Twisted fibres boost efficiency of water harvesting from fog

Fog harvesting is a simple method of collecting fresh water from the air. You string up a mesh material, and water droplets form around the threads and run down to a receptacle. But now a bit of maths and a handful of experiments have shown that we can probably improve enormously on the efficiency of this system. The same science might also give us better microfluidic devices, and even clothes that dry themselves much more efficiently. University of Oslo mathematician Andreas Carlson has discovered that if two threads are twisted together, at the point where the two meet a gully is formed. When water droplets form on such a twisted pair, unlike with a single thread where they tend to get stuck and don’t go into your collecting tin, the droplets are guided into the gully and gravity rapidly pulls them down the thread. He reckons this simple alteration could boost the water output of fog harvesting systems, which can be a lifeline in arid places, by at least a fifth…

Andreas - There has been this concept called the fog nets, which is essentially like a fishing net that you string up. And fog is just very, very small droplets that are dense and they collide with these nets. They drain off and you get water. And what really intrigued us is can we rethink a little bit how we can design this to get better yield when extracting water from the atmosphere?

Chris - Can we start Andreas by thinking about just the single threads that are in those, you dub them, fog nets or fishing net-like structures. When a droplet of water lands on that single thread, what does it do

Andreas - When you have these droplets that kind of collide with the thread. What happens is that you form a drop that most of the time actually gets quite stuck because it cannot just move because there is some roughness on the surface that kind of doesn't really allow the gravity to pull the droplet down along the tread, which then kind of limits their efficiency.

Chris - So if you look at such a thread, you would see the water sort of circumferentially, the droplet would wrap itself around the thread and effectively just lodge there, sticking to the thread.

Andreas - Exactly. So if you have this nylon or plastic threads, what you then get is the water that would cling entirely around the tread, and it would grow so it would become actually quite big. And as it grows big, unfortunately what happens is that the efficiency of these nets go down. So you would like to have these small droplets collide with the net and be very quickly and effectively kind of drain through the net.

Chris - So all the time there's droplets stuck on the net. They're not running into your receptacle giving you fresh water. They're sticking to the net and that's harming the efficiency. So the challenge then is, well, can we make these structures any better at channelling those droplets off of the net and into our pipe?

Andreas - Yeah. There, so there are three things. One, you, you would like to have a very quick drainage and you would like to avoid large droplets because they reduce efficiency in the sense that they can get retrained by the wind. So they just get pulled off the net. And also it turns out that you would like to have rather a lot of small droplet instead of just some larger ones on this net because then the efficiency effectively goes down

Chris - And now you're going to tell me how you think you can do it. So what do you think you've got here? What have you discovered?

Andreas - What we discovered is that if we now generate a groove, which we can then easily do by putting two fibres together, similar to a structure of ropes, which then allowed us to transport water and droplets along this kind of groove structure as we twisted the fibres together.

Chris - Got you. So if I imagine my index finger and then the next finger to it stuck together and that there is a crease between the two by winding two threads together, you create that groove and the droplet falls in there rather than forming that almost barrel shaped circumferential droplet round the single thread.

Andreas - Yes. And really the fascinating part with that is that we can then show that you can then entirely control which shape the droplet would take and also how it would move about as it kind of pulls down by gravity. If you put one droplet on these twisted structures.

Chris - It's a bit like a helter skelter that you see at the fairground then, isn't it? Where the slide goes down, the droplet is effectively in the slide of the helter skelter. That slide being the groove created between the two coiled threads as they wind around each other. How does the size of the threads and also the frequency or the rate at which they're twisting make a difference. Because you could have a helter skelter with lots of twists of the slide or a very shallow twist to the slide. So what's the optimum?

Andreas - This really depends on your kind of application. So if one wants to think about harvesting water from there, we saw that the more we twist the better it is. So the more winds your helter skelter has, the better it would be.

Chris - And what sort of impact do you think this would make if you took fog harvesting, this is where we started and you said your aim was to try to understand and then improve on that. How much better is the water harvesting potential, the efficiency? If you put this twisted configuration into play, what sort of improvement can you achieve, do you think?

Andreas - What we did in our article is that we really illustrated that this is a concept that has a potential to work. Our next step is now of course to kind of explore this in a wider range of wind and fog conditions because I think this is kind of the first proof of principle. But what our results indicate is that we can have a significant improvement if you compare with the single fibre. So at least probably around 20% or more.