As disturbing reports warn of the water crisis facing the world, scientists are turning to the idea of extracting water vapor from the air to quench our thirst. Just this week, a prototype was demonstrated to work even in Phoenix, Arizona, but we’ll need to do better than the volume produced. A combination of the best of different natural processes to pull water from thin air could do just that.
Dr Xianming Dai and Dr Tak-Sing Wong of Pennsylvania State University noted that many animals and plants condense water vapor, but attempts to imitate them have suffered a trade-off. Some capture the water well but transport it poorly. Others have the reverse problem.
Dai and Wong have announced in Science Advances a surface that wins on both counts. Unlike the metal-organic framework tested in the Arizona desert, this work is yet to leave the laboratory. Ultimately, however, it could allow large surface areas, for example the sides of buildings, to harvest dew to drink even during the harshest droughts.
Inspired by the lotus leaf, some previous efforts have created surfaces that condense water vapor and, all going well, repel water droplets so they flow into a basin. Unfortunately, the infused air that prevents the water catching in place have proven to be easily damaged, thereby interrupting transportation.
Other efforts, inspired by the slippery surfaces of the pitcher plant, infused hydrophobic liquid lubricants into the surface. These efficiently collect the water that forms, but only a small part of their surface area triggers condensation, minimizing the water formed in the first place.
Inspired by the Namib beetle, Dai and Wong used hydrophilic (water-attracting) surfaces, but added the slipperiness of pitcher plants. The combination is achieved through shaping their surface at the scale of a billionth of a meter to make grooves that both collect water and transport it in the desired direction, helped along by an oil lubricant.
To work, the lubricant needs to meet three criteria: It must not mix with water, lest it be quickly lost and pollute the product. The lubricant must stick to surfaces so that they are entirely covered, and it must maintain a stronger attraction to them than water does, so that it wets the surface while water stays on top.
Finding such a combination was far from easy, but having done so, Dai and Wong have demonstrated it outperforms alternative surfaces under lab conditions.
"Furthermore, the material is scalable," Dai said in a statement. "Unlike the beetle-inspired surfaces where droplet creation only occurs in specific areas, we can create small or large surfaces, all with the ability to trap and move water quickly everywhere on the surface."