New Material Lifts Technology From Desert Beetle, Harvests Water From Air
Nature offers some pointers on dealing with our parched new climate.
Image: James C Weaver/ Dylan Wainwright/Joanna Aizenberg
The global water scene should strike fear into the hearts of even the most privileged of middle-class Americans. There will be good years and there will be worse years, but the planet is generally getting drier: When it comes, rain will happen in often cataclysmic downpours, while the dry periods in between will grow longer and longer. There will be less seasonal runoff, lower water tables, and emptier aquifers. At the same time, there will be much, much more demand.
The future will bring floods, but only those offering limited relief and coming at the cost of much damage. The southern US and the Great Plains will have to get used to permanent drought, which means the rest of us will feel water scarcity shockwaves in any number of ways. Clearly, we need to be adapting.
Humans don't have the millenia required for evolution to the job, but we can take some cues from those species that have been working on the problem for a lot longer than us. To that end, materials scientists from Harvard University have published a paper today in Nature describing a new material that can collect and direct water harvested from the air, a hybrid inspired by the bumpy shells of the Namib desert beetle, the V-shaped spines of cacti, and the slippery surfaces of carnivorous pitcher plants.
The general problem faced by researchers is of facilitating and controlling what's known as dropwise condensation. Condensation is easy enough to make happen when the collection material is already wet with the condensate, but not so much when the material is dry. Finding "nonwetting" agents that can promote condensation turns out to be a big problem in industrial applications as these agents tend to break down quickly.
The Harvard paper summarizes: "Controlling dropwise condensation is fundamental to water harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic tradeoffs that make it difficult to optimize both growth and transport at once."
In other words, it's necessary to collect as much water on the surface as quickly as possible while also moving that collected water away as quickly as possible. You can see how these might be competing demands.
As chemist and materials scientist Joanna Aizenberg and her team note, their hybrid material, a synergism of the three evolution-produced designs, substantially outperforms other synthetic water-harvesting materials. Insights came in part from a new, deeper look at the role of the bumpy physical structure of the beetle's shell; previous research has tended to look instead at the shell's surface chemistry instead.
Aizenberg's material takes these bumps and arranges them in patterns similar to those employed by cacti to guide the flow of harvested water droplets. "Lastly, the negligible friction of the slippery coating of pitcher plants inspired us to coat the bumps with molecularly smooth lubricant immobilized on nanotexture to facilitate these topography-based mechanisms," the group explains.
"We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces," Aizenberg and co write. "We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications."
Of course, this isn't a miracle material that can create water where it isn't. The beetle in question harvests water effectively in a desert, but that desert also happens to be prone to foggy conditions. But we can at least imagine it maximizing water harvesting where water is, even if it's present in relatively small amounts.