Biomechanics: February 2004

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Windblown water (blue arrow) becomes a drink for a carefully positioned Namib Desert beetle. Facing into the breeze, with its body angled at forty-five degrees, the beetle catches fog droplets on its hardened wings. The droplets stick there to hydrophilic bumps, which are surrounded by waxy, hydrophobic troughs. Droplets accumulate and coalesce until their combined weight overcomes the water's attraction to the bumps as well as any opposing force of the wind; in a ten-mile-an-hour breeze, such a droplet would stick to the wing until it grows to roughly two-tenths of an inch in diameter; at that point it would roll down the beetle's back to its mouth parts, quenching a desert thirst.

Roberto Osti

Follow the southwestern coast of Africa north from Cape of Good Hope 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 insect 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 Stenocara 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. Each drop slides down the wings toward the beetle’s mouth like a bead of rain on the hood of a freshly waxed car.

It’s a neat trick, but it hardly seems practical to have teeming hoards of beetles harvesting fog for water. What would you do with them for the fogless rest of the day? And how would you keep them from drinking the water themselves, rather than donating it to crops, livestock, or people? Fortunately, Parker and Lawrence have a solution. They developed a surface to mimic the beetle’s elytra that seems to work as well as the beetle’s wings do. The two investigators partly embedded dozens of glass spheres, each about the diameter of a poppy seed, in a thin layer of wax. After de-waxing the top of each glass sphere with alcohol, they had an array of hydrophilic bumps in a hydrophobic field. In tests, they found that neatly ordered arrays of beads caught more mist than random, disordered ones did. But both kinds of array caught more than did smooth, waxy surfaces; most water drops just bounced off the latter. Water landing on bare glass drained in unpredictable directions.

Many techniques that imitate nature—collectively known as biomimetic technologies—are prohibitively expensive. This one might well be a commercially viable exception. There are a number of ways to create either flexible or rigid surfaces with hydrophilic-hydrophobic patterns. Perhaps the simplest would be to print hydrophilic dots onto sheets of such hydrophobic materials as polyethylene. On a camping tent, for instance, such a printed pattern, combined with suitable guttering, could gather the day’s water supply from the early morning mist.

Parker and Lawrence’s effort isn’t the first time people have tried to harvest water from fog. People in remote areas of South America have already relied on fog for their water. According to the government of Canada (which collaborated, with an organization called FogQuest, on a plan to develop technologies to harvest fog), collectors near the village of Chungungo, Chile, gathered an average of 4,000 gallons of fog-borne water a day for several years in the past decade. The fog would collect on the threads of fine nets before it dripped into gutters leading to cisterns.

In other, windier areas, droplets could blow through the nets or get blown off again after landing. Parker and Lawrence’s work shows that those drawbacks could be minimized by substituting solid panels for the nets. I even hold out the hope that solid panels could become so efficient that they could act as highly localized fog busters. For those of us who are routinely delayed as we pass through such perennially fogbound airports as San Francisco’s, fog-busting itself might be the technology’s “killer app.” But my biggest hope for this discovery is that it will spur new efforts to preserve biodiversity. After all, you never know where you’ll find your next drink.