Thursday, July 5th, 2007--

If packed correctly, tiny copper rods will swarm and swirl like a school of fish, according to a study published in the journal Science this week. The study indicates that swarming behavior—previously seen only in biological systems—may, at some level, be mindless.

“I would say the broad lesson is that you can get this kind of behavior without complex biological mechanisms at work. I think we’re sort of defining a baseline with which brainless objects act,” says Narayanan Menon, a physicist at the University of Massachusetts Amherst, and co-author of the study.

Menon and his colleagues Vijay Narayan and Sriram Ramaswamy packed a single layer of rods—each about 4.5 mm in length, .8 mm in diameter—in a solution-filled container. The container was shaken, causing the rods to move.

But the rods didn’t just move; they moved in unison, swarming around the dish. "It really looks like they have some volition, as if they are heading in some direction because they have a reason to go there," Menon says. "Of course, they have no reason!"  

To swarm, the rods must first align themselves so they are parallel, which they do naturally. This type of automatic alignment is commonly associated with liquid crystals and is called nematic order.

“Liquid crystals tend to order this way for packing reasons. It’s like logs in a stream, if there are enough of them, they’ll tend to want to point one way,” Menon says. The rolling-pin-shaped rods in this experiment align for the same reason, according to Menon. The only force that governs the movement of the rods is that no two rods can be in the same spot at the same time.

Yet the pointy rods, unlike liquid crystals, swarm into tightly packed clumps. “I think that is the most remarkable thing here—that there are big variations in populations from spot to spot.” It is not entirely clear why the swarming occurs, but it is related to how the rods line up. They move in one direction because of "small imperfections in the ordering and the alignment at any given spot," Menon says.

Although Sriram Ramaswamy, co-author on the paper, created a mathematical model a few years ago that predicted swarming, one surprise was that not all rods swarm. Originally, the researchers tried using rods that were not shaped like rolling pins. These tubular rods did not swarm, nor even align themselves.

To find out why some particles swarm and others do not, "physicists will increasingly subject rice grains, rods, needles, disks, and other non-spherical objects to shaking and shearing in the coming years," writes Martin van Hecke, a physicist from Leiden University in the Netherlands, in a corresponding "Perspectives" piece in Science.

Studying flocking in inanimate populations could inform how we understand similar behavior in biological populations, Menon says. "Some of the things that we find, we hope, can be exported back into the arena of population dynamics or whatever it is that biologists might be interested in."

What did you think of the story? Send us some feedback.

--Flora Lichtman

Sources

Narayanan Menon
Department of Physics University of Massachusetts Amherst Amherst, MA

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