The physics of fire ant rafts could help engineers design swarming
robots

Date:
March 2, 2022
Source:
University of Colorado at Boulder
Summary:
Fire ants survive floods by forming rafts made up of thousands of
wriggling insects. New research reveals how these creepy-crawly
lifeboats change shape over time.



FULL STORY ==========================================================================
Noah rode out his flood in an ark. Winnie-the-Pooh had an upside-down
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 lays out
the simple physics-based rules that govern how these ant rafts morph
over time: shrinking, expanding or growing long protrusions 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 fix damaged spots.

The results appeared recently in the journal PLOS Computational Biology.

"The origins of such behaviors lie in fairly simple rules," said Franck Vernerey, primary investigator on the new study and professor in the
Paul M.

Rady Department of Mechanical Engineering. "Single ants are not as smart
as one may think, but, collectively, they become very intelligent and
resilient communities." Fire ants form these giant floating blobs of
wriggling insects after storms in the southeastern United States to
survive raging waters.

In their latest study, Vernerey and lead author Robert Wagner drew on mathematical simulations, or models, to try to figure out the mechanics underlying these lifeboats. They discovered, for example, that the
faster the ants in a raft move, the more those rafts will expand outward,
often forming long protrusions.



========================================================================== "This behavior could, essentially, occur spontaneously," said Wagner,
a graduate student in mechanical engineering. "There doesn't necessarily
need to be any central 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 waters. Then they waited.

"We left them in there for up to 8 hours to observe the long-term
evolution of these rafts," Wagner said. "What we ended up seeing is
that the rafts started forming these growths." Rather than stay the
same shape over time, the structures would compress, drawing in to form
dense circles of ants. At other points, the insects would fan out like
pancake batter on a skillet, even building bridge-like extensions.



==========================================================================
The group reported that the ants seemed to modulate these shape changes
through a process of "treadmilling." As Wagner explained, every ant
raft is made up of two layers. On the bottom, you can find "structural"
ants who cling tight to each other and make up the base. Above them are
a second layer of ants who walk around freely on top of their fellow colony-members.

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

"The whole thing is like a doughnut-shaped treadmill," Wagner said.

Bridge to safety In the new study, he and Vernerey wanted to explore
what makes that treadmill go round.

To do that, the team created a series of models that, essentially,
turned an ant raft into a complicated game of checkers. The researchers programmed roughly 2,000 round particles, or "agents," to stand in for
the ants. These agents couldn't make decisions for themselves, but they
did follow a simple set of rules: The fake ants, for example, didn't
like bumping into their neighbors, and they tried to avoid falling into
the water.

When they let the game play out, Wagner and Vernerey found that their
simulated ant rafts behaved a lot like the real things.

In particular, the team was able to tune how active the agents in their simulations were: Were the individual ants slow and lazy, or did they
walk around a lot? The more the ants walked, the more likely they were
to form long extensions that stuck out from the raft -- a bit like people funneling toward an exit in a crowded stadium.

"The ants at the tips of these protrusions almost get pushed off of the
edge into the water, which leads to a runaway effect," he said.

Wagner suspects that fire ants use these extensions to feel around their environments, searching for logs or other bits of dry land.

The researchers still have a lot to learn about ant rafts: What makes ants
in the real world, for example, opt to switch from sedate to lazy? But,
for now, Vernerey says that engineers could learn a thing or two from
fire ants.

"Our work on fire ants will, hopefully, help us understand how simple
rules can be programmed, such as through algorithms dictating how robots interact with others, to achieve a well-targeted and intelligent swarm response," he said.

Video: https://youtu.be/IrLc-uDv7GU ========================================================================== Story Source: Materials provided by
University_of_Colorado_at_Boulder. Original written by Daniel
Strain. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Fire_ants_in_a_raft ========================================================================== Journal Reference:
1. Robert J. Wagner, Franck J. Vernerey. Computational exploration of
treadmilling and protrusion growth observed in fire ant rafts. PLOS
Computational Biology, 2022; 18 (2): e1009869 DOI: 10.1371/
journal.pcbi.1009869 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220302185954.htm

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