AsianScientist (Jul. 28, 2015) – Scientists from Beihang University have figured out the mechanism of water repulsion in the legs of water striders using high speed camera imaging and microscopy. This finding, published in Proceedings of the National Academy of Sciences, may serve as a foundation for future design of water repellent or antifogging surfaces, the researchers say.
Water repulsion is a key survival mechanism for water striders. They thrive on ponds and in humid environments, where they need to navigate through a large body of water in the pond and also combat against water vapor of their humid environments. Water repulsion helps the insect stay afloat and dry at the same time.
The legs of water striders are made up of a central cylinder decorated by tapered setae (or hairs). Using high speed cameras and microscopy techniques, the researcher captured the process of water repulsion from the legs at nanoscale resolution.
The water repulsion mechanism occurs in three steps. First, water starts condensing or gathering at the tip of the setae, where is slides downwards towards the base of the setae. There the water droplet continue to grow in size until it bends the flexible setae.
The elastic energy of the setae grows as the droplets grows. The water droplet become asymmetrical and flees to the surface of the setae. At this point, the elastic energy converts into kinetic energy.
“Importantly, this mechanism does not require any additional energy,” Professor David Quéré, one of the corresponding authors of the paper, told Asian Scientist Magazine.
This finding has an impact on the future design of water repellent or antifogging surfaces. It serves as a basis and inspiration for such future applications such as repelling dew.
Design for repulsion of water at such nanoscale is “more demanding” than the scale of raindrops, said Quéré. Another potential application of this finding is to use it to harvest water from clouds. One more potential application is the calibration of water droplets.
“As they are expelled, [water] drops have more or less the same size, based on the distance between proximal setae. It is not easy to calibrate drops (in air) of such size,” Quéré explained.
The article can be found at: Wang et al. (2015) Self-Removal of Condensed Water on the Legs of Water Striders.
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Copyright: Asian Scientist Magazine; Photo: Wang Qianbin and Hu Xiaotian/Beihang University.
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