Water
technologies
Membrane distillation
Tackling water scarcity, which currently affects every continent and ~3 billion people around the world, is one of the greatest challenges in this century. Desalination, which separates freshwater from saline or contaminated water, proves to be one of the most promising methods to ease global water stress. As a hybrid thermal/membrane desalination technology, membrane distillation (MD) has recently gained much attention owing to its simple separation mechanism with low operating temperature and pressure. MD can utilize low-grade or waste heat to separate water vapor from high salinity waters, therefore be regarded as an energy-efficient strategy to ease water stress for the less developed regions.
The key component for efficient MD desalination is a suitable hydrophobic membrane with a low resistance to vapor transfer while keeping a high resistance to liquid wetting. These requirements pose a paradoxical issue to the membrane design – large membrane pore size leads to an enhanced distillation capacity, but it also increases the susceptibility to wetting. To date, it remains a challenge to create a polymeric membrane that can simultaneously enhance distillation flux and wetting resistance via a simple processing method.
At the AECR Lab, we are currently studying a new approach to create super liquid-repellent membranes with hierarchical porous structures. By coating a thin layer of silicone nanofilament network on top of microporous membrane matrix, our developed hierarchical membrane showed a concurrent enhancement of the wetting resistance and distillation performance. The nanofilament outer layer can effectively prevent membrane wetting under an extremely high hydrostatic pressure (>11.5 bar) without compromising vapor transport. Meanwhile, the large micro-pores inside the membrane increased the distillation flux by up to 60% over the state-of-the-art commercial membranes. Compared with polyethylene and polytetrafluoroethylene membranes, the nanofilament-coated membrane not only exhibited superior thermal stability in long-term exposure to hot saline but also greatly improve the thermal efficiency in desalination, from 84% to 93%. Our next step is to further develop this novel strategy and enable it to be applied in distributed water treatment systems.
Moisture harvesting
For almost all the water harvesting installations in desert lands (e.g., desalination, dewing system, adsorbent-based water collection, etc.), condensation is the ultimate means to capture water from moisture. The efficient condensation strategy is of critical importance for the arid regions to relieve water shortages. The fog-basking desert beetle with hydrophilic bumpy structures has long been considered an excellent biological example with an extraordinary water harvesting rate. While to date, real implementations using beetle-mimetic surfaces are still limited owing to the laborious fabrication of contrasting wetting characteristics in large-scale manufacturing. Additionally, in industrial applications, the benefit of water capture on microscale hydrophilic structures is often outweighed by the liquid pinning effect.
At the AECR Lab, we are studying a facile and scalable approach to fabricate a biphilic nanoscale topography that successfully balances the tradeoff between water nucleation enhancement and rapid liquid transport. By minimizing the hydrophilic bumpy structures to nanoscale, the novel biphilic surface completely changes the random condensation mode, and enables a tunable water nucleation phenomenon. We reveal a strong correlation of characteristic water nucleation spacing on biphilic structural topography. This nucleation tuning effect realizes a synergetic effect for water nucleation and liquid transport on biphilic nanostructures, enabling a threefold water collection rate in arid climates (relative humidity below 40%) as compared to the state-of-the-art superhydrophobic surface.
Our outcome expands the horizon of designing surfaces for tuning condensation dynamics according to different environmental conditions and may lead to important advances in multifunctional applications, particularly for water harvesting in drought-affected regions.