Water Vapour Harvesting by the Namib Desert Beetle

The wing covers of the Namib desert beetle gather water from the air using nanoscale bumps.

The Namibian Beetle (Stenocara gracilipes) lives in one of the driest deserts in the world, the Namib on the southwest coast of Africa, but obtains all of the water it needs from ocean fog due to the unique surface of its back. Microscopic bumps with hydrophilic (water attracting) tips and hydrophobic (water repelling) sides cover its hardened forewings, which it aims at oncoming fog each morning. Water droplets materialize out of thin air on its back, then slide down channels into its awaiting mouth. Synthetic surfaces mimicking the beetle’s back have been created that are several times more effective than existing fog-catching nets, and could be used to generate clean freshwater supplies in arid regions, refugee camps, and at the tops of skyscrapers, requiring no pumping.

The Namib Desert is characterized by high temperatures, strong winds, and negligible rainfall, but it does experience nocturnal and early morning fogs that move in from the Atlantic Ocean. The fog droplets are unusually small, about 1-40 microns in diameter (human hair is about 50 microns in diameter) so could easily be carried off by the hot desert wind. To capture these droplets before they’re gone, the darkling beele faces into the wind and unabashedly sticks its rear end high in the air as if mooning all the beautiful creatures of the world, and starts to literally drink the fog as the droplets readily condense on its bumps and role down deliciously into its mouth. Observations suggest each beetle gains an average of about 12 percent of its body weight after fog-basking.

These clever darklings cover each 100-500 micron diameter bump with material that attracts water droplets (hydrophilic), while the area covering the 500–1,500 micron distance between each bump repels water (hydrophobic). As the tiny fog droplets cling to the top of each bump, they start to aggregate into bigger and bigger water droplets. At diameters of 4-5 mm, the weight of the droplet exceeds the forces attracting it in the first place, as well as the force of the desert wind, so it simply rolls off. Once in the trough between the forest of bumps, the droplet has no choice but to keep moving forward due the angle of the beetle’s body and the waxy pavement. The highway system born of the configuration of bumps and troughs directs all traffic to the beetle’s mouth. Take a closer look and you’ll see that the waxy pavement of the troughs covers a microscopic hexagonal pattern of 10 micron diameter hemispheres, which together creates a superhydrophobic surface. In the hyperarid Namib Desert climate, he who fog-basks, laughs last. (Biomimicry Guild 2008).

Application Ideas: Create passive devices to collect and harvest water in dessicated environments for local consumption particularly in poor countries or for anyone needing to spend time in desert environments.

Source: asknature.org

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