As efforts to harvest clean water in arid regions continue, a team of scientists has developed crystals that create water from air without using energy.
The team, from New York University Abu Dhabi’s Smart Materials Lab, Jilin University in China and the Centre for Smart Engineering Materials, developed elastic organic crystals – named Janus. Inspired by desert plants and animals, these crystals are capable of capturing moisture from the air.
The findings, published in October in the Journal of the American Chemical Society, also showed how this technology could accelerate water collection on an industrial scale, potentially offering an alternative to energy-intensive desalination, which provides most of the UAE’s water.
According to US Geological Survey data, Earth’s atmosphere holds about 13,000 cubic kilometres of water.
“This is an untapped source of water that is just present, but there are no efficient ways to utilise this water,” said professor of chemistry at NYU Abu Dhabi, Pance Naumov, who led the research. “We would like to capitalise on the fact that we have humidity present in the air, but what we need is a more efficient way to harvest that water and convert it into potable water.
“That water does not contain any salts, so we don’t need to use any energy to desalinate it. What we need is efficient technology to harvest it and convert it into liquid water.”
Prof Naumov added that the opportunities in Abu Dhabi are significant given the coastal location and the way that humidity fluctuates significantly during the course of the day, adding that their research is “very effective”.
“It is a record holder in the amount of water that it can collect over a certain period of time and over the surface area of the material.”
What’s the science behind it?
Part of the surface of the crystals that Prof Naumov and his team have worked on is hydrophobic or water repellent, while the other part is hydrophilic, meaning they no longer repel water. Water is collected in a hydrophilic area and transported by a hydrophobic region to a receptacle.
This twin feature of the crystals is why it was named after a Roman god Janus who was often depicted with two faces.
Another key feature of the Janus crystals is their optical transparency, so if a laser or light is shone one side, it passes through to the other side. This property is useful to monitor water collection, Prof Naumov said.
“If the crystal does not have any water droplets on its surface, we have normal transduction of light, but when we have drops accumulating, because of different interactions between the crystal and the water, this affects the light passing through the crystal,” he explained. This makes the process “smart or advanced compared to other materials that do not have self-sensing capability”.
Alternatives to Janus crystals?
Any organic crystal that can grow in elongated shapes allowing some parts to be covered with a hydrophobic coating could potentially be used to collect water, Prof Naumov said. “In principle we can use environmentally-friendly or benign or even biodegradable crystals for the same purpose so they will be used to collect water and after that discarded and not have any footprint on the environment,” he added.
“The next stage would be for this technology to be developed further – to optimise the composition, for example, to compare different materials, to assess different crystals against existing water-collection materials and finally to scale this up on an industrial level.”
Desert beetles show ingenuity of nature
Beetles in Africa’s Namib Desert collect water from fog on their shells, a phenomenon that has intrigued researchers for years. The insects feature hydrophilic bumps that accumulate moisture from the air and hydrophobic channels that direct water to their mouths.
In a paper published early this year in PNAS Nexus, researchers in the US highlighted the importance for water collection of tiny surface irregularities on the backs of desert beetles with the scientific name Onymacris bicolour.
The scientists compared the water-collection capabilities of dead beetles that had their shells coated with gold paint (which affects wettability), nail polish (which covers surface irregularities) and both paint and polish. These were then placed in tiny channels through which fog passed.
It showed that the ones with gold coating had little impact on the amount of water collected, while those that were not covered by nail polish showed greater water harvesting.
Dr Hunter King, an assistant professor at Rutgers University in the US, said that the “impaction efficiency”, which is a key factor affecting water accumulation, “is significantly increased by the texture of the surface”.
“We have also clearly seen that dramatically changing the surface chemistry or wettability of the surface has no effect on impaction,” he told The National.
“I’m hopeful that our takeaway – that the shape and texture of an aerosol-intercepting target … can significantly affect collection – could be used to enhance relevant technologies,” he said, adding that it will be helpful for developing devices that collect water.
