Fire ants may be the scourge of southern states like Georgia and Texas, but scientifically, they’re endlessly fascinatingfor example of collective behavior. Afew fire ants spaced well apart behave like individual ants. But pack enough of these closely together, plus they act similar to an individual unit, exhibiting both solid and liquid properties. They are able to form rafts to survive flash floods, arrange themselves into towers, and you may even pour them from the teapot just like a fluid.
“Aggregated, they are able to almost be regarded as a material, referred to as ‘active matter,'” said Hungtang Ko, now a postdoc at Princeton University, who began monitoring these fascinating creatures as a Georgia Tech graduate student in 2018. (And yes, he’s got been stung many, often.) He’s a co-author of two recent papers investigating the physics of fire ant rafts. The initial, published in the journal Bioinspiration and Biomimetics (B&B), investigated how fire ant rafts behave in flowing water in comparison to static water conditions.
The next, accepted for publication in Physical Review Fluids, explored the mechanism where fire ants get together to create the rafts to begin with. Ko et al. were somewhat surprised to get that the principal mechanism is apparently the so-called “Cheerios effect“named honoring the tendency for all those last remaining Cheerios floating in milk to clump together in the bowl, either drifting to the guts, or even to the outer edges.
An individual ant includes a specific amount of hydrophobia, i.e., the opportunity to repel water. This property is intensified if they link together, weaving their health similar to a waterproof fabric. The ants gather up any eggs, make their solution to the top via their tunnels in the nest, so when the flood waters rise, they chomp down on each other’s bodies making use of their mandibles and claws until a set raft-like structure forms. Each ant behaves as an individual molecule in a materialsay, grains of sand in a sand pile.The ants can make this happen in under 100 seconds. Plus, the ant raft is “self-healing”: it’s robust enough that when it loses an ant occasionally, the entire structure can stay stable and intact, even for months at the same time.
In 2019, Ko and colleagues reported that fire ants could actively sense changes in forces acting upon their floating raft. The ants recognized different fluid flow conditions and may adapted their behavior accordingly to preserve the raft’s stability. A paddle moving through river water will generate a number of swirling vortices (referred to as vortex shedding), evoking the ant rafts to spin. These vortices may also exert extra forces on the raft, sufficient to break it apart. The changes in both centrifugal and shearing forces functioning on the raft are very smallmaybe 2 percent to 3 percent the force of normal gravity. Yet somehow, the ants can sense these small shifts making use of their bodies.
Earlier this season, researchers at the University of Colorado, Boulder,identified several simple rules that appear to govern how floating rafts of fire ants contract and expand their shape as time passes. As we reported at that time, sometimes the structures would compress into dense circles of ants. Other times, the ants would begin to fan out to create bridge-like extensions (pseudopods), sometimes utilizing the extensions to flee the containers.
How did the ants achieve those changes?The rafts essentially comprise two distinct layers. Ants on underneath layer serve a structural purpose, creating the stable foot of the raft. However the ants on top of the layer move freely along with the linked bodies of these bottom-layer brethren. Sometimes ants move from underneath to top of the layer or from top of the to underneath layer in a cycle resembling a doughnut-shaped treadmill.
Ko et al.‘s B&B study is somewhat related in focus, except the Boulder study viewed the broad collective dynamics instead of interactions between individual ants. “You can find thousands of ants in the open, but nobody really knows what sort of couple of ants would connect to one another, and how that affects the stability of the raft,” Ko told Ars.
With such large rafts, repeatability is definitely an issue. Ko wished to gain a bit more control over his experiments and in addition study the way the ants adapted to different flow scenarios in water. He discovered that the ants employ a dynamic streamlining strategy, changing the form of the raft to lessen drag. “So maybe it requires less force, or less metabolic cost, to carry onto the vegetation than should they stuck with the initial larger pancake shape,” said Ko.