18:42 GMT04 December 2020
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    Raindrops can be a deadly threat to fragile organisms such as insects, with the force of the blow equivalent to a person being struck by a bowling ball. The danger is even greater in the case of a high-speed impact.

    A team of biologists has studied the effect of raindrops on minute organisms, such a butterflies, that often struggle to survive the impact.

    In the peer-reviewed study, “How a raindrop gets shattered on biological surfaces”, published on 8 June in Proceedings of the National Academy of Sciences of the United States of America (PNAS), biological and environmental engineer Sunghwan Jung, from Cornell University in New York, Seungho Kim, and their colleagues analyzed how different animals and plants ward off the potential threat of falling raindrops.

    “Getting hit with raindrops is the most dangerous event for this kind of small animal," said Sunghwan Jung.

    The scientist explains that besides the force of impact itself, the rain can upset an insect’s flight momentum, or strip birds of their warmth. Accordingly, these organisms have to come up with means of dodging contact with raindrops as much as they can.

    To look into just how this is achieved, the team used a high-speed camera that captures from 5,000 to 20,000 frames per second to study the impact of water falling onto butterflies, moths, dragonflies, gannet feathers, and katsura leaves.

    While there had been similar studies before, they had based their observations on drop impacts at speeds much lower than real raindrops, which can reach up to 10 metres (33 feet) per second.

    The new study went further, analyzing water falling on their subjects at high speeds.

    In the process of the research, different impact dynamics resulting from the impact with the rain were recorded.

    The team discovered that as rain collides with the surface of a leaf or a butterfly wing, it hits microscopic spikes that generate shock-like waves through the drop.

    A car drives slowly in the rain on an icy rural road Sunday, Jan. 18, 2015, near Newtown, Pa.
    © AP Photo / Mel Evans
    A car drives slowly in the rain on an icy rural road Sunday, Jan. 18, 2015, near Newtown, Pa.

    As the waves interfere with each other, they force the rain drop to take on a wrinkled pattern as it spreads, with different thicknesses across its volume.

    Due to the wave effect, at the instant when the drop is ready to bounce away, the spikes on the surface of the wing poke holes through the water.

    The drop, accordingly, ruptures into tiny fragments.

    As a nanoscale-structured wax layer on the natural surfaces also aids in repelling the water, coupled with the fragmentation of the drop itself the effect reduces the contact time between liquid and surface by up to 70 per cent, claims the study.

    As this slashes the amount of heat and momentum transfer, this could be a game-changer for insects who desperately require some warmth in their muscles to be able to fly away from predators.

    "By having these two-tiered structures - one microscale (the rough bumpy structure) and the other nanoscale (the wax structure), these organisms can have a super hydrophobic [water-repelling] surface," explained Jung.

    Kim and colleagues also caught these fragmented rain shards smuggling pathogenic fungal spores - revealing how fungi can use plant defences to enhance their own dispersal powers.

    "This is the first study to understand how high-speed raindrops impact these natural hydrophobic surfaces," said Jung.

    Underscoring that the research into how the micro-spikes on butterfly wings shatter raindrops could be used by engineers to develop more advanced waterproofing materials.

    "There's a huge market for these kinds of surfaces," said Jung.

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    butterfly, Cornell University
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