The physics of fire ant rafts can help engineers design swarm robots

Fire ants form a protrusion of an ant raft. Credit: Vernerey Researcher Group at CU Boulder

Noah rode out his flood in an ark. Winnie the Pooh had an upturned umbrella. Fire ants (Solenopsis invicta), meanwhile, form floating rafts made up of thousands or even hundreds of thousands of individual insects.

A new study by engineers at the University of Colorado Boulder explains the simple, physics-based rules that govern how these ant rafts change over time: shrinking, expanding or growing from long projections like an elephant’s trunk. The team’s findings could one day help researchers design robots that work together in swarms or next-generation materials in which molecules migrate to repair damaged spots.

The results recently appeared in the magazine PLOS Computational Biology.

“The origins of such behavior lie in fairly simple rules,” said Franck Vernerey, lead researcher on the new study and professor in the Paul M. Rady Department of Mechanical Engineering. “Lone ants are not as smart as you may think, but together they become very intelligent and resilient communities.”

Fire ants form these giant floating blobs of writhing insects after storms in the southeastern United States to survive raging waters.

In their latest research, Vernerey and lead author Robert Wagner used mathematical simulations, or models, to try to figure out the mechanics underlying these lifeboats. For example, they found that the faster the ants move in a raft, the more those rafts will expand outward, often forming long protuberances.

“This behavior can essentially occur spontaneously,” said Wagner, a mechanical engineering graduate student. “There doesn’t necessarily have to be centralized decision-making by the ants.”

Treadmill time

Wagner and Vernerey discovered the secrets of ant rafts almost by accident.

In a separate study published in 2021, the duo dropped thousands of fire ants into a bucket of water with a plastic rod in the middle — like a lone reed in the middle of stormy water. Then they waited.

“We left them there until eight hours to observe the long-term evolution of these rafts,” Wagner said. “What we finally saw is that the rafts started forming these growths.”

Instead of maintaining the same shape over time, the structures would compress and pull inward to form dense circles of ants. At other points, the insects would fan out like pancake batter on a frying pan and even build bridge-like extensions.

The physics of fire ant rafts can help engineers design swarm robots

Fire ants form an ant raft. Credit: Vernerey Researcher Group at CU Boulder

The group reported that the ants appeared to modulate these shape changes through a “treadmill” process. As Wagner explained, each ant raft consists of two layers. At the bottom you will find “structural” ants that cling tightly to each other and form the base. Above that is a second layer of ants that roam freely on top of their fellow settlers.

Over a period of hours, ants can crawl up from the bottom, while free-roaming ants will fall down to become part of the structural layer.

“The whole thing is like a donut-shaped treadmill,” Wagner said.

Bridge to Safety

In the new study, he and Vernerey wanted to investigate what makes that treadmill run.

To do that, the team created a series of models that essentially turned an ant raft into an intricate game of checkers. The researchers programmed about 2,000 round particles, or “agents,” to replace the ants. These agents couldn’t make decisions on their own, but they did follow some simple rules: the fake ants, for example, didn’t like to bump into their neighbors and tried to avoid falling into the water.

When they got to play the game, Wagner and Vernerey found that their simulated ant rafts behaved a lot like the real thing.

In particular, the team was able to fine-tune how active the agents were in their simulations: were the individual ants slow and lazy, or were they roaming around a lot? The more the ants walked, the more likely they would form long spurts sticking out of the raft — a bit like people running to an exit in a crowded stadium.

“The ants at the ends of these protrusions are pushed almost off the edge into the water, leading to a runaway effect,” he said.

Wagner suspects that fire ants use these extensions to grope in their environment, looking for logs or other patches of dry land.

The researchers still have a lot to learn about ant rafts: For example, why do ants in the real world choose to switch from calm to lazy? But for now, Vernerey says engineers can learn a thing or two from fire ants.

“Our work on fire ants will hopefully help us understand how to program simple rules, such as through algorithms that dictate how robots interact with others, to achieve a well-targeted and intelligent swarm response,” he said.


Fire ants have been found to make ‘appendages’ on homemade rafts when placed in water


More information:
Robert J. Wagner et al, Computational exploration of treadmills and protrusion growth observed in fire ant rafts, PLOS Computational Biology (2022). DOI: 10.1371/journal.pcbi.1009869

Robert J. Wagner et al, Treadmills and Dynamic Protrusions in Fire Ant Rafts, Journal of The Royal Society Interface (2021). DOI: 10.1098/rsif.2021.0213

Provided by the University of Colorado at Boulder

Quote: The physics of fire ant rafts can help engineers design swarm robots (2022, March 2), retrieved August 25, 2022 from https://phys.org/news/2022-03-physics-ant-rafts-swarming-robots. html

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