This Theme promotes innovations in water purification/desalination and reuse through rational design, synthesis and application of new (nano)materials, membranes, and multi functionalized surfaces. The ongoing projects include fouling resistant membranes, pore size control of membranes for selective filtration, smart surfaces for selective oil-water separation, photothermal membranes for solar-driven water desalination, and (photo)catalytically reactive membrane filtration.
PI: Himanshu Mishra
A clear understanding of wetting at interfaces of graphene and graphene oxide with water is essential to drive applications in desalination. Surprisingly, however, wetting behavior of graphene is not entirely clear. In particular, effects of underlying substrates on the surface energy of graphene have remained a subject of debate. Similarly, mechanisms of flow of water through graphene oxide membranes are not well understood. Researchers have invoked nanocapillary flows through sub-nanometer spaces, where continuum descriptions surface tensions, contact angles, and Laplace pressure break down. Adsorption of water vapor on to graphene flakes of different thickness at relative humidity is being investigated. Experiments will be supplemented with theoretical calculations of van der Waals interactions. In another study, the research team of Himanshu Mishra is exploiting nano-scale features of porous anodic alumina (PAA), produced via a lithography-free simple and scalable process, for membrane distillation. After a surface functionalization step PAA membranes are rendered superhydrophobic and, as a consequence, exhibit liquid entry pressures in the range of 5-20 atm. Effects of membrane characteristics, such as diameter, thickness, along with feed water temperature, pressure, and flow rates, are being investigated.
A SOLAR HEATER INSPIRED BY THE LOTUS FLOWER
PI: Peng Wang
A point-of-use solar distillation device that can clean up saltwater and wastewater without producing greenhouse gases was constructed in this study. The key to the new technology is a floating membrane coated with a special light-absorbing polymer that repairs its hydrophobic skin when damaged. 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 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. Inspired by the lotus flower, a plant that restores damage to its hydrophobic leaves through the migration of waxy molecules, researchers at WDRC 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, enabling it to supply additional hydrophobic chains to damaged regions through biomimetic self-migration. The new device nearly tripled the output of freshwater from typical solar stills. It also exhibited remarkable damage resistance; after using 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 PPy-coated 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 for integration of two types of functions into one distillation device. This has great potential to be employed in point-of-use potable water production.