The KAUST researchers have designed a principle whereby a new membrane made from water-wet materials has especially designed gas-entrapping pores that allow it to simultaneously separate hot, salty from cool, pure water while facilitating the transfer of pure vapor from one side of the other. The team also indicated that this dynamic could lead to greener, cheaper desalination membranes.
At present, scientists popularly use super-water-repellent perfluorocarbon membranes for a desalination process known as membrane distillation (MD). KAUST post-doctoral fellow, Ratul Das explained that perfluorocarbons are, however, expensive, nonbiodegradable and vulnerable to fouling and damage at higher temperatures.
Himanshu Mishra and his team of researchers at KAUST's Water Desalination and Reuse Center, to develop perfluorocarbon-free alternatives, drew inspiration from two insects; springtails that live in wet sold and sea skaters that live in open oceans. The two insects have mushroom-shaped microtextures covering their cuticles and hairs that can spontaneously entrap life-sustaining air if the insects become submerged in water. Mishra noted that they mimicked those features onto water-wet (nonwater resistant) materials. The resulting robustly trap air upon immersion in liquids. The idea of gas-entrapping membranes was born.
The team developed protocols for creating vertical pores within thin sheets. The diameters of the pores inlets and exits were smaller than the pore channels. Mishra said that they started by toying with thin wafers of silicon to develop pores with these re-entrant edges. These edges prevent liquids from intruding into the pores. He explained further that they were able to achieve the function of perfluorinated membranes by harnessing this bio-inspired texture using water-wet materials, which might seem to defy conventional wisdom. When the team immersed a silicon membrane with simple cylindrical pores in water, it is filled within one second. Silica gas-entrapping membranes (GEMs), on the other hand, trap air robustly within their pores when immersed in water, and can remain intact for more than six weeks.
Then, the researchers explored applying the same principle to a cheaper, easily manufactured water-wet material called poly(methyl methacrylate) (PMMA). A research technician in Mishra's team, Sankara Arunachalam, explained that PPMA-GEMs robustly separated streams of hot, salty feed from cold water for more than 90 hours with a salt rejection of 100 percent.
Furthermore, Mishra said that to their knowledge, this case is the first-ever demonstration of MD membranes derived from intrinsically wetting materials. The benefits are apparent; conventional water-wet plastics such as PMMA are significantly cheaper than perfluorinated ones, are environmentally friendly and can withstand harsher operational conditions. Interdisciplinary investigators are needed to assess the scalability and reliability of this approach. The results of the research could unlock the possibility of common water-wet materials for greener, cheaper desalination.
