Georgia Institute of Technology

Engineers are studying their swarm behavior in hopes of one day developing self-healing material.

When news spread that South Carolina was infested with “squirming mats of fire ants” after its historic flooding in October, the Internet went wild. As it turns out, these living rafts are more than just a viral sensation.

Engineers, biologists, and physicists have been studying the collective behavior of fire ants for years. Unlike other species, these ants build rafts during floods to stay afloat for days—as well as construct bridges, ladders, and walls—by linking their bodies together. The hope is that someday people can learn from ants to develop building material that can better respond to nature, and perhaps even heal itself.

That’s still ages away, says David Hu, a mechanical engineer at Georgia Institute of Technology who’s been studying fire ants for nearly a decade. But his latest study on the mechanical properties of ant swarms, published last week in Nature Materials, brings the world one tiny step closer to making all that a reality.

Hu and his colleagues applied different pressures on groups of ants and found that, collectively, they can respond like both a solid and a liquid. When the researchers pushed a raft of ants underwater, they sprang back up and returned to their original shape, like a solid. And when researchers dropped a penny into a wall of ants, the tiny critters unlinked their bodies to let the coin pass through and then relinked to fill the gap behind the coin, similar to liquid.

To measure the flow of ants under pressure, the team sandwiched clumps of ants between two plates of a rheometer—a device used to measure the viscosity of liquids—and gently wiggled the top plate at different speeds. At higher speeds, meaning more stress, the ants acted more like liquid. In a way, the ants are like ketchup, which gets thinner and flows more easily the harder you shake the bottle.

A swarm of fire ants behaves like active or living material, which is robust because it respond to its environment. Currently, active material exists only on a microscopic level, says Alberto Fernandez-Nieves, a physicist at Georgia Tech who worked on the study. Think skin cells that heal themselves after a cut, or water molecules that speed up or slow down according to the temperature.

Engineers are trying to figure out how to mimic that behavior to one day develop self-healing material. "It's an active research problem that they've been working on it for the last 20 years,” says Hu. “In the U.S., we have a lot of problems with our infrastructure getting old, and everything we've built has to be repaired after a certain number of years: pipes, buildings, bridges—anything that has to support large loads.”

The basic idea is to build “a bucket of stuff that we can pour out and that stuff will just make arbitrarily large robots,” says Hu. Before that can happen, though, there’s still a lot of questions to answer. Hu and his colleagues are trying to understand how each individual ant behaves within a swarm, and how the need for oxygen and communication affect swarm behavior. “When you build small robots, they’re also going to need energy and ways to communicate with each other,” he says.

“The thing is, materials that have their own energy and move around, it’s a totally different set of rules for them,” Hu adds.”no one knows how to even describe them or how to predict what they do. It’s kind of like a new branch of physics.”

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