Insect, bird and plant: protection against the destructive power of rain through nanostructures

Someone likes rain, someone drives him into spleen, and someone likes to watch the rain from the comfort of his home. Rain, being part of the planet’s water circulation system, is crucial for flora and fauna. However, for some creatures, such as insects, it could be a harbinger of imminent death, if not for evolution. Scientists from Cornell University (USA) conducted a study in which they examined how rain drops interact with the surfaces of various biological materials (feathers, plant leaves, insect wings, etc.). What did the analysis show, how did the butterflies differ and how can the acquired knowledge be put into practice? The answers to these questions await us in the report of scientists. Go.

Study basis

As already mentioned, rain is an integral and extremely important part of the life of the planet and all its inhabitants. For people, rain is mostly an assistant (in farming, in water supply, etc.). However, for wildlife representatives, rain is often fraught with many dangers: flooding homes or food gathering places, changing the temperature regime, the inability to effectively hide from predators or, conversely, hunting, etc. And this is quite logical, because rainy weather is not dominant in nature, from which evolutionarily it was not the main factor in the adaptation of living organisms to a particular region of habitat. However, there was still adaptation, otherwise the first rain would have destroyed many species unprepared by evolution.

Rain can hardly be called something painful. Well, water is dripping from the sky, think. If drops of falling water would leave physical damage on human skin, no one would invent a shower. But many species of creatures have a different attitude to rain. For example, insects, whose wings often consist of extremely fragile and light elements necessary for flight. There are still birds that could not fly normally if their feathers got wet to the skin. Even plants would suffer if their leaves were damaged by raindrops.

However, we all know that the butterflies did not die out even in the tropics, the birds gave birth to the proverb “like water from a goose”, and the leaves of plants do not look like a sieve. All this is due to the unusual structure of the wings of insects, feathers and leaves, which allows you to break drops at the moment they fall to the surface.

From the point of view of physics, this can be called one word – superhydrophobicity, i.e. the ability to repel water, rather than absorb it like a sponge. Previously, as scientists say, many studies have been conducted on the hydrophobicity of biological surfaces, but the effect of water was “soft.” But during rain, water droplets demonstrate a much more complex dynamics that have not been previously studied.

One of the authors of the study talks about his work.

Scientists note that superhydrophobic structures at the nanoscale prevent liquid from entering the nanostructure. Microstructures, in turn, lead to the fixing of the liquid, allowing it to penetrate into the gaps between the microstructures. Because of this, the residence time (contact) of the bouncing drop on the solid increases. This leads to the fact that between the drop and the surface improves the transfer of mass, momentum and heat. The negative effect is to reduce the functions of self-cleaning, anti-icing, anti-fog and super-hydrophobicity.

Therefore, nanostructures are much more interesting in terms of superhydrophobicity than microstructures.

Nevertheless, previous studies have shown that certain microstructures are able to exhibit asymmetric stretching and retraction, as well as a rebound of a drop in the form of a pancake, which ultimately leads to a quick separation of the drop from the surface with a significant reduction in contact time. But, again, this study was carried out with slow drops, whose speed did not correspond to that observed in real rain.

In this work, scientists decided to test precisely the fast (shock) drops and how they interact with the surfaces of various biomaterials. As a result, it was found that a shock drop at high speeds can generate shock surface waves in the presence of a certain surface morphology at the microscale. The upper interface between the liquid and air in the spreading liquid is broken due to shock waves and becomes vulnerable to ruptures of the water film. In other words, the unusual morphology of the surfaces of biomaterials leads to the fact that the shock drop is divided into parts that cannot cause significant damage, unlike the whole drop.

As a result, the contact time decreases by about 70%, and, accordingly, the transfer of heat and momentum of the shock drop to the surface also decreases.

Research results

Various biomaterials were prepared for observation, including bird feathers, insects, and plant leaves. The samples were exposed to water droplets with a radius of ® from 1.1 to 2.0 mm with a movement speed (U) from 0.7 to 6.6 m / s.

Relevant Weber number * (We = pU²(2R) / γ) ranges from 15 to 2000, which corresponds to the typical precipitation We. The density was 1000 kg / m³and a surface tension of 72 mN / m.

Weber Number * – a similarity criterion in hydrodynamics, which determines the ratio of fluid inertia to surface tension.

Drop dynamics was recorded by a high-speed camera with a frame frequency of 5000 to 20 000 s^-1 and a resolution of 1024 by 672 pixels.

Image No. 1

On the image 1A The effect of a drop on a bird feather, which is superhydrophobic with surface roughness at various scales, is shown.

The pictures show that there is a hierarchical structure: microscopic barbs extend from the barbs attached to the rod.

The structure of the pen: 1 – the fan; 2 – rod; 3 – barbs (consist of intersecting barbs clinging to each other with microscopic hooks); 4 – down part; 5 – ochin (the hollow part of the shaft located in the skin).

When a droplet hits the pen at high speed (We 1000), the liquid-air interface of the propagating droplet is broken and generates hundreds of V-shaped shock waves. A typical shock wave is observed when a compressible fluid experiences a density discontinuity and forms a V-shaped wave pattern. In this case, the observed wave is not a classical shock wave, since the liquid in these experiments is incompressible at speeds and has a uniform density. However, similar V-shaped waves are the result of a rupture in the film thickness of a spreading drop, and not of density changes. Therefore, even if the fluid does not compress, scientists call the wave pattern “shock waves” in their work.

