In arid regions, fog can often be a hidden source of fresh water, and some communities have turned to "fog harvesters" to collect it. These devices typically use a mesh or web-like structure to capture tiny water droplets from the air, which then drip into containers below. Scientists have long been working to improve the efficiency of these systems—some experimenting with hydrophilic and hydrophobic materials to enhance water collection. Now, a new breakthrough has emerged: a team of researchers has developed a cotton-based material that can switch between being fully hydrophilic and hydrophobic, making it far more versatile than traditional fog collectors.
The innovation comes from a collaboration between scientists at Eindhoven University of Technology in the Netherlands and Hong Kong Polytechnic University. The project started with regular cotton fabric, which naturally absorbs about 18% of its weight in water. But when coated with a special polymer called PNIPAAm (poly(N-isopropylacrylamide)), the material gains remarkable properties. At temperatures up to 34°C (93°F), PNIPAAm behaves like a hydrophilic sponge, allowing the cotton to absorb up to 340% of its own weight in water droplets. However, once the temperature rises beyond that point, the polymer undergoes a phase change, becoming hydrophobic and releasing the collected water as droplets.
This unique behavior makes the material ideal for passive water collection, without the need for wind or mechanical shaking—unlike conventional fog harvesters. The water collected is pure and safe, and the polymer coating can be reused multiple times, adding to the sustainability of the system.
Cotton is an affordable and widely available material, and PNIPAAm is also relatively inexpensive. The researchers envision using this treated cotton as a low-cost solution for rural or drought-prone areas. It could be laid over crops or placed in containers to collect moisture from the air at night. Additionally, the material could be used in innovative ways, such as in tents designed to gather water or in sportswear that helps manage sweat.
Professor John Xin from Hong Kong Polytechnic University and Dr. Catarina Esteves from Eindhoven University of Technology are now working on refining the material further. Their goal is to enhance the hydrophilic properties of the polymer and lower the temperature threshold at which it switches to hydrophobic. This would make the technology even more efficient and adaptable to different climates.
With continued development, this smart cotton-based material could become a game-changer in water-scarce regions, offering a sustainable and cost-effective way to access clean water.
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