Billions of people around the world lack access to safe, clean drinking water, with water shortages especially acute in the desert environments of North Africa and the Middle East.
Chemists in China are now taking inspiration from spiders to develop systems that collect water from air using no energy. They have designed novel artificial fibers that efficiently gather fog much like spider webs trap dew. The fibres could lead to a low-cost, green way to produce potable water on a large scale.
According to the United Nations, two-third of the world’s population will face water shortages by 2050. The two conventional methods used to produce freshwater on are desalination—which involves removing salt from seawater—and wastewater treatment. But both consume a large amount of energy and require complex technical equipment. Plus, desalination also only works in coastal areas, not arid and rural regions.
Yet, even the most arid parts of the world have a tiny amount of water vapor in the air. Researchers have developed many different technologies to harvest this water from the atmosphere in the form of fog, dew or moisture. These include gels and other water-absorbing materials, nets that can trap fog via condensation, and specially engineered surfaces that draw water.
Some teams have tried to mimic spider silk. The silk threads trap water because of their rough surfaces and periodic spindle-shaped knots that help collect and direct the condensed droplets. This simplicity and potential low cost has inspired researchers to develop several different artificial spider silk-like fibers that harvest fog.
The fibers reported so far have surface properties similar to natural spider silk and can harvest freshwater from fog, but only to a certain extent, write chemist Yongmei Zheng of Beihang University and her colleagues in a recent paper in Chemical Engineering Journal. Only water droplets near or on the spindle knots can be easily collected, reducing water-harvesting efficiency.
Zheng and colleagues took advantage of other unique features of spider silk that previous studies have ignored. One is that the main axis of the silk between the knots consists of two separate, parallel threads with a small gap in the middle, and the other is that the spindly knots have a spiral or helical shape.
The researchers designed a novel dual-thread fiber that more closely mimics spider silk. They start by dipping nylon fibers in water-attracting polymer. By controlling the concentration of the polymer solution and pulling speeds during dipping, they create spindle-shape bulges in the fiber of specifically calculated heights and spaces between them. Then, as reported in the journal Advanced Functional Materials, they heat the fibers at a high temperature to crack the bumps and create a spiral or helix shape.
The unique structure of the fibers boosted the efficiency with which nets made of the fibers collected fog vapors, the team observed. First, tiny droplets collected on the fiber get sucked into the tiny channel between the two threads to form a liquid film. Then this liquid film spreads toward both sides until the channel is filled with water and adjacent spindle knots are connected by the liquid film.
When captured water droplets come in contact with the liquid film, they get quickly transported to spindle knots and fused with droplets already there to form larger drops. The fibers capture water droplets along their entire length, not just at the spindles, and the larger droplets that quickly form are easier to collect, leading to a 590 percent increase in fog harvesting efficiency compared with normal spider silk fibers. In the fibers with the helical knots, each knot could carry over 2,100 times more water than the volume of the knot itself.
The new fibers are low cost, durable, strong and flexible, the team writes, making them extremely promising for large-scale, high-efficiency fog harvesting systems.
Sources:
Jinmu Huan et al. Special fog harvesting mode on bioinspired hydrophilic dual-thread spider silk fiber, Chemical Engineering Journal, 2023.
Shaomin Wang et al. Bioinspired Robust Helical-Groove Spindle-Knot Microfibers for Large-Scale Water Collection. Advanced Functional Materials, 2023.
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