Twisted fibres are more efficient at capturing and transporting water from foggy air than straight ones. This finding, from researchers at the University of Oslo, Norway, could make it possible to develop advanced fog nets for harvesting fresh water from the air.
In many parts of the world, fresh water is in limited supply and not readily accessible. Even in the driest deserts, however, the air still contains some humidity, and with the right materials, it is possible to retrieve it. The simplest way of doing this is to use a net to catch water droplets that condense on the material for later release. The most common types of net for this purpose are made from steel extruded into wires; plastic fibres and strips; or woven poly-yarn. All of these have uniform cross-sections and are therefore relatively smooth and straight.
Nature, however, abounds with slender, grooved and bumpy structures that plants and animals have evolved to capture water from ambient air and quickly transport droplets where they need to go. Cactus spines, nepenthes plants, spider spindle silk and Namib desert beetle shells are just a few examples.
From “barrel” to “clamshell”
Inspired by these natural structures, Vanessa Kern and Andreas Carlson of the mechanics section in Oslo’s Department of Mathematics placed water droplets on two vertical threads that they had mechanically twisted together. They then recorded the droplets’ flow paths using high-speed imaging.
By changing the tightness, or wavelength, of the twist, the researchers were able to control when the droplet changed from its originally symmetric “barrel” shape to an asymmetric “clamshell” configuration. This allowed the researchers to speed up or slow down the droplets’ flow. While this is not the first time that scientists have succeeded in changing the shapes of droplets sliding down fibres, most previous work focused on perfectly wetting liquids, rather than partially wetting ones as was the case here.
Once they understood the droplets’ dynamics, Kern and Carlson designed nets that could be pre-programmed with anti-clogging properties. They then analysed the twisted fibres’ ability to collect water from fog flowing through an experimental wind tunnel, plotting the fibres’ water yield as a function of how much they were twisted.
Grooves that work as a water slide
The Oslo team found that the greater the number of twists, the more water the fibres captured. Notably, the increase was greater than would be expected from an increase in surface area alone. The team say this implies that the geometry of the twists is more important than area in increasing fog capture.
“Introducing a twist allowed us to effectively form grooves that work as a water slide as it stabilises a liquid film,” Kern explains. “This alleviates the well-known problem of straight fibres, where droplets would get stuck/pinned.”
Refrigerator works by twisting and untwisting fibres
The twisted fibres would make good fog nets, adds Carlson. “Fog nets are typically made up of plastic fibres and used to harvest fresh water from fog in arid regions such as in Morocco. Our results indicate that these twisted fibres could indeed be beneficial in terms of increasing the efficiency of such nets compared to straight fibres.”
The researchers are now working on testing their twisted fibres in a wider range of wind and fog conditions. They hope these tests will show which environments the fibres work best in, and where they might be most suitable for water harvesting. “We also want to move towards conditions closer to those found in the field,” they say. “There are still many open questions about the small-scale physics of the flow inside the grooves between these fibres that we want to answer too.”
The study is detailed in PNAS.
