An analysis of high-speed raindrops hitting biological surfaces like insect wings reveals how the highly water-repelling veneers reduce the water's impact.
A lot of us has been hit on the face by drops of rain water. And while some might get a little bit upset about it, it's actually very much not a big deal. But to insects on the other hand, drops of rainwater are dangerous. Imagine the sky decided to rain down golf balls on us; Yea, it sounds painful right?. Well that's how it is for insects when drops of rain hit them.
Sunghwan "Sunny" Jung, who is a biological and environmental engineer from Cornell University in New York, made it clear that insects are fragile to rain drops.
"[Getting hit with] raindrops is the most dangerous event for this kind of small animal," he said.
He noted that there are other problems raindrops can cause for fragile living things. Apart from the force of imapct, rain for instance, in most cases, can wreak havoc on insects' flight momentum and can deprive birds of their warmth.
Sunghwan "Sunny" Jung along with Seungho Kim and some colleagues decided to take a deeper look at how various animals and plants reduce this potential danger. They made use of a high-speed camera that is able of capturing between 5,000 and 20,000 frames per second to study the impact of water falling onto butterflies, moths, dragonflies, gannet feathers, and katsura leaves.
Before now, other research had made similar observations on drop impacts, but at speeds much lower than real raindrops which is about 10 metres (33 feet) per second. However, in this new research the team dropped water on their subjects at high speeds, and recorded the various impact dynamics that took place.
They discovered that when a drop of water collided with the surface of a butterfly wing, it fell onto microscopic bumps or spikes that created shock-like waves through the little amount of water. These waves interfere with each other, thereby causing the drops of water to form a wrinkled pattern as it spreads, with different thicknesses across its volume.
Then, just as the drop is about to bounce away, the wave effect allows the spikes on the surface of the wing to poke holes right through the water film, which ruptures the drop into tiny fragments. (This effect was further investigated by dropping water onto an artificial surface that mimicked the surface spikes.)
A nanoscale-structured wax layer on these natural surfaces 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. In turn, that reduces the amount of heat and momentum transfer. This would make a huge difference for insects who need to retain some warmth in their muscles to be able to fly and escape 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." Jung explains.
Kim and the team also noticed these fragmented rain shards smuggling pathogenic fungal spores - exposing 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 said.
With this new understanding of how micro-spikes present on butterfly wings shatter raindrops, engineers could be able to make more advanced waterproofing materials.