It may not be quite as impressive as photons being both particles and waves, but a new study has found ants can combine features of liquids and solids in order to survive threatening situations such as floods. The observations of how they behave are providing clues to how we can make materials capable of self-assembly.
Earlier this month a remarkable video appeared of fire ants forming a raft to survive floods. The ants achieve this feat by secreting an oily substance that allows them to float above the water. Ant collaboration is also observed in the formation of load-bearing bridges. However, a paper in Nature Materials demonstrates that this is only the start of ant collective ingenuity.
Dr Alberto Fernandez-Nieves of the Georgia Institute of Technology put thousands of ants into a rheometer, a device that measures the way liquids or semi-liquids respond to forces.
Both live and dead ants were tested at different speeds, but at the right pressures it didn't matter. The live ants acted as if they were dead when forced to flow through the rheometer. The ants behaved as a non-Newtonian fluid, letting go of each other under pressure so that they flowed more easily.
“It's not unlike ketchup," said Fernandez-Nieves, in a statement. "The harder you squeeze, the easier it flows. But with ants, this happens much more dramatically than with ketchup." This contrasts with some other non-Newtonian fluids, such as mixtures of cornflour and water, that flow less easily under pressure.
“Ants seem to have an on/off switch in that they let go for sufficiently large applied forces, said co-author Dr David Hu in the statement. “Despite wanting to be together, they let go and behave like a fluid to prevent getting injured or killed.” The team demonstrate the same effect in a video.
The fire ants give way before a penny as it falls, self-assembling once it has passed them. Credit: Georgia Tech.
“This is the hallmark of viscoelastic behavior,” said Fernandez-Nieves. “The ants exhibit a springy-response when probed at short times, but behave fluid-like at longer times.”
These capacities are of more than curiosity value. Hu points out in another video, “Imagine I have glass window and I throw a brick through it... Imagine if the window was made of ants...The glass shards would reform, self-heal, allow the brick to pass and then form a complete new window...This is the dream of active matter.”
Making furniture out of ants has some obvious drawbacks, but Fernandez-Nieves noted that similar behavior is seen in polymers, and it may be possible to produce materials that replicate these self-healing behaviors. Whether this is an entirely a good idea is a different matter, of course. Hu gives the example of the liquid metal T-1000 from Terminator 2 as the sort of capacities being sought, something some people appear disturbingly close to achieving at home.