Butterfly Wings Have Secret Structures That Protect from Deadly Raindrops

For butterflies, a drop of rainwater is equivalent to a person being struck by a bowling ball falling from the sky. Now we know how they protect themselves.

Butterfly wing mitigating the danger posed by a falling raindrop. Kim et al., PNAS, 2020

“[Getting hit with] raindrops is the most dangerous event for this kind of small animal,” explains biological and environmental engineer Sunghwan “Sunny” Jung, from Cornell University in New York. But the force of impact alone isn’t the only problem raindrops can pose for these fragile beings. Rain can wreak havoc on insects’ flight momentum and take away their warmth, so limiting time in contact with each raindrop is critical for these animals.

Jung is part of a research team that has recently taken a closer look at how different animals and plants mitigate this potential danger. Their findings have recenly been published in a peer-reviewed study titled “How a raindrop gets shattered on biological surfaces.”

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, they had to come up with some means to avoid contact with raindrops as much as they can.

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

While there had been similar studies before, they all 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 and recording the different impact dynamics coming into play.

The team discovered that as rain collides with the surface of a leaf or a butterfly wing, it hits microscopic bumps or spikes that generate shock-like waves through the falling drop of water. These waves then interfere with each other, causing the droplet to form a wrinkled pattern as it spreads, with different thicknesses across its volume.

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

At that point, the drop ruptures into tiny fragments – and the danger is literally driven away.

This diagram shows a post-impact rippling drop on super-hydrophobic surface. (Kim et al, PNAS, 2020)

A nanoscale-structured wax layer on these natural surfaces also helps to repel the water; this, along with the fragmentation of the drop, reduces the contact time between liquid and surface by up to 70 percent, the researchers found.

As this slashes the amount of heat and momentum transfer, this can 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,” explains Jung.

The research team 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,” Jung notes.

(Cornell University)

Enhanced understanding of how micro-spikes on butterfly wings shatter raindrops could help engineers to develop more advanced waterproofing materials, an area which has already drawn from nature – think of the water-repelling coating on clothes originally inspired by lotus leaves.

“There’s a huge market for these kinds of surfaces,” said Jung, but if you want to make “engineering products inspired by this material, durability is the biggest issue.”

Indeed, butterflies are ephemeral, and so is the technique to protect their brief existence.

Sources: 1, 2, 3


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