For the last several decades, the lotus leaf has been the model for slippery, water-repellent surfaces. The leaf is covered in micro-scale pillars that are decorated with tiny structures, like a microscopic forest, explains Penn State materials scientist Tak-Sing Wong. The density of the “trees” is relatively low, creating a thin layer of air between the lotus leaf surface and the canopy. When a water droplet lands on the lotus leaf, it sits on the pocket of air and becomes very mobile, like a puck on an air hockey surface.
It stands to reason that other water-repellent organisms might also use this strategy, but to the surprise of Wong and his colleagues, that’s not what they found in their latest investigation mosquito eyes, cicada wings, and springtail bodies, described in Science Advances today (July 17). “In the tree analogy, there is no tree, just grassland,” explains Wong. Instead of a sparse forest, these insects’ surfaces are covered in dense, nanoscale spikes, like blades of grass.
“Nature is always full of surprises and counterexamples. Every time we understand something, nature shows us that there’s a whole other thing that we missed,” says David Hu, a biomechanics professor at Georgia Tech who was not involved in the work.
The mathematical theory that describes the air-pocket repulsion of water droplets goes back to 1944, with the work of Arnold Cassie and S. Baxter of the Wool Industries Research Association. This explains the water-repellent properties of the lotus leaf and other surfaces in nature, such as duck feathers. “It was a big deal because there’s a huge number of commercial applications, like Rain-X for the windshield, waterproof paint, and GoreTex, that are based on the lotus leaf concept,” says Hu.
Wong’s lab has been studying water-repellent surfaces in nature for more than a decade. In their latest examination of how insects keep dry, it was puzzling that the scanning electron micrographs of various critters revealed surface nanostructures that appeared inconsistent with Cassie-Baxter, because the grassland configuration does not produce the air pockets.
To find out why the insect surfaces were structured this way, Wong’s group made synthetic nanostructures mimicking what they saw under the microscope with varying sizes and densities. Then they performed water droplet impact tests. Basically, they released water droplets onto the surfaces at varying speeds and watched what happened.
What they found is that the water droplets bounced a couple of milliseconds faster away from the surfaces with tightly packed nanoscale structures than from the surfaces with the spread-out, larger microscale structures, says Lin Wang, a graduate student who led the project in Wong’s lab. A couple of milliseconds may not sound like much, but for an insect that is flying in the rain, shaving off that time “is actually a big deal,” he says. It could be the difference between hitting the ground and continuing flight.
Falling raindrops represent a significant problem for insects. They descend at speeds in the range of 1–2 meters/second and can easily weigh 20 percent of an insect’s body mass. “If you get hit by a droplet that is 20 percent of your own weight and you can’t shake it off quickly, that’s pretty dooming,” says Mathias Kolle, who leads a laboratory of biologically inspired photonic engineering at MIT and was not involved in this study.
Existing models didn’t predict that water would bounce off the densely packed nanostructures faster than off those with sparse microstructures. To explain this unexpected property of the nanostructures, Wong and colleagues proposed a new theoretical model, building on Cassie and Baxter’s air pocket theory, that highlights the importance of the interface between air, liquid, and solid, a so-called triple contact line. Because the microstructures are sparsely spread out on the leaf, there are relatively few intersections where all three substances—water, air, and plant—meet. On the insect surfaces where nanostructures are densely packed, there are many touch points, and it’s this property—as-yet not fully understood—that appears to account for the differences between the dense nanostructures and the sparse microstructures.
“This is like discovering a new property, like temperature or pressure,” explains Hu, who wrote the 2018 book, How to Walk on Water and Climb up Walls, which includes a discussion of how water-striders walk on water and how mosquitoes handle raindrops. Still, the triple contact line remains mysterious. “No one’s really measured its properties, its magnitude,” he explains. “I think this will give people more motivation to actually measure this thing which exists, but is difficult to measure because it is so small and even difficult to imagine.”
One reason the nanoscale textures need to be present in high density is that they have to withstand the high pressure of the water droplet landing, Wong and Wang suggest. When structures are sparsely distributed, like in the lotus leaf, the impact of falling droplets can displace them, destroying the pocket of air and causing the water to adhere to the surface. Given the requirement that the tiny structures have to be packed together, water repellency can still be optimized by making nanostructures that will form long triple contact lines when a water drop lands.
Kolle says that these biological surfaces are interesting because they typically serve many functions. For example, butterfly wings are covered with scales whose colors signal to woo potential mates and to scare predators; the wings also need to repel water, help with thermoregulation, and of course, enable flight. Now that the water repelling properties of insects are better understood, the next big question is how the different functions are balanced when they need to be optimized together, Kolle says.
L.R. Wang et al., “Compact nanoscale textures reduce contact time of bouncing droplets,” Science Advances, 6:eabb2307, 2020.