Freshwater capture for water extraction

On the surface

There’s a natural example of a surface that’s engineered to facilitate the extraction of water from the atmosphere. It’s a beetle (Physosterna cribripes) that lives in the Namib Desert in Africa. It exposes hydrophilic (“water loving”) bumps on its back to the prevailing winds at dawn, when the fog comes in. These structures capture freeform fog droplets in the air. The rest of its back is hydrophobic, so the droplets can roll off easily and reach the beetle’s mouth. This way it can survive in an environment that has no ground water.

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Physosterna cribripes, Sossusvlei, Namibia. Credit: WikiMedia

Around the world there’s been interest in replicating this idea. I’ve been working on freshwater capture for about 11 years now. The idea is to use the power of surfaces to achieve an advanced function – and there is no more important objective than to extract water for use by people, animals and plants.

But we’ve recently moved away from that particular beetle example, because it turns out there’s very little volume that you can get out of that structure. In the last three years or so, we’ve moved on to a new project that we call ACWA: advanced capture of water from the atmosphere.

Within one cubic metre of air at 70% humidity and temperature of 30˚C, there is approximately one tablespoon of water. If the humidity is higher, there is obviously more water. The idea is to extract this water from the atmosphere and use it where we need it, when we need it.

The idea is to extract this water from the atmosphere and use it where we need it, when we need it.

We’ve designed surfaces that, when exposed to the sky, actually become cooler than the environment around it. By being cooler, they condense the water that is in vapour form. The idea is similar to what happens when you get a bottle of milk out of the fridge – droplets form on the outside of the bottle because water in the air condenses on the coldness of the glass. Obviously, the volume of water that condenses is proportional to the area of the surface, so if we want to do this on a large scale, we need to capture water over large surface areas, without having to use any energy so that is sustainable and affordable.

The concept is called daytime radiative cooling. Basically, you’re trying to design materials that have particular optical properties so that they reflect solar light, and don’t heat up under the Sun. And secondly, they emit heat at a particular wavelength called the “atmospheric window”, so that heat is not reabsorbed near the surface, and therefore it spontaneously cools.

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We’ve designed nano-porous polymer surfaces that, due to their structure and composition, can be cooler than the air around them. So if the humidity is high, in the order of 70% or above, you can reach the conditions of dewpoint – the point at which vapour changes to liquid. If you then design the surface so the droplets readily roll off, you can collect them, giving access to a small but precious amount of water that can supplement other existing supplies. It’s the same type of water quality that you would get from rain.

We’ve been running prototypes installed on the roof of one of the buildings here at Sydney University. Our best collection rates are so far in the order of 350ml of water per square metre of surface per day, but we’re developing the materials to improve this.

The surface doesn’t look anything special – just like plain white paint. This is because the structure is at nanoscale, within the bulk of the film. You need a scanning electron microscope to see the structure.

Our best collection rates are so far in the order of 350ml of water per square metre of surface per day, but we’re developing the materials to improve this.

We are close to commercialisation. The idea is that it will be applied like paint to the roofs of buildings or warehouses or farm buildings, where a small but predictable supply of water throughout the year can make the difference between survival and failure. It could be used to feed water to high-value plants, or to support animals that need a constant amount of water. There is any number of applications.

In its final form, it will be a standard paint formulation. So it’s not particularly energy intensive to make, and uses very common chemicals that are already in the market.

We’re also working on functional lubricant-infused surfaces that are designed to be slippery, so they don’t allow any liquids or solids to attach to them. With this feature, we can do two important things that are really important for sustainability of flow processes.

Firstly, we can prevent the attachment of bacteria and larger organisms. This is especially relevant in the marine environment, where you get fouling from the attachment of algae, barnacles and larger organisms that occurs whenever a boat or any sort of underwater infrastructure is exposed to the sea. Basically, we have a liquid-like coating that doesn’t allow anything to stick to it. It won’t actually kill the species, it just doesn’t allow them to hook onto the surface. You can also prevent fouling by species such as proteins or cells onto biomedical devices.

There’s very, very promising data showing that it works. Importantly, in the marine application, it uses non-toxic liquids: we use silicone oil, which is not something that you want bucketloads of in the ocean, but it’s not toxic to marine life. And the volume that was shown as necessary for this effect is actually minute – of the order of 0.2 millimetres of the oil per square metre of surface.

It’s very exciting where that interest is leading.

The second application of lubricant-infused surfaces is in drag reduction. Any object moving through water requires energy to drive it because there’s frictional drag, that is resistance to flow. If you can allow the water to slip on the surface, reducing the frictional resistance on the surface, then you need less energy to drive that object. We’ve shown on a microfluidic scale that we can reduce drag by close to 20% relative to a flat surface.

We are in collaboration with a small enterprise based in Sydney, MicroTau, to fabricate these structures on a larger scale. It might open the scope for application on ship hulls.

Chemistry had always been a subject I am comfortable with. Both my parents are chemists – my Dad was a university professor, and my Mum was a high school chemistry teacher. It has always been a natural fit for me. I was preparing to choose my Honours project at the University of Florence in Italy when a professor there purchased a new type of instrument called an atomic force microscope. I became fascinated by the idea that you could image surfaces at the nanoscale, and that’s when my interest in patterns and structure on surfaces started. It’s very exciting where that interest is leading.

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