Like Water off a Beetle's Back
Summers, Adam, Natural History
An African insect could show how to wring moisture from the fog-and let the sun shine on cloudy airports.
Follow the southwestern coast of Africa north from Cape Horn toward Namibia's gemstone-rich Skeleton Coast, and you come to the Namib Desert. Home to the world's highest sand dunes, the Namib is also a cornucopia of biomechanical marvels: a spider that rolls like a wheel; a gecko that dances on the hot sand; and the bizarre, two-leafed Welwitschia mirabilis, which looks like a wrecked airplane planted in the sand and can live more than a thousand years. The environment is a harsh one. Annual rainfall in the Namib typically measures less than an inch, and on most days the only source of moisture is the early morning fog that rolls off the chilly Atlantic, tantalizing the denizens of the parched sands.
In the slaty light of one such foggy dawn, a long-legged Namib beetle (genus Stenocara) stands on a small ridge of sand. Its head faces upwind, and its stiff, bumpy outer wings are spread against the damp breeze. Minute water droplets from the fog gather on its wings; there the droplets coalesce, until they finally grow big enough to release their electrostatic grip on the wing surfaces and roll down to the beetle's mouth parts, giving the animal an early morning drink. In such an arid environment that drink is vital, for once the Sun burns off the fog, there is little the in sect can soak up except blistering heat. Besides being helpful to the beetle, the water-gathering mechanism-only recently understood by investigators-might someday become the basis for large-scale, artificial schemes to gather water from the air.
There's plenty of water in a fog bank; the hard part is getting hold of it. The water droplets in fog are, on average, just one one-thousandth of an inch across, and the largest ones are only twice that size. The droplets are so small, in fact, that they often don't fall downward; instead they get carried sideways or even upward by currents of wind.
The trick to drinking fog is getting the droplets to aggregate, so that wind and electrostatic forces no longer overwhelm gravity. When a wind-blown fog droplet lands on a hydrophilic (water-loving) surface, such as clean glass or stone, the drop flattens out because of the electrostatic attraction between the molecules of water and those of the surface. The cross section of the flat drop is too small for the wind to pick it back up. And, because water molecules so strongly attract each other, the flat drop also presents a highly hydrophilic surface to which other droplets can attach.
Andrew R. Parker, a zoologist at the University of Oxford, and Chris R. Lawrence, an investigator at the defense research firm QinetiQ, headquartered in Farnborough, England, discovered that Stenocam beetles take advantage of those basic properties of water. On the beetle's elytra-its hardened, outer pair of wings-there is a pattern that alternates hydrophilic bumps, just one-fiftieth of an inch across, with waxy, hydrophobic (water-averse) valleys. A fog droplet collects on each little bump, and further droplets attach to the first. The droplets coalesce and grow until they reach about two-tenths of an inch in diameter. At that size, because the insect's back slopes at roughly forty-five degrees to the horizontal, the drops are heavy enough to unstick from the bumps and buck the wind. …