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Image of a Fire ant up close
Fire ants could inspire molecular machines. (Credit: Insects Unlocked)

Think of it as mathematics with a bite: Researchers at CU Boulder have uncovered the statistical rules that govern how gigantic colonies of fire ants form bridges, ladders and floating rafts. The findings may also help researchers explore designs for new dynamic networks, including molecular machines that deliver drugs directly to cells.

The research takes a unique look at one of the strangest, and potentially painful, networks in nature. Fire ants (Solenopsis invicta) are resourceful builders, using their own bodies to create gigantic structures made up of hundreds to thousands of insects and more.

In the new study, a team set out to lay out the engineering principles that underlie these all-ant structures — specifically, how they become so flexible, changing their shapes and consistencies within seconds. The group used statistical mechanics to calculate the way that ant colonies respond to stresses from the outside, shifting how they hang onto their neighbors based on key thresholds.

The findings may also help researchers understand other “dynamic networks” in nature, including cells in the human body, says Vernerey, an associate professor in the Department of Mechanical Engineering.

Such networks “are why human bodies can self-heal,” Vernerey says. “They are why we can grow. All of this is because we are made from materials that are interacting and can change their shape over time.”

The team wanted to find out how the ants govern its internal cha-cha in response to outside pressures. To do that, they used a mathematical tool that allowed them to average out the behavior of the hundreds to thousands of ants in a colony.

The researchers discovered that as the forces on ant colonies increase, the insects pick up their speed. If the force on an individual ant’s leg hits more than eight times its body weight, the insect will compensate by switching between its neighbors twice as fast. That behavior explains why ant colonies are classified as “shear-thinning” fluids, or materials that get thinner the more force you put on them — think stirring a can of paint.

But if you keep increasing the forces on the ants, they can no longer keep up. When that happens, the ants will stop letting go of their neighbors and instead hold on for dear life.

The researchers explained that they’ve only just scratched the surface of the mathematics of fire ant colonies. But their calculations are general enough that researchers can already begin using them to explore designs for new dynamic networks, including molecular machines that deliver drugs directly to cells.

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