Scientists from Yanshan University, Jilin University and Yancheng Institute of Technology in China assisted by colleagues from University of Lisbon created a plant capable of simultaneously producing water from fog and generating electricity. Unlike the majority of the existing designs, where some devices gather moisture effectively, but do not produce electricity, while others generate current from falling drops, but hardly give any water, the new plant combines both functions in a single system.
The development was inspired by several natural mechanisms at a time. First, scientists used the principle of a beetle from the Namib desert which collects moisture owing to a combination of hydrophilic and hydrophobic sections on its carapace. Second, they borrowed a conical shape of the wild cactus barbs creating the gradient of curvature and guiding a drop of water to its foundation. The third source of inspiration is nepenthes (pitcher plant), a carnivorous plant with a special slippery surface covered with microdepressions through which liquid may quickly move virtually without resistance.
As a result, all these principles were integrated in one structure.
To this end, scientists used a laser to apply a ramified dendritic painting with wedge-like microchannels on a titanium plate. Then, using heating and ultraviolet radiation they formed sections with different wettability on the surface – from superhydrophobic repelling water to superhydrophilic where a drop virtually instantly spreads over the surface.
The structure they obtained works as follows:
Tiny drops of fog settle on the hydrophilic side branches and gradually merge into larger ones. Then, owing to the Laplace pressure difference (a higher pressure in narrow parts of the wedge-like channels) and to the action of capillary forces drops begin to independently move towards the central channel and then downward. Gravitation and the gradient of wettability help water to quickly glide over the surface without forming a solid film which usually degrades the effectiveness of fog collection. Experiments showed that such geometry provides the most effective transport of drops.
The best results were shown by the surface that combined two types of microstructures. The speed of moisture collection in such a case reached 9.7 g of water per square centimeter per hour. This is approximately 4.3 times more as compared with the unprocessed titanium surface and more than twice more as compared to usual hydrophilic coatings.
Then the collected water under the action of the gravity force gets onto the surface of the second device – a droplet triboelectric nanogenerator. Its working layer is made of a composite based on MXene and polydimethylsiloxane. The first one is highly capable of accumulating a charge, the second one makes the surface superhydrophobic with wetting angle about 152°.Owing to this drops quickly roll down and do not hinder further generation of electricity.
When a drop hits the generator surface, it gets deformed and causes redistribution of electrons between the material layers. As a result a short electric pulse arises. Scientists selected optimum operating conditions: drop falling height about 25 cm, surface inclination angle 45° and drop volume about 30 microliters. In such a mode the generator produced voltage up to 5 V and current about 0.25 microamperes. The system withstood more than one thousand working cycles without any noticeable deterioration of characteristics, while the protective polymer layer protected the material from the action of water and ultraviolet.
To demonstrate the operation of the system the researchers assembled a module with an area of about 0.35 m2 consisting of 25 elements for collecting the fog and a built-in generator. Under the laboratory conditions imitating a dense fog the plant collected about 1.6 l of water within 10 hours – about 83% of the human daily need for drinking water for arid regions. Simultaneously the system generated enough electricity to feed a light-emitting diode which can be used, for instance, for supplementary lighting of plants.
As an experiment scientists used water collected to grow barley. Seeds began to come up in 48 hours, and a week later the sprouts irrigated with water and produced by the plant visibly overtook in height the test group without irrigation.
The analysis of the climatic data shows that the new technology is especially suitable for regions with high air humidity, but with non-regular precipitation, for example, for Salvador or Bangkok, where relative humidity may reach 96%. According to the estimates of the researchers, in these cities the plant can produce 4.6 l of water per square meter of the surface per day.
