National Geographic : 2008 Apr
tenebrionid beetle flourishes in the Namib Desert in southwestern Africa, one of the world's hottest, driest environments. The beetle drinks by harvesting morning fogs, facing into the wind and hoisting its behind, where hydrophilic bumps capture the fog and cause it to coalesce into larger droplets, which then roll down the waxy, hydrophobic troughs between the bumps, reaching the beetle's mouth. Parker imported sev eral dozen beetles from Namibia, which prompt ly scampered all over the lab when he opened the box, but eventually settled contentedly on the dune. There, using a hair dryer and various misters and spray bottles, Parker simulated the conditions in the Namib Desert well enough to understand the beetle's mechanism. He then replicated it on a microscope slide, using tiny glass beads for the bumps and wax for the troughs. For all nature's sophistication, many of its clever devices are made from simple materials like keratin, calcium carbonate, and silica, which nature manipulates into structures of fantastic complexity, strength, and toughness. The abalone, for example, makes its shell out of calcium carbonate, the same stuff as soft chalk. Yet by coaxing this material into walls of stag gered, nanoscale bricks through a subtle play of proteins, it creates an armor as tough as Kevlar- 3,000 times harder than chalk. Understanding the microscale and nanoscale structures respon sible for a living material's exceptional properties is critical to re-creating it synthetically. So today Andrew Parker had arranged to view the skin of a thorny devil museum specimen under a scanning electron microscope, hoping to find the hidden structures that allow it to absorb and channel water so effectively. With a microscopist at the helm, we soared over the surface of the thorny devil's skin like a deep-space probe orbiting a distant planet, dip ping down now and then at Parker's request to explore some curious feature of the terrain. There seemed to be little of interest in the Matterhorn like macrostructure of an individual thorn, though Parker speculated that it might wick away heat from the lizard's body or perhaps help cap ture the morning dew. Halfway down the thorn, however, he noticed a series of nodules set in rows, which seemed to grade down to a larger water-collection structure. Finally we dove into a crevasse at the base of the thorn and encoun tered a honeycomb-like field of indentations, each 25 microns across. "Ah-ha!" Parker exclaimed, like Sherlock Holmes alighting upon a clue. "This is clearly a superhydrophobic surface for channeling water In 1982 botanist Wilhelm Barthlott of the University of Bonn in Germany discovered in the lotus leaf a naturally self-cleaning, water-repellent surface. The secret lies in waxy microstruc tures and nanostructures that, by their contact angle with water, cause it to bead and roll away like mercury, gathering dirt as it goes. Barthlott patented his discovery, calling it the Lotus Effect. It has found commer cial application in products like the biomimetic paint Lotusan (on blocks at right). Infused with microbumps, the paint is reputed to repel water and resist stains for decades.