that insight, Michael Triantafyllou, is a naval architect and a professor of ocean engineering at the Massachusetts Institute of Technology. With his colleagues he has designed a model that wriggles through water with an efficiency near that of living fish.
Walking alongside the tanks in his lab, Triantafyllou points to a container of particles that fluoresce when they are thrown in the water and illuminated with laser beams. “We throw thousands of particles in here,” he says. “When we shine the laser it’s like looking at the night sky, with thousands of little stars. Any motion is immediately reflected. Then we take photographs of that, which give us the velocities and what’s happening in the water around the fish.”
The MIT fish is a five-foot-long model tuna, with an aluminum skeleton and joints sheathed by a smooth skin of latex foam covered with Lycra, the material used for swimsuits. The many joints let it undulate back and forth much as real fish do. The Robotuna team employed directed evolution to refine its design through some two thousand generations. Computers also figure in a feedback system now in development that will help it to adjust to changed conditions in the water.
Robotuna approaches the swimming efficiency of a living tuna, which reaches speeds of up to forty knots. A comparably powered research sub, driven by propellers and lacking fishy streamlining, might reach six knots. That could be one reason funding for this work comes from the National Oceanic and Atmospheric Administration and the Naval Research Laboratory. Someday schools of swimming robofish may sample ocean nutrients or probe volcanic vents, test for toxic spills or go exploring through sunken wrecks.
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