Highlights from the American Physical Society meeting

Highlights from the APS March Meeting, Baltimore, March 18-22, 2013

Fire ants need damp soil
Despite its name, the fire ant isn’t afraid of a little water. In fact, the insects need just the right amount of moisture in soil to build superior nests, new experiments show.

TUNNEL ANTICS Fire ants are excellent diggers, but excavating tunnels (shown in artificial lab soil) in their underground lairs is easier if the soil is a little wet, new experiments show. Nick Gravish

Researchers from Georgia Tech in Atlanta filled 14.5-centimeter-deep aluminum tubes with an artificial soil made up of tiny glass beads and various amounts of moisture. The team then gave the stinging, invasive insects 20 hours to get their nest on. CT scans revealed that at about 10 percent moisture content — a medium amount — the ants (Solenopsis invicta) built tunnels twice as long as they did in parched soil. The researchers speculate that the dash of water makes the soil particles glom together in a manner that makes for easy digging.

 “If you keep your backyard dry, they may leave and dig in your neighbor’s backyard,” said Daria Monaenkova, who presented the research on March 21 at the American Physical Society meeting. When the researchers tried varying the size of the soil particles, it didn’t influence nest construction: The fire ants chugged along fine with small or large grains, at least until the soil particles became as big as small worker ants. — Rachel Ehrenberg

A tree stressed in the forest really does make a sound
Experiments that aim to explain the high-pitched screams emitted by thirsty trees have linked the distress calls to bubbles that form within the sap. But there may be more to the sounds than meets the ear.

When trees experience drought or really cold weather, they sometimes produce sounds. Most such noises are too high in frequency for the human ear to detect. But the sounds can reach 140 decibels, as loud as a jet-engine level rumble, Alexandre Ponomarenko of Grenoble University in France explained March 21.

To get at what produces the noises, Ponomarenko and his colleagues took sections of larch and pine, sandwiched them inside a porous gel and applied pressure. Along with about half of the noises came bubbles within the plant cells, microscope images reveal. The researchers speculate that the remaining noises might come from the bubbles moving through other cells, or perhaps even the wood fracturing. — Rachel Ehrenberg

Nanocrystals slither through tiny tubes
Nothing can squeeze through tight spaces quite like a nanocrystal. Physicists watched a small iron crystal slide through a tube with an opening one-quarter of the diameter of the crystal itself. The finding could lead to remotely controlled nanomachines that navigate through confined quarters.

Using an electric field, the researchers drove an iron crystal 20 nanometers in diameter through a carbon nanotube like a train in a tunnel. The surprise came when the tube constricted to 5 nanometers in diameter: The iron nanocrystal altered its shape and squeezed through. Microscope images showed that the iron remained a crystalline solid.

“It was mind boggling to watch,” said Sinisa Coh, a physicist from the University of California, Berkeley, who presented the results March 21.

Coh and his team attribute the crystal’s dexterity to the collective movement of its atoms. The researchers think that only the surface atoms move — the ones at the back of the crystal migrate along the tube’s edges to settle at the front, allowing them to conform to the size of the tube. — Andrew Grant

Editor’s Note: The size of the nanocrystal was corrected in this story on March 26.

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