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Specially-coated cotton collects water from desert fog – and releases it as liquid

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January 21, 2013

Eindhoven University of Technology’s Dr. Catarina Esteves, with a piece of the fog-harvest...

Eindhoven University of Technology’s Dr. Catarina Esteves, with a piece of the fog-harvesting cotton

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In arid places where fog occurs overnight, some people utilize so-called “fog harvesters” to acquire fresh water. These are typically pieces of netting that collect fog droplets, which then roll down into a container below. Various researchers have tried to increase the efficiency of these harvesters, such as by making them from a combination of hydrophilic (water-absorbing) and hydrophobic (water-repelling) materials. Now, a team of scientists have done something a little different – they’ve created a cotton-based fog-harvesting material that switches between being entirely hydrophilic and entirely hydrophobic.

The scientists – from The Netherlands’ Eindhoven University of Technology and Hong Kong Polytechnic University – started with ordinary cotton fabric. Ordinarily, plain cotton can only absorb about 18 percent its own weight in airborne water droplets. The team then applied a coating of a polymer known as PNIPAAm.

At temperatures up to 34ºC (93ºF), PNIPAAm has a spongy, hydrophilic structure. This allowed the coated cotton to absorb a whopping 340 percent of its weight in droplets. Once the temperature gets any higher, however, the polymer’s structure “closes up” and becomes hydrophobic. In the case of the treated cotton, this caused the absorbed water to be released in liquid form.

The treated cotton in its hydrophilic (left) and hydrophobic states

The treated cotton in its hydrophilic (left) and hydrophobic states

That water was reportedly pure and safe to consume, plus the polymer was able to stand up to repeated cycles of use. Additionally, while traditional fog harvesters rely partially on wind to help shake the collected droplets loose, the new material works even in completely still conditions.

Cotton is already pretty cheap, and the PNIPAAm is likewise said to be fairly inexpensive. The idea is that sheets of the treated cotton could be laid directly over crops (or receptacles, presumably) at night, then left to absorb and release water on their own. It has also been suggested that the material could be used to make water-harvesting tents, or perspiration-wicking athletic clothing.

The researchers, led by Hong Kong Polytechnic’s Prof. John Xin and Eindhoven’s Dr. Catarina Esteves, are now looking at ways of increasing the polymer’s hydrophilic qualities, and lowering the temperature at which it becomes hydrophobic.

Source: Eindhoven University of Technology

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away.   All articles by Ben Coxworth
7 Comments

Interesting but I think desalination is more practical and better for the environment.

Slowburn
22nd January, 2013 @ 01:41 am PST

@Slowburn, that is, assuming you have a source of saline water. We may be talking actually desert conditions, as in Namibia, where a fog rolls in night, and disappears as the day warms up.

Another possible solution would be to have a fabric with a very rough surface. Water vapour condenses on points, so the more points of contact, the more water gets deposited. It doesn't need to be hydrophilic. Maybe nano fibres could come into play here.

David Colton Clarke
22nd January, 2013 @ 05:51 am PST

@Slowburn,

What would you do with the mountains of salt that would accumulate? There is only so much salt that humans can consume, or lay on the roads during winter to help de-ice them (and lets face it- salt is neither all that effective for that nor kind to road vehicles or roadside vegetation and water-courses). Further, storing vast amounts of salt woud itself be ecologically damaging.

And as David Colton Clarke pointed out, you'd have to be near a sea (not to mention an abundant source of cheap energy).

bergamot69
22nd January, 2013 @ 12:21 pm PST

Now we can colonize Arrakis.

seabear
22nd January, 2013 @ 09:51 pm PST

re; bergamot69

Assuming that I could not sell it at a profit putting salt mines out of business I would put it back into the sea before the salt dunes become a problem.

Solar power will work well if the land is cheap enough.

Taking water out of the air 'steals' it from the land that it would have otherwise watered.

Slowburn
23rd January, 2013 @ 10:57 pm PST

Seems to me that this development is also a passive dehumidifier. Just hang it up in a damp room, blow a fan across it, change the temperature and out comes the water. Might need some energy to change the temperature of course but maybe the day-night transition could do this for you.

warren52nz
29th January, 2013 @ 03:47 pm PST

". . . looking at ways of increasing the polymer’s hydrophilic qualities, and lowering the temperature . . ."

Little point to either, IMO. 340% its own weight seems plenty enough---any more, and the cotton is likely to deteriorate over a number of cycles. It's better to make it cheap, and use a lot of it.

As for lowering the transition temperature, 34ºC appears to me to be easily obtainable in the desert, in full sun. On second thought, evaporation will complicate things---it will lose precious condensate while keeping the cotton too cool.

So, yeah, maybe lowering the transition temperature is worthwhile, as long as the price is kept low.

Freederick
19th April, 2014 @ 02:45 pm PDT
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