New research from the Georgia Institute of Technology demonstrates that groups of ants behave in ways that are like both liquids and solids. For example, if you drop a penny into an ocean of ants, they will initially absorb the impact like a solid spring. But then the penny will begin to sink through the group as if they were a liquid; they'll release their grips on one another and move around to fill in the void on top.

"Remarkably, the observed behavior is similar to what is seen in materials that are not alive, like polymer gels right at the point when they become a gel," says Alberto Fernandez-Nieves, an associate physics professor at Georgia Tech.

To perform their experiments, the researchers poured thousands of writhing fire ants into a rheometer—a device used to measure the flow of liquids or slurries (semiliquid mixtures, such as cement) when force is applied to them— and tested at speeds from 0.0001 rpm to 100 rpm. When the speed got high enough, live ants would abandon their normal behavior and let go of each other to behave like dead ants, decreasing the viscosity of the group and allowing them all to flow more like a liquid.

"It's not unlike ketchup," Fernandez-Nieves explains. "The harder you squeeze, the easier it flows. But with ants, this happens much more dramatically than with ketchup."

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David Hu, an associate professor at Georgia Tech, describes the behavior as a survival method. "Ants seem to have an on/off switch in that they let go for sufficiently large applied forces," he says. "Despite wanting to be together, they let go and behave like a fluid to prevent getting injured or killed."

In nature, ants will assemble rafts from their bodies to float and survive during floods, and they can self-assemble bridges to get across gaps. But when forces are applied to them, the group can behave more like a malleable semi-liquid to avoid being crushed into an actual liquid.

"If you cut a dinner roll with a knife, you're going to end up with two pieces of bread," says Hu. "But if you cut through a pile of ants, they'll simply let the knife go through, then reform on the other side. They're like liquid metal – just like that scene in the Terminator movie."

Hu imagines using the new research to create a "self-repairing" material that can reform its original shape after it is broken or changed. Just imagine armies of T-1000 fire ants swarming towards you. Or actually, maybe don't.

The research group's findings were recently published in the journal Nature Materials.

Source: Georgia Institute of Technology

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