It can be quite a shock when a particularly fat raindrop catches you in the eye, so you can only imagine how alarming navigating a torrential downpour as a flying insect must be. New research published in Proceedings of the National Academy of Sciences shows how microstructures on plants and the wings of insects such as butterflies effectively shatter raindrops, reducing the contact time between the water drops and the wing or leaf surface.
To investigate how different wing surfaces interacted with falling raindrops, researchers from Cornell University used a high-speed camera capturing between 5,000 and 20,000 frames per second to see how drops reacted to the wings of butterflies, moths, dragonflies, gannet feathers, and katsura leaves.
"[Getting hit with] raindrops is the most dangerous event for this kind of small animal," said senior author Sunghwan Jung in a statement, noting the relative weight of a raindrop hitting a butterfly wing would be similar to that of a bowling ball falling from the sky on a human.
Their observations revealed that when a drop collides with a leaf or a butterfly wing, it’s met with microscopic bumps or spikes that create shock-like waves through the miniature body of water. These waves clash with each other, wrinkling the drop so it has different thicknesses across its volume. The bumps and spikes then essentially rip up the drop by poking through the water and shattering it into fragments. They then used artificial surfaces to see if they could replicate the surface-drop interaction by mimicking the surface spikes.
By creating a nanoscale-structure wax layer on top of the synthetic surface the researchers could repel the water which, when combined with surface spikes breaking up the droplet, meant they were able to reduce the contact time between synthetic wing or butterfly wings by up to 70 percent. A reduction in contact time like this reduces the amount of heat transferred when flying and also didn’t sap their momentum. This adaptation would be hugely advantageous for birds not wanting to lose heat whilst in flight or insects who need to maintain their ability to maneuver mid-air so as to escape predators.
The team hopes to later be able to use this two-tier structure to engineer artificial surfaces for use in product design and innovation. This isn't the first time that plant and animal properties have been utilized for creating better materials, for example, the lotus plant which has leaves that repel water was used to formulate superhydrophobic nanomaterials that can act as a non-toxic treatment for water-proofing clothing.
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