Fire ant rafts form on the water due to the 'Cherios Effect'

Fire ant rafts form on the water due to the ‘Cherios Effect’

  • Scientists have found that 10 or more fire ants can stick together to build a life-saving raft in a flood.
  • Less than 10 ants cannot form a stable raft because the “Cherios effect” of fluid dynamics is not enough for very few struggling ants.
  • This effect can be used to engineer nanoparticles for biofluidic emulsions, and ant movements can determine the design of swarming microrobots.

How many ants are needed to build a raft?

This sounds like a mystery, and in a way it is. Scientists whose interests intersect in insects and fluid mechanics wonder how fire ants hold together in water. When their nests are flooded, as they often do in subtropical homes, these ants can avoid drowning by clumping together and forming “rafts.” Is it a sign of a collaborative effort?

In fact, fire ants do not seek to huddle together in the water. Instead, they try to push each other away. But research conducted by the Georgia Institute of Technology in Atlanta, over the past few years, may have an answer to the mystery. When 10 or more ants come close to each other in the water, the property of surface tension is called “Cheerios effect“He holds them together anyway, which saves their lives—despite their best efforts to avoid contact. Then their legs intertwine, making their hold steady. Voila, a raft of fire ants was born.”

This strange situation that researchers described in the fall of 2021 with a number APS Fluid Dynamics DepartmentAnd the Not only applies to fire ant species Solenopsis invicta From the wetlands of Brazil. If you’re a fan of Cheerios, you may notice that individual cereal pieces tend to stick together in the bowl of milk. Even other objects, like paper clips, stick together as long as they can float. Cheerios – and ants – are less dense than water, so they are buoyant.

If you look at your cereal with a magnifying glass, you will notice that the milk around each piece forms an upward slope, compared to the surface of the rest of the liquid. This “meniscus effect” causes the U-shaped fluid to stick to the body floating in it, because the molecules of the fluid are more strongly attracted to the edges of solids than to each other. Cheerios, ants, and paper clips are all pushed up slightly to the highest point in the meniscus. Very quickly, they end up finding mutual high points together on the invisible film of surface tension that holds the top layer of water molecules together. This is how the Cheerios effect pulls ants and small floating objects together.

Fire ants in Georgia tech experiments begin forming a raft thanks to the Cheerios effect

Georgia Tech’s fire ants begin to form a raft thanks to the “Cherios Effect.”

Andre Maggiore, courtesy of Georgia Institute of Technology

“I thought that when two ants met each other on the surface of the water, they would stick together effectively,” said the paper’s first author, Hangtang Koh Tells popular mechanics in a letter. Coe, a mechanical and aerospace engineer at Princeton University, worked on fire ant raft studies with mechanical engineering professor David Ho while completing his Ph.D. at Georgia Tech. However, the interaction time seems to be too short for them to realize that they are next to a neighbour. So they ended up kicking them all out of the way! ”

Fortunately, the Cheerios effect forces the ants to stay together. Hu’s team floated different numbers of ants in a tank of circulating water. “We observe that ants swim in random directions even when they are around neighbors and kick each other away upon contact,” the authors wrote. “These repulsive effects cause the small rafts to break down.” But when 10 ants are in the water at once, they can bond and remain stable.

Previously, researchers studying the “ant raft” phenomenon believed that the ants were actively cooperating to save the colony. Many types of animals choose to work together to accomplish complex tasks without a clear leader. Members of the fire ant species Solenopsis invicta, which are found in both South America and the subtropical regions of the United States, work together to form camps for hundreds of thousands of ants, often reaching a height of more than 30 bodies. When they encounter relatively tall structures like grass, they are Create ant towers that lay on vegetation while it floats.

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How ants survive ingenious during floods | Super heating | BBC Earth


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Hu Jintao had been investigating fire ant rafts for years. These ants regularly assemble “ladders, chains, walls, and rafts, composed exclusively of individual ants bound together” to survive the floods, according to Hu. previous papers On ant rafts from 2011. “The raft is one of the tallest living structures, allowing ants to navigate it for several months as they migrate and colonize new lands.”

In their experiments, the flowing water created the greatest pressure at the leading edge of the raft, where the ants faced the flow directly. The ants at the rear end of the raft felt the slightest amount of pressure. To compensate, the shape of the raft constantly changed so that all the ants could stick together, and not a single ant had to bear the brunt of the oncoming stream. In similar experiments, Hu’s research team found Posted in Biology open In February 2022, rowing ants consumed nearly 43 percent more energy than dryland ants. But the larger the raft, the less energy the ant would have to expend.

So what happens when the velocity of the water flow increases? In Hugh’s later experiments, the ants’ rafts would “elongate from circular shapes to more streamlined shapes” in response to faster water. The ants experienced a 48 percent drop in drag when their raft extended. The way insects move could “provide insights for designing intelligent robotic swarms that can adapt to fluid flows,” he and colleagues wrote in July 2022 Article in Bioinspiration and biomimetic.

The Cheerios effect may have a cute name, but it’s just a form of capillary motion, which forces fluid in a narrow channel to flow, even against gravity. Surface tension and gravity combine between liquid and solid materials to push the liquid forward through the channel. Chemical engineers use this property of liquids for experiment Nanoparticles It can be self-assembled in water and oil interface. For example, they can improve the crude refining process and work with biofuel emulsions (think mixing eggs and oil together, an emulsifier that produces mayonnaise).

“I don’t think we know of a very large number of cases where insects have intentionally used the Cheerios effect yet. But capillary force is ubiquitous for floating insects, such as a water slider,” says Koo.

Fire ants seem to have found the best use of this property of fluid mechanics – sticking together to save their lives.

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