Anthropocene reports :
"A massive supply of fresh water exists as vapor above oceans. Scientists have an idea to tap it.
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 Zhengof 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.
