< !--Google Tag Manager-- >
How Life-Forms Mitigate Raindrop Impact Damage

How Life-Forms Mitigate Raindrop Impact Damage

Have you ever wondered how birds and insects can fly through heavy rainfall without adverse consequences? Researchers have discovered that their wings are exquisitely designed to repel water—a feature that points to an intelligent designer.

Raindrops can cause a lot of damage. Where I grew up in coastal British Columbia, raindrops can get as large as a quarter inch diameter. Such drops free falling from clouds a thousand feet high or higher can strike with considerable impact force. I have seen such raindrops rip an umbrella to shreds.

How, then, do delicate plant tissues, insect wings, bird feathers, and other life tissues survive and function under such fierce rainfall conditions? A team of five biological engineers and entomologists led by Seungho Kim answer this question in a recent article published in the Proceedings of the National Academy of Sciences USA.1

Kim’s team performed a number of experiments where they prepared (mounted) a variety of biological tissues, including insects, plant leaves, and bird feathers. They then subjected these tissues to raindrops ranging from 2.2 to 4.0 millimeters in diameter and impacting at velocities ranging from 0.7 to 6.6 meters per second. They captured the dynamics of the raindrop impacts on the various tissues with a camera with a frame rate of 5,000–20,000 frames per second. Readers can watch a 5-second video clip of what happens to a 1.7-millimeter raindrop falling on the wing of a tiger moth here: https://movie-usa.glencoesoftware.com/video/10.1073/pnas.2002924117/video-3.

Kim’s team demonstrated that all the tissues they studied possess nanoscale (requiring a powerful microscope to see) superhydrophobic (super water repellent) surfaces. These surface structures prevent the penetration of liquid water into the tissues.2 The experiments showed that when the impacting raindrops approached the tissue surfaces, the surfaces generated shock-like surface waves. The shock waves disrupted the spreading raindrops at the point of where air meets liquid. These perturbations then triggered ruptures and holes, breaking the falling raindrops in each case into dozens of tiny satellite droplets.

The result is that contact time between the falling raindrops and the tissues was very much reduced. The superhydrophobic surfaces can be so water repellent that raindrops bounce off the surface in only a few milliseconds. Because these raindrops break up into tiny droplets, heat and momentum transfers from the impacting raindrops to the tissues are reduced to tiny fractions.3 This means that birds or insects do not lose body heat as fast as they would otherwise.

If it were not for the amazingly designed superhydrophobic surfaces on the feathers of birds, exposure to rain would lower the body temperature of birds to a degree where they would die from hypothermia. Likewise, the same design feature on the wings of flying insects ensures that the flights of such insects during falling rain do not destabilize and either ground or kill such creatures. Similarly, this same shock wave, water repellent feature on the leaves of many vascular plants means that such leaves won’t be ripped to shreds during rainstorms. Kim’s team also noted that superhydrophobic surfaces enhance the dispersal of spores.

These kinds of research achievements give us yet more evidence that the more we study nature the more evidence we uncover for the many intricate and ingenious ways God designed Earth’s life to thrive. They also give us opportunities to copy nature’s designs to make better water repellent materials for our own use.

Endnotes
  1. Seungho Kim et al., “How a Raindrop Gets Shattered on Biological Surfaces,” Proceedings of the National Academy of Sciences USA 117, no. 25 (June 23, 2020): 13901–07, doi:10.1073/pnas.2002924117.
  2. Tae-Gon Cha et al., “Nanoscale Patterning of Microtextured Surfaces to Control Superhydrophobic Robustness,” Langmuir 26, no. 11 (February 12, 2010): 8319–26, doi:10.1021/la9047402.
  3. Samira Shiri and James C. Bird, “Heat Exchange between a Bouncing Drop and a Superhydrophobic Substrate,” Proceedings of the National Academy of Sciences USA 114, no. 27 (July 3, 2017): 6930–35, doi:10.1073/pnas.1700197114.