How to make a self-filling water bottle

Generating water out of the air is tricky, but it can be done thanks to the Namib Desert Beetle, which figured out the trick long ago, and a bit of biomimicry.

Biomimicry is the use of inspiration from nature to create new technologies. A good example is Velcro, which is based on burrs from plants and their habit of sticking to socks. In this case though, the kudos for cleverness really does belong to the beetle in question.

The Namib Desert Beetle, showing its long, spindly legs and its black wing casings covered with water-attracting bumps (click to embiggen)
Namib Desert Beetle, Stenocara dentata, with its black wing casings covered with water-attracting bumps. Click to see the bumps up close (Photo by Hans Hillewaert, via Wikimedia Commons)
This beetle belongs to the genus Stenocara and it lives in the Namib Desert, on the coast of Namibia in south west Africa. The desert is very dry, receiving less than 10 millimetres of rain per year, but about six times per month there are morning fogs that sweep across the sand. And it’s from the fog that Stenocara gets its water (Parker AR & Lawrence CR 2001, “Water capture by a desert beetle”, Nature, vol. 414, no. 6859, pp. 33-34, doi:10.1038/35102108).

It’s a member of the family Tenebrionidae, or the darkling beetles, so-called because they have black wing casings. In the Stenocara these wing casings (which are actually the forewings, or elytra) are fused together, and the whole thing is covered with little bumps about half a millimetre in diameter.

The trick is that these bumps are hydrophilic, meaning that water sticks to them, much like it clings to and spreads out on smooth glass. But the areas in between the bumps are waxy and hydrophobic, i.e., they repel water.

What happens is that in the morning the beetle stands on its long, spindly legs facing into the breeze, with its body angled at about 45 degrees. As the fog flows past, tiny droplets accumulate on the hydrophilic bumps. When they get to about 5 millimetres in diameter, they become too big to stick to the bumps and they detach and roll down the hydrophobic grooves to the beetle’s mouth.

In a single day, the beetle can collect 12% of its weight in water. So it’s not surprising that people have been trying to make similar materials artificially, usually by depositing drops of hydrophilic substance on a hydrophobic substrate.

One company, called NBD Nano, is trying to commercialise it and create efficient, artificial fog harvesters. Of course, this isn’t the first technology that’s been tried for this purpose.

Dehumidifiers are a familiar example, taking water out of the air by cooling it and causing condensation. But of course that requires quite a bit of energy to run – although in 2011 an Australian inventor won the James Dyson Award for a device that cools the air using underground coils, so you only need enough energy to pump the air down a pipe.

Another low energy technique is that used in the Atacama Desert in Chile, and is basically just a piece of mesh strung between two poles, with a trough under it. They get a fog too, and as it flows through the mesh, drops of water condense and run down into the trough.

But NBD Nano claim that the beetle’s skin is several times more efficient than the Chilean method. So far they claim a square metre of beetle-inspired material, at 21 °C and 75% relative humidity, can produce 3 litres of water per hour.

Energy is still used to blow air across the collector, but so far they’ve gotten away with using solar panels and a rechargeable battery. So using this as a covering for a greenhouse would be a very simple way to generate enough water for the plants at fairly low cost.

Of course, you don’t need a fan to blow the air if there’s a foggy breeze, like in Chile or Namibia. But it’s also unnecessary if the collector itself is moving, like on a car or a boat, or even a marathon runner.

This is where the idea for a ‘self-filling water bottle’ comes in: imagine running along, carrying a bottle with a panel of this material, and your motion is what allows it to generate water.

That’s a long way off, but it’s a good reminder that the air carries a lot of water that’s just waiting to be harvested. All we need to do is copy the Namib Desert Beetle, and we can do it too.

(This story aired on 6 December 2012 – you can listen to the podcast.)

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One thought on “How to make a self-filling water bottle

  1. That is incredible! I didn’t know about this species before, but I have wondered about the possibility of “self-filling” water bottles since the atmosphere in many parts of the world contains significant amounts of water vapor. It’s only a matter of capturing it. Although it’s a long ways off, this beetle could certainly help achieve this goal.

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