Chemical engineers have developed a polymer coating that renders cotton super-absorbent, and are proposing that it be used to develop a desert fog water-collecting system or sportswear that keeps athletes dry.
John Xin of Hong Kong Polytechnic University and Catarina Esteves of Eindhoven's University of Technology (TU/e) built the coating after taking a leaf out of the the humble Namib desert beetle's book. The beetle has risen to fame in recent years for inspiring several other water collecting devices, including the 2011 Dyson Award-winning Airdrop and NBD Nano's self-filling water bottle. The beetle survives in the arid southwest African desert -- where just 1.3cm of rain falls on average each year -- by making use of a humid ocean fog that regularly envelops much of the region. The beetle aims it wings at the incoming breeze, and as it hits its hard shell droplets condense. These collect before running down its back, straight into its mouth. This is possible because of tiny bumps on the beetle's back which are water-attracting at their tips and water repelling at their sides.
While the Airdrop had to borrow heavily from the Namib beetle, Xin and Esteves' design does not rely on the presence of wind -- its water-repelling and attracting properties are temperature reliant. This was made possible by applying a coating of Poly(N-isopropylacrylamide) -- or PNIPAAm. PNIPAAm was first engineered in the 50s, and goes from being highly absorbent at lower temperatures, to dehydrating and shedding all its hydration at higher temperatures. By applying this to cotton, the pair found it increased the material's absorbency to 340 percent its own weight (it was 18 percent without the coating, so quite the adaptation). When the temperature hits 34 degrees celsius however, the change occurs and the polymer acts as a kind of waterproof barrier; the cotton becomes hydrophobic (water repellant) and sheds all the water it has absorbed.
Like the fog harvesting polypropylene mesh nets already in use, this cotton-absorption method would work well in mountainous or desert regions, where humid mists sweep in and are one of the main sources of rare moisture. According to a statement by TU/e, their method does not rely on wind, the water shed by the cotton is pure and it can be repeated over and again with the same results.
Seen as it is heat dependant, however problems could arise with the system. Sea fogs often occur when the temperatures are low, so this is the period during which the polymer coating is doing its work and making the most of condensation in the air. As soon as the temperature rises the water will be shed -- making it ideal for hydrating crops at sporadic times throughout the day. However, temperatures in the Namib can be unpredictable and soar to above 50 degrees celsius, while its lowest daily temperature is often just a few degrees different from its highest. This means there could be long periods where the cotton fills and sheds its water just once a day, if that, and it could miss out on a large part of a fog which occurs 180 days of the year. This is in fact an area of research that the duo will be focusing on going forward, to see if they can manipulate the polymer's properties to allow the dehydration to occur at a lower temperature.
Nevertheless, it's an interesting concept, and the duo are already toying with uses that range beyond irrigation systems -- to sportswear that keeps the wearer dry or tents that collect water overnight.
The study detailing Xin and Esteves' find will be published in a February addition of Advanced Materials.
