The beetle, the cactus and an animal-trapping plant help engineer water handling, says S.Ananthanarayanan.
Getting droplets of water to form from vapour and then moving the water away has industrial importance. Whether it is water-harvesting, condensation in a steam turbine or fogging of a windscreen, there is the need for a surface that combines both the ability to form droplets as well as effectively draining the condensation centre.
Kyoo-Chul Park, Philseok Kim, Alison Grinthal, Neil He, David Fox, James C. Weaver and Joanna Aizenberg, at Harvard University, in their report in the journal, Nature, say that existing approaches, based on very fine scale texturing of surfaces, have not been able to simultaneously optimise both how fast a droplet grows and also how fast it flows away. But the example of the structure and composition used by three different biological species, the authors of the paper say, has led them to design a surface that does a lot better than synthetic surfaces that have been tried out so far.The inspiration for the drop forming structure comes from the Namib desert beetles, which have evolved to squeeze water out the nearly moisture-free air in the Namib desert, on the Atlantic side of southern Africa, an area which receives as little as 1 cm of rain in the year. With no other source of water, these beetles are known to strike a pose with their wings and rear end exposed to the fog and the wind, to harvest precious drops of water, which the wings’ microstructure channel to the creatures’ mouths.
While the backs and wings of these beetles have a distribution of small bumps, most studies, the Nature paper says, have attributed their water gathering property to the surface chemistry – of the bumps being ‘water attracting’, while the surroundings are ‘water repelling’, rather than the topography of the surface. The emphasis has been on micro and nanoscale structure and pits or depressions have been considered to be better water gatherers than protuberances, like the millimeter sized bumps that the beetles have, the paper says.
The Harvard researchers, however, note that the entire bumpy surface has been found to be covered with water repelling wax, which means that it is not the surface chemistry that helps condensation. On the other hand, the researchers say, even in the absence of a microstructure, the geometry of the millimeter sized dome could be an agent of condensation, by acting to concentrate the flow of vapour over the surface at the top of the dome.To test out this possibility, which was suggested by some members of the group in Jan 2015, an experimental bumpy surface of the same millimeter dimensions was created by pressing thin aluminum sheet and treating the surface to be water repelling. The sheet, along with a bump-free control sheet, was then exposed to a temperature and humidity regulated environment, where the convective movement of air close to the surface was negligible, so that the main effect near the surface was diffusion of air, due to molecular movement. While there was droplet formation, due to condensation, what was seen is that the largest droplets, which indicate concentration of vapour, occurred at the apex of bumps in the bumpy sheet. A number of control trials were carried out, to eliminate the role of roughness or chemical properties of the surface, and it was established that it is the curvature of the bump that leads to condensation, which increases if the bump is more pointed. There is hence an optimum size, and even modification of the shape, to be rectangular, for the most efficient condensation.
Draining of condensate<
While this trial showed that bumps may be a good way to promote droplets, the next question was how to drain the droplets away, so that the water could be collected and was not lost to evaporation. Here again, nature showed and optimised a design for drawing the water off, even against the force of gravity. The method is by using capillary forces, the kind that make the water in a glass vessel creep up at the edges, as exemplified in the spines that are found on cacti.
Cacti are a category of plants found in desert and arid regions, where the priority is conservation of water. Thus, cacti have thick stalks and leaves, which have less surface area as a proportion of their volume, and they are partly covered by thin spines. The spines have little water content and so present no avenue for leakage, but studies, by Jie Ju, Hao Bai, Yongmei Zheng, Tianyi Zhao, Ruochen Fang and Lei Jiang have shown in a paper in the journal, Nature Communications in 2012 that the spines have barbs with a conical shape, with microscopic grooves that get smoother as they approach the base, to help channel water that may condense from fog or as dew. The Harvard researchers work it out that when the bump has a non symmetrical slope which grows wider as it descends, the drop that forms at the apex would move down the slope and keep moving even as it widens, as it grows in size as it coalesces with other droplets. The effect of guiding the droplet along the slope is regardless of even the force of gravity, in case the bump is oriented so that the flow is upwards.
The effect is dependent on no friction or very little friction being there while the droplet moves. To simulate this property, the Harvard group relies on a stratagem of another optimisation in nature, a category of carnivorous plants that trap animal prey by luring them to the slippery rim of a deep cavity, the Nepenthes pitcher plants, so called because the insect trap is shaped like a pitcher. Following the lead, the Harvard group lubricated the bumps and the asymmetric slope so that droplets could move just as gravity and capillarity would drive them.
The result of trials with all three features – bumps, the slope and lubrication, being provided, was that larger drops formed faster and then slid down more effectively than with other surfaces that had not been constructed in this topography. “This combination of short response time and reliable, high-volume long-term performance is critical in numerous applications ……such as heat exchange, dehumidification, and desalination systems,” the authors of the paper say.
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