Ants in a rheometer (© Georgia Tech)

What do fire ants and ketchup have in common?

This is not a joke (nor a joke answer, though hard to believe), but according to a study published in Nature Materials earlier this week, they’re both runny when squeezed.

In the study, scientists from Georgia Institute of Technology subjected a handful of fire ants to mechanical testing. Their results suggest that ants can be collectively liquid-like or solid-like, depending on how much force is applied. The property may explain why ants are able to build rather impressive structures in nature.

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Aside from delivering a nasty sting, fire ants are known for linking their bodies into enormous ant-bridges across ditches, and floating ant-rafts when their nests are destroyed by floods. The researchers were interested in the how these structures stay together.

Their experiments used a rheometer, a machine that tests how ‘liquid-like’ a material is by smearing it around using two spinning plates. (Don’t worry, the ants had room to move – there was no ant-paste.)

At low spinning speeds, linked ants maintained their rigid structure. But at high speeds, they fell apart. This is where the comparison to ketchup comes in.

Some substances are solid-like at rest (like ketchup in a bottle) but liquid-like when force is applied (like ketchup flowing uncontrollably out of a thumped bottle). Author Alberto Fernandez-Nieves said that this effect of force was even greater in ants.

Co-author David Hu, an associate professor in the George W. Woodruff School of Mechanical Engineering, explained the effect by the fact that ants “let go” of each other when forced. “Despite wanting to be together, they let go and behave like a fluid to prevent getting injured or killed,” he said.

He explained that being able to hold on or let go allows ants to make strong physical structures like rafts, but also to avoid being broken into “smithereens” when pulled apart. He added, “They’re like liquid metal — just like that scene in the Terminator movie.”

Fire ant body armour, anyone?


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