Fire ants may be the scourge of southern states like Georgia and Texas, but scientifically speaking, they are endlessly fascinating as an example of collective behavior. A few fire ants placed at a good distance from each other behave like individual ants. But pack enough of them tightly together, and they act more like a single entity, exhibiting both solid and liquid properties. They can form rafts to survive floods, arrange themselves in towers, and you can even pour them from a teapot as a liquid.
“Aggregated, they can almost be seen as one material, known as ‘active material,'” said Hungtang Ko, now a postdoc at Princeton University, who began studying these fascinating creatures as a graduate student at Georgia Tech in 2018. (And yes, he has been stung many, many times.) He is the co-author of two new papers examining the physics of fire ant rafts. The first, published in journal Bioinspiration and Biomimetics (B&B), investigated how fire ant floats behave in flowing water compared to static water conditions.
The other one, approved for publication in Physical Review Fluids, explored the mechanism by which fire ants join together to form the rafts in the first place. Cow et al. were somewhat surprised to find that the primary mechanism appears to be the so-called “The Cheerios effect“— named in honor of the tendency of the last remaining Cheerios floating in milk to clump together in the bowl, either drifting to the center or to the outer edges.
A single ant has a certain amount of hydrophobicity, ie the ability to repel water. This the property is intensified when they link together, their bodies weave much like a waterproof fabric. The ants collect any eggs, make their way to the surface via their tunnels in the nest, and as the flood waters rise, they chop down each other’s bodies with their mandibles and claws until a flat raft-like structure forms. Each ant behaves like an individual molecule in a material – say grains of sand in a pile of sand. The ants can accomplish this in less than 100 seconds. What’s more, the ant raft is “self-healing”: it’s robust enough that if it loses an ant here and there, the overall structure can remain stable and intact, even for months at a time.
In 2019, Ko and colleagues reported it fire ants could actively sense changes in forces acting on their floating raft. The ants recognized different fluid flow conditions and can adapt their behavior to maintain the stability of the raft. A paddle moving through the river water will create a series of swirling eddies (known as vortex shedding), causing the ant rafts to spin. These eddies can also exert additional forces on the raft, sufficient to break it apart. The changes in both centrifugal and shear forces acting on the raft are quite small—perhaps 2 percent to 3 percent of normal gravitational force. But somehow the ants can sense these small shifts with their bodies.
Earlier this year, researchers at the University of Colorado, Boulder, identified some simple rules which appears to control how floating rafts of fire ants contract and expand their shape over time. As we reported at that time the structures sometimes came to be compacted into dense circles of ants. Other times, the ants began fanning out to form bridge-like extensions (pseudopods), sometimes using the extensions to escape the containers.
How did the ants achieve these changes? The rafts essentially consist of two distinct layers. Ants on the bottom layer serve a structural purpose and form the stable base of the raft. But the ants on the upper layer move freely on top of the linked bodies of their brethren on the lower layer. Sometimes ants move from the bottom to the top layer or from the top to the bottom layer in a cycle similar to a doughnut-shaped treadmill.
Cow et al.s B&B study is somewhat related in focus, except that the Boulder study looked at the broad collective dynamics rather than interactions between individual ants. “There are thousands and thousands of ants in the wild, but nobody really knows how a pair of ants would interact with each other and how that affects the stability of the raft,” Ko told Ars.
With such large fleets, repeatability can be an issue. Ko wanted to gain a little more control over his experiments and also study how the ants adapted to different flow scenarios in water. He found that the ants use an active streamlining strategy, changing the shape of the raft to reduce drag. “So maybe it takes less force, or less metabolic cost, to stick to the vegetation than if they stuck to the original larger pancake shape,” Ko said.
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