ROBERTO ORECCHIA / ALAMY STOCK PHOTO
BIOLOGICAL AN D EN VIRON MEN TAL SCIEN CE AN D EN GIN EER IN G DIVIS IO N
A solar heater inspired by the lotus flower Chemical tricks improve the efficiency and durability of photothermal membranes that use sunlight to turn water into steam.
point-of-use solar distillation device that can clean up saltwater and wastewater without producing greenhouse gases has been constructed by a research team from KAUST1. The key to the new technology is a floating membrane coated with a special light-absorbing polymer that repairs its hydrophobic “skin" when damaged. For centuries, attempts have been made to use the sun’s heat to distill clean water from polluted sources. Simple solar stills, such as a glass plate placed over a waterfilled box, are inexpensive to operate but are also notoriously inefficient. This is because water is a poor light absorber, and any captured heat tends to distribute uniformly through the still instead of localizing at surfaces where evaporation occurs. To combat these problems, researchers are developing floating “solar generator" materials that heat up quickly in sunlight and then trap heat at air–water interfaces for steam production. These devices are usually coated with
Researchers found inspiration for the new distillation device in the floating lotus flower, which is able to send waxy molecules to repair damage to its hydrophobic leaves.
water-repellant waxy molecules, such as fluorinated alkyl chains, for better floating. However, damage from ultraviolet rays and oxidative chemicals can degrade the hydrophobic layers, causing the generator to sink.
“A simple one-hour treatment in sunlight was sufficient to restore the mesh’s selffloating capability.” Inspired by the lotus flower, a plant that restores damage to its hydrophobic leaves through the migration of waxy molecules, Peng Wang and colleagues from the University’s Biological and Environmental Science and Engineering Division developed a self-healing solar generator. The researchers coated a tightly woven stainless steel mesh with polypyrrole (PPy), a light-absorbing polymer with high photothermal conversion efficiency and bumpy surface microstructures. The team modified the PPy film with fluoroalkylsilane chains, enabling it to act as a reservoir that supplies additional hydrophobic chains to
damaged regions through biomimetic self-migration. The new device nearly tripled the output of freshwater from typical solar stills, thanks to a significant jump in temperature at the air–water interface and a conversion efficiency of close to 60 percent. It also exhibited remarkable damage resistance: after the team used a plasma source to oxidize the mesh and make it sink to the bottom of a beaker, they found a simple one-hour treatment in sunlight was sufficient to restore its self-floating capability. The team’s first prototype — a transparent plastic condensing chamber and solar fan mounted on top of a PPycoated mesh — floats lightly on the surface of seawater and distills a steady stream of water for more than 100 consecutive hours. “Careful material selection allowed us to integrate two types of functions into one distillation device,” Wang said. “This has great potential to be employed in pointof-use potable water production.” 1. Zhang, L., Tang, B., Wu, J., Li, R. & Wang, P. Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating. Advanced Materials, 17 July 2015 doi: 10.1002/adma.201502362
From curiosity to innovation