Slippery-Rough Surface Can Harvest Water From Air

In the image, the left panel is a directional slippery rough surface (SRS, this study), the middle panel is a slippery liquid-infused porous surface (SLIPS) and the right panel is a superhydrophobic surface. This image shows a comparison of water harvesting performance of SRS vs other state-of-the-art liquid repellent surfaces. (Xianming Dai/Nan Sun/Jing Wang/Tak-Sing Wong, Penn State)

(CN) – A newly designed surface inspired by rice leaves and pitcher plants outperforms cutting-edge liquid-repellent surfaces in water-harvesting applications, according to a study published Friday in the journal Science Advances.

In the report, a team of researchers presents a slippery rough surface (SRS) that could help nations across the world combat water scarcity, which promises to increase as the effects of climate change worsen.

“With an estimated 4 billion people living in a situation of water scarcity during at least some part of the year, an inexpensive method for harvesting water from water vapor or from fog droplets in air could have enormous practical applications, and will help alleviate the water scarcity issues in many regions of the world,” said lead author Tak-Sing Wong, an assistant professor of mechanical engineering at Penn State University.

The efficiency of many water-harvesting technologies is limited because when water is attracted to a hydrophilic surface, it typically forms a sheet and clings to the surface, making it difficult to remove.

However, Wong’s post-doctoral scholar, Simon Dai, was experimenting with combining various biological strategies to develop a slippery solution for water harvesting.

“With SRS, we combined the slippery interface of a pitcher plant with the surface architecture of a rice leaf, which has micro/nanoscale directional grooves on its surface that allows water to be removed very easily in one direction but not the other,” said Dai, who is now an assistant professor at the University of Texas at Dallas.

Dai created a pitcher plant-influenced slippery surface with hydrophilic chemistry. Simultaneously, he added directional grooves and gave the surface a microscale roughness that expanded the surface area. The pace of fog and water harvesting is directionally proportional to the size of the surface area on which droplets can form. The rice leaf-inspired grooves then sweep the droplets away via capillary action or gravity.

In experiments conducted at Penn State, the team demonstrated that these surfaces can gather miniature water droplets from the air at a pace quicker than many state-of-the-art surfaces. Molecular dynamics simulations carried out at UT Dallas by Steven Nielson, Dai’s colleague, clarify why the hydrophilic surface was particularly adept at water harvesting.

“If the SRS material is produced at scale, we estimate that we can collect over 120 liters of water per square meter of the surface per day, and we can further increase the water harvesting rate by optimizing the SRS,” said co-author Nan Sun, a graduate student in Wong’s group at Penn State.

The team is currently trying to optimize and scale up the SRS so that efficient water-harvesting systems can be placed in regions where water is scarce.

The research was financially supported by the National Science Foundation, the Materials Research Institute at Penn State, the Advanced Research Projects Agency-Energy, Covestro LLC, and the Office of Naval Research.


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