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.
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.