To a butterfly, getting hit by a raindrop is roughly analogous to a human getting hit by a bowling ball made of water, as the scientist Sunghwan Jung notes in this video. So how do butterflies — pretty delicate creatures — keep from getting wrecked by rain?
To figure it out, Jung and a team recorded high-speed video of raindrop-wing impacts; shots of it are in that video above, and quite gorgeous to behold — check it out.
What they saw is that the butterflies have two layers of protection: Microscale bumps that shatter the raindrops into smaller pieces, and a nanoscale layer of wax that repels the smaller balls of water. Their full paper is here.
"Consider the micro-bumps as needles," Jung said. If one dropped a balloon onto these needles, he said, "then this balloon would break into smaller pieces. So the same thing happens as the raindrop hits and spreads."
This shattering action reduces the amount of time the drop is in contact with the surface, which limits momentum and lowers the impact force on a delicate wing or leaf. It also reduces heat transfer from a cold drop. This is important because the muscles of an insect wing, for example, need to be warm enough to fly.
"If they have a longer time in contact with the cold raindrop, they're going to lose a lot of heat and they cannot fly very easily," Jung said, making them vulnerable to predators, for example.
Repelling water as quickly as possible also is important because water is very heavy, making flight in insects and birds difficult and weighing down plant leaves.
"By having these two-tiered structures," Jung said, "[these organisms] can have a super hydrophobic surface."
Obvs, quite apart from the coolness of the raw scientific knowledge, these sort of analyses are useful in helping us figure out possible new biomimicked materials.
(Photo of a butterfly from their experiment courtesy Cornell University; tip of the hat to "Things You Wouldn't Know If We Didn't Blog Intermittently" for pointing out this one)