Sap-sucking aphids dwelling within cramped confines inside trees risk drowning in their own sticky excrement. Now scientists have shown that the tiny bugs avoid a gooey doom by rolling their excretions into nonstick liquid marbles. This slick tactic might someday inspire designs for speedy valves or compact parcels for nanotech devices.
Aphids can spend their entire lives in galls, hollow lumps in branches and leaves that trees build around irritants such as germs, fungi, or bugs. Hundreds of aphids can live in one gall, creating enough excreted honeydew to doom them all. Fossils show that aphids have lived since the age of the dinosaurs, though, so they must have some way to guard against this sticky situation.
The tiny insects spin out fibers chemically similar to candle wax that disintegrate into needles. A team led by applied mathematician Lakshminarayanan Mahadevan of the University of Cambridge, U.K., discovered that the wax is extremely water-repellant: Water droplets float on top of the needle tips much as ascetics rest on beds of nails. The aphids coat themselves and the inside of the tree gall with this powder, and cover their honeydew with it the moment droppings leave the bug's body. The aphids easily shuffle the stick-proof fluid balls around the nest, the team reports in the 22 June issue of the Proceedings of the Royal Society.
Because little force is needed to scoot around the waxy globules, they'd make "remarkably quick" valves and plugs for nanotechnology, says Mahadevan, or similar wax could be used to encapsulate chemicals in easy-to-move parcels. The idea is "absolutely fascinating," says chemical engineer Manoj Chaudhury of Lehigh University in Bethlehem, Pennsylvania. He says the problems with such a technique, such as directing the balls' motion, are probably not insurmountable.