In the first serious study of the physics of fire-ant rafts, researchers have described how the insects form floating, waterproof islands.

In nature, the rafts allow fire ants to survive epic rainstorms in their native Brazil. In the lab, they could help inspire designs for small, swarming robots that might someday be used to explore inaccessible areas or even clean up oil spills.

“The ant raft, up to this point, has been little more than just categorized and documented,” said mechanical engineer Nathan Mlot of the Georgia Institute of Technology, lead author of a paper in the April 25 Proceedings of the National Academy of Sciences. “We were coming at it from an engineering perspective.”

Even though ants’ exoskeletons naturally repel water, a lone ant dropped in a bucket will flounder. But whole colonies of fire ants can float downstream for weeks at a time when flushed from their underground nests. Mlot and his graduate advisor, David Hu, wondered what held the dense mass afloat — and whether it could be harnessed for other applications.

“How are the ants actually linking in the raft?” Mlot said. “We could speculate all we wanted, but the only way to know for sure was to get visual data.”

Mlot’s team collected thousands of fire ants (Solenopsis invicta) by roadsides in Atlanta, where the stinging pests are an invasive species. They immediately noticed that clumps of ants take on the consistency of soft playdough. Ant masses flow like honey or ketchup, and can be described using equations usually found in fluid dynamics.

“You could pick up a cluster of these ants and mold it in your hand. You could form it into a ball and toss it up in the air, and all the ants would stay together in one ball,” Mlot said. “They’re almost like a material.”

To set up a reproducible experiment, the team molded ants into balls by swirling them in a beaker. The ants’ natural tendency to stick together made them clump into near-perfect spheres.

Then the researchers placed balls of 500 to 8,000 ants into a water-filled filled container. Th ant sphere almost immediately relaxed into a flat, pancake-shaped raft, with ants on bottom forming a stable layer for the rest of the colony to rest on.

Surprisingly, the whole swarming mass remained delicately balanced atop the water’s surface. When the researchers tried to submerge the raft, water underneath deformed like a stretchy fabric, conforming to the raft’s underside contours.

Focusing on the details of this phenomenon, the researchers subjected their ants to a battery of bizarre tests. To measure how much force one ant could apply to another, they glued live ants to the bottom of a glass slide, then harnessed other ants to them with elastic bands. They painted ants with identification marks and charted their path across rafts. To investigate how the mechanics of raft formation in high resolution, they froze an entire ant raft in liquid nitrogen, then looked at it under a scanning electron microscope.

The images revealed that fire ants grip each other with their mandibles, claws and sticky pads at the end of their feet. Together, they form a tight weave similar to waterproof fabrics like Gore-Tex, which enhances the natural water-repelling properties of their bodies.

The team also built a simple mathematical model of raft formation. It might be used to inspire programs guiding cooperative robots.

“Robotics has often looked at insect communities for inspiration,” said roboticist James McLurkin of Rice University, who designs and builds robot swarms.

Roboticist Seth Goldstein of Carnegie Mellon University suggests that groups of small robots forming antlike rafts could be used to explore sewer lines or waterlogged caves. McClurkin even floated the idea, so to speak, of cleaning oil spills in the Gulf of Mexico.

As for Mlot and his ants, he didn’t lose any sleep over their fate. “After you get bit a couple of times, you lose your sympathy for them,” he said, adding that the experiments are simple enough that anyone can try them at home, “if they’re brave enough.”

Images & video: Nathan Mlot, Georgia Institute of Technology.

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Citation: “Fire ants self-assemble into hydrophobic rafts to survive floods.” Nathan J. Mlot, Craig A. Tovey and David L. Hu. PNAS, published online April 25, 2011. DOI: 10.1073.pnas.1015568108