The novel membrane's pores trap air upon immersion in water, separating the liquids on either side and only allowing the transport of pure water vapor from the hot side to the cold side. Image: 2019 Ivan Gromicho.A new membrane made from water-wet materials has specially designed gas-entrapping pores that allow it to simultaneously separate hot, salty water from cool, pure water while facilitating the transfer of pure vapor from one side to the other. This principle, developed by researchers at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia, could lead to greener, cheaper desalination membranes. The researchers describe their work in a paper in the Journal of Membrane Science.
Currently, super-water-repellent perfluorocarbon membranes are popularly used for a desalination process known as membrane distillation (MD), in which the membranes block liquid water but allow water vapor to pass through. But perfluorocarbons are expensive, nonbiodegradable, and vulnerable to fouling and damage at higher temperatures, explains KAUST postdoctoral fellow Ratul Das.
With the aim of developing perfluorocarbon-free alternatives, Himanshu Mishra and his team of researchers at KAUST's Water Desalination and Reuse Center drew inspiration from two insects: springtails that live in wet soils and seaskaters that live in open oceans. Both have mushroom-shaped microtextures covering their cuticles and hairs that can spontaneously entrap life-sustaining air if the insects become submerged in water.
"We mimicked those features onto water-wet [non-water repellent] materials. The resulting surfaces robustly entrap air upon immersion in liquids. The idea of gas-entrapping membranes was born," explains Mishra.
Mishra's team developed protocols for creating pores within thin sheets, in which the diameters of the inlet and outlet of each pore are much smaller than the pore channels.
"We began by toying with thin wafers of silicon to develop pores with these re-entrant edges. These edges prevent liquids from intruding into the pores," Mishra explains. "We 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 a silicon membrane with simple cylindrical pores is immersed in water, it becomes completely full of water within 1 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.
The team then explored applying the same principle to a cheaper, easily manufactured water-wet material called poly(methyl methacrylate) (PMMA). "PMMA-GEMs robustly separated streams of hot, salty feed from cold water for more than 90 hours with a salt rejection of 100%," says Sankara Arunachalam, a research technician in Mishra's team.
"To our knowledge, this is the first-ever demonstration of MD membranes derived from intrinsically wetting materials," says Mishra. "The benefits are obvious: common water-wet plastics, such as PMMA, are significantly cheaper than perfluorinated ones, are environmentally friendly, and can withstand harsher operational conditions. Interdisciplinary investigations are needed to assess the scalability and reliability of this approach."
This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.