Ants may seem tiny and weak when they're alone, but together they can form a sort of "super-organism" -- one with superpowers. Researchers at the Georgia Institute of Technology have found that a jumbled crowd of fire ants acts like both an elastic solid and a viscous liquid -- a rare and remarkable property that holds the secrets of self-healing materials.
The discovery could one day help scientists design self-repairing bridges and self-assembling modular robots, said co-author David Hu, a mechanical engineer at Georgia Tech, at the American Physical Society's fluid dynamics conference in Pittsburgh.
Fire ants have a vicious, stinging bite and a ferocious instinct for survival that has turned them into a notorious invasive species in North America.
"They live in these places that are flooded, and they have to build things that are resilient to forces because they live in a very abusive environment," Hu said. "And we want to understand what the properties of this material [are] that allows them to respond to these environments."
Originally from Brazil, the ants survive flash floods by linking together to form a living life raft that can travel great distances over months at a time before settling down. They can build bridges out of their own bodies, making them both "the bricks and the construction workers," Hu said.
"It's kind of between material science and modular robotics," Hu said. "You study one ant, it's basically a robot. But then you combine them all together; it's not just a swarm, it's also a building material."
While studying these ant bridges, the scientists noticed something strange. Instead of falling apart like a normal bridge, the ant structures would actually get stronger, by shrinking and growing very stiff.
They also noticed that this ant "material" had both solid and fluid properties: A ball of ants could also bounce back to its original shape if compressed, like an elastic solid. And yet, when the scientists tossed a stick into a clump of ants, they seemed to flow around it like a liquid.
To find out how these critters seemed to switch phases on command, the scientists subjected live ant clumps to a rheometer, which is so sensitive that it can pick up the force from a single insect. In this device, scientists put whatever material they're studying in between two round metal plates, like the cream in an Oreo cookie. And then, as one does with Oreos, they twist the plates and see how the 'cream' reacts.
Cream is solid, because it fractures and breaks (as you can see any time you halve an Oreo by twisting it). If the stuff in the middle were honey, which is fluid, the metal plates would keep twisting and twisting.
Now, as shown in the bridge and the ball examples, ants can act pretty solid as a group. But in between the rheometer's plates, they acted more like honey would. In fact, the scientists found that the ants, en masse, can act like both a solid and a fluid to equal degrees, putting them in a rare class of material. (Dead ants, when subjected to the rheometer, acted like a plain old solid.)
The scientists think the ants' secret morphing power lies in how they join together, using jaws and arms and legs to link together, sort of like the molecules in a material. But unlike non-living materials, these ants are constantly moving, forming, breaking and re-forming these connections.
This ability allows the ants to store energy and dissipate it, depending on the situation. The findings could be useful in developing self-healing materials for bridges or buildings -- or for designing modular robots that can build new shapes without fitting together perfectly.
"The way nature does things is very different from the way we do," Hu said. "We build things out of usually solid blocky square structures that have to be rebuilt every time they're broken. And biology is totally opposite. They're usually squishy, usually round, and usually they self-heal."
For now, Hu says they want to understand each ant's individual role in creating this remarkable super-material, and then see how it applies to the structures they build.
"We'll basically consider it a victory when we know what one ant has to do to make these bridges not fall," Hu said. "Right now, we just showed that they're doing interesting things."
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