Then, the decaying drop is sharply broken / fragmented shortly after the rupture of the liquid film and the holes formed. A similar morphological transition of the shock drop was also observed for other biological surfaces, such as insect wings and plant leaves (1B and 1C)

All these samples have hierarchical superhydrophobic structures, where there are many micrometer-sized tubercles with nanoscale structures.

To further study the nuances of the structure of the shock wave, two types of surfaces with different wettability *.

Wettability * – the ability of a fluid to maintain contact with a solid surface, which is controlled by a balance between the intermolecular interactions of the adhesive type (liquid to surface) and the cohesive type (liquid to liquid).

The first type is a hydrophilic glass surface, the second is a superhydrophobic surface covered with hierarchical micro- and nanostructures.

Image No. 2

On a smooth hydrophilic surface, the drop simply spreads, forming a rim that expands in the radial direction (2A)

On a superhydrophobic glass, a drop with a low impact velocity propagates, retracts, and bounces (2B), while a droplet with a high shock velocity demonstrates shock surface waves and fracture dynamics (2C)

At a high U value, hundreds of shock-like surface waves were generated in the presence of microscale uneven structures (2C, second image on the left). The drop spreads and then suddenly bursts as holes form (2C, a picture in the center). As a result, the propagating drop is divided into smaller satellite drops (2C, image on the right) as the holes become larger and merge.

A similar dynamics was observed on a surface with microstructures (type III) with a constant interval and height of the protrusions (2D)

These data allowed us to confirm that when a drop collides with hierarchical superhydrophobic surfaces with a high impact velocity, micro-scale irregular structures can perturb the propagating drop, creating numerous shock-like waves at the liquid-air interface, and ultimately the initial drops are broken into smaller ones.

Such distribution and fragmentation significantly reduces the residence / contact time of a drop on a solid surface (almost twice).

Image No. 3

On the image 3A A diagram of how a shock-like wave is generated over a micro-convexity on the surface during droplet propagation is shown. It was experimentally confirmed that the half angle of the shock-like wave increases with time (insertions on 3B) and decreases with the radial distance from the center to the bulge (r_b) On the chart 3C the dependence of ψ on t and r is clearly visible_b for various hierarchical superhydrophobic surfaces.

The half angle of the shock-like wave (ψ) can be determined by the following formula: sin ψ = u_w / u = 1 / Ma *, where uw is the wave propagation velocity, u is the fluid velocity, and Ma is the equivalent Mach number*, since Ma = u / u_w.

Mach number* – the ratio of the speed of the object in the medium to the speed of sound in this medium.

Here u can be approximated as r / t, where r is the radial distance from the center of the drop. Regarding the wave propagation velocity (u_w): the wave propagates under the action of capillaries (microstructural surface irregularities), which leads to u_w = √2γ / (ph), where γ is the surface tension, h is the layer thickness of the spreading drop. Therefore, ψ can be expressed as follows:

sin ψ = 1 / Ma * = √2γ / (ph) / (r / t)

The layer thickness h (r, t) can be approximated as R3 / (U2t2) (Ut / r) n, where n = 3 is the early stage, 2 is the middle stage and 0 is the final stage of the drop propagation. Based on the formula above and the spreading drop layer thickness model, the scattered data on 3C perfectly aligned under one line on 3D.

Image No. 4

Above critical impact velocity (U_c) many shock-like waves are formed that collide with each other. As a result, shock-like waves create an inhomogeneous thickness of the spreading drop layer (4A) with a certain amplitude (| Ƞ |), which leads to the formation of a wrinkled structure on the spreading drop. Then, holes / tears suddenly form in those places where shock-like waves during the fall stage. As a result, the water layer breaks when interacting with microscopic bulges on the surface of biomaterial samples. This fact introduces an additional criterion – h

It was also necessary to analyze the hole nucleation time (t_hole), defined as the time interval between the contact of the drop with the surface and the formation of the first hole. On the 4B thole is shown to decrease with impact velocity (U). This implies two possible ultimate scenarios.

The first scenario – the impact velocity is much higher than the critical velocity (U ﹥ U_c), and holes form on the spreading drop when the radius of the drop is close to its maximum radius (2C and 2D)

In this case, there is a certain “wrinkling” of the spreading drop of extension. This indicates that the amplitude of the perturbed interface (| Ƞ |) is large enough to play a significant role in the rupture of the film.

The second scenario is when U ≈ U_c, a single hole appears immediately before the drop bounces. Thus, the hole formation time, thole, approaches the time of contact with the capillary. In this scenario, the shock wave at the microconvex has a small amplitude and quickly dissipates.

For U> U_cas soon as holes nucleate on a spreading droplet, they expand rapidly to reduce their surface energy on hydrophobic nanostructured surfaces. The propagation velocity of holes on superhydrophobic surfaces was 1 m / s until the holes merged together, which led to a complete rupture of the spreading drop.

Such dynamics of a nanostructured hydrophobic surface also leads to a 70% decrease in the time of contact of a drop with it.

Video No. 2: the effect of a drop of water with a radius of 1.7 mm on other biological surfaces: a sailing butterfly, a cecropia butterfly and a dragonfly.