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How a raindrop gets shattered on biological surfaces
Edited by Joanna Aizenberg, Harvard University, Cambridge, MA, and approved May 14, 2020 (received for review February 15, 2020)

Significance
Rainfall on biological superhydrophobic surfaces is ubiquitous in nature. Previous studies in a laboratory setting have focused only on low-speed impacts, which can be quite different from rain conditions in nature. In this study, we reported unexpected and interesting shock-like patterns when a drop impacts biological surfaces at high speeds. These shock-like waves trigger sudden drop fragmentation into smaller satellite droplets and lead to a more than twofold decrease in contact time. Our findings may elucidate biological advantages (hypothermia risk reduction for birds, flight stability for insects, spore dispersal on plants) of superhydrophobic surfaces triggered by microstructures.
Abstract
Many biological surfaces of animals and plants (e.g., bird feathers, insect wings, plant leaves, etc.) are superhydrophobic with rough surfaces at different length scales. Previous studies have focused on a simple drop-bouncing behavior on biological surfaces with low-speed impacts. However, we observed that an impacting drop at high speeds exhibits more complicated dynamics with unexpected shock-like patterns: Hundreds of shock-like waves are formed on the spreading drop, and the drop is then abruptly fragmented along with multiple nucleating holes. Such drop dynamics result in the rapid retraction of the spreading drop and thereby a more than twofold decrease in contact time. Our results may shed light on potential biological advantages of hypothermia risk reduction for endothermic animals and spore spreading enhancement for fungi via wave-induced drop fragmentation.
Superhydrophobic structures at the nanoscale are known to prevent the penetration of the liquid toward the nanostructures (1). However, structures at the microscale cause a liquid-pinning behavior by allowing the liquid to penetrate into the gaps between the microstructures (2), thereby leading to the increase in the residence/contact time of a bouncing drop on a solid (3). As a result, mass, momentum, and heat transfers are enhanced between the drop and the substrate (4⇓–6), while hindering other known functions such as self-cleaning (7), anti-icing (8), antifogging (9), and robust superhydrophobicity (10). Therefore, nanostructures have been considered to be more valuable to achieve such prominent superhydrophobic functionalities, compared to microstructures.
Recently, drop impact on engineered microscale structures has been known to exhibit asymmetric spreading (11) and retraction (4) and a pancake-shaped rebouncing (12), finally leading to the rapid drop detachment with a significant decrease in the contact time (4, 12). However, the role of microscale structures during drop impact was underestimated since most studies have focused on drop impacts at low speeds (4, 5, 13, 14), much lower than real raindrop impact speeds in natural events.
In this present study, we demonstrate that an impacting drop at high speeds can generate shock-like surface waves in the presence of surface morphology at the microscale. The top air–liquid interface of the spreading liquid is perturbed due to the shock waves and becomes vulnerable to film ruptures via hole nucleation. Finally, the holes grow in time and coalesce with each other. As a result, the contact time is reduced about 70%, and, correspondingly, the heat and momentum transfers of the impacting drop onto the substrate are reduced (5). Therefore, our findings may elucidate functional benefits in terms of hypothermia risk, flight stability, and spore dispersal of biological surfaces triggered by the microscale structures.
Results
Experiments.
We prepared various biological specimens including bird feathers, insects, and plant leaves (Materials and Methods). Then, a water drop is released to impact these biological surfaces. Here, the drop radius, R, and the impact velocity, U range from 1.1 to 2.0 mm and 0.7 to 6.6 m/s, respectively. The corresponding Weber number, We
(A–C) Biological surfaces for drop impact experiments (Left) and sequential events after the impact of a water drop of 1.7 mm in radius on the biological surfaces (from Left to Right). (A) Northern gannet feather, where [U, We] = [4.6
Drop Impact on Biological Surfaces.
Fig. 1A shows a drop impact on a bird feather, whose surface is superhydrophobic with surface roughness at different scales (17). Fig. 1A, Center Left Inset shows the hierarchical structure: Microscale barbules emerge from barbs attached to a millimeter-scale rachis. When a drop impacts the feather at a high speed (We
Drop Impact on Artificial Surfaces.
To further investigate details of the shock-like wave structure, we prepared two types of substrate with different wettability. One is a hydrophilic glass surface, and the other is a superhydrophobic surface coated by hierarchical micro- and nanostructures (Materials and Methods). On the smooth hydrophilic substrate, a drop merely spreads by forming a radially expanding rim as in Fig. 2A. On the superhydrophobic glass, a drop spreads, retracts, and bounces at a low-impact speed (Fig. 2B), whereas a drop with a high-impact speed exhibits shock-like surface waves and destructive breakup dynamics as in Fig. 2C.
Bottom-view images of an impacting drop with 1.7 mm in radius on artificial surfaces. (A) Drop impact on a smooth glass for high-impact velocity, where [U, We] are [3.8 m/s, 680]. (B and C) Drop impact on a glass coated by hierarchical superhydrophobic structures (type I) for low-impact velocity (B) and for high-impact velocity (C). Corresponding [U, We] are [0.9 m/s, 40], [3.8 m/s, 680], respectively. Insets in B represent the side-view image of drop impact achieved by a synchronized high-speed camera. Insets in C are magnified images of the local area to clearly show the microbump, the shock-like wave on the microbump, and the nucleated hole followed by the shock-like wave, respectively. For low U, the impacting drop merely spreads, retracts, and rebounds, whereas for high U, hundreds of shock-like waves are generated on the spreading drop (Center Left), and then a number of holes are abruptly nucleated (Center), which grow in time simultaneously (Center Right). Finally, the contact time between the drop and the substrate gets shortened by breaking into smaller satellite droplets (Right). (D) Similar dynamics were observed on a different superhydrophobic surface with regularly spaced microbumps (type III), where [U, We] are [3.8 m/s, 680]. The corresponding videos are in Movies S4 and S5, respectively.
At a high U, hundreds of shock-like surface waves were generated in the presence of microscale bumpy structures as in Fig. 2C, Center Left. A drop spreads and then is suddenly ruptured, as holes are nucleated (Fig. 2C, Center). Eventually, the spreading drop is shattered into smaller satellite droplets (Fig. 2C, Right) as the holes get bigger and coalesce. Similar dynamics were observed on a micropatterned surface (type III) with constant spacing and height of bumps as in Fig. 2D. Therefore, we confirmed that, when a drop impacts hierarchical superhydrophobic surfaces with a high-impact velocity, microscale bumpy structures can perturb the spreading drop to generate the numerous shock-like waves on the liquid–air interface and eventually break into smaller droplets.
Such spreading and fragmentation behaviors greatly lower the residence/contact time of a drop on a solid substrate. For a typical bouncing drop (Fig. 2B) with low-impact velocities, the measured contact time (≃18 ms) almost follows the theoretical capillary contact time, 19 ms, estimated from
Shape of Shock-Like Waves.
Fig. 3A shows the schematic of how a shock-like wave is generated above a microbump on the surface as a drop spreads. Experimentally, we confirmed that the half angle of the shock-like wave increases with an elapsed time (Fig. 3B, Insets) and decreases with the radial distance from the center to a bump
(A) Schematic of the formation of shock-like waves by microbumps, where
In analogy to the Mach shock wave, the half angle of the shock-like wave, ψ, might be determined by the distance ratio of
Here, u can be approximated by
Hole Nucleation Criterion.
Above a critical impact velocity,
(A) Schematic of shock-induced liquid–film rupture due to superhydrophobic microstructures. (B) Critical time for hole nucleation,
Hole Nucleation Timescale.
Now, we estimate the hole nucleation time,
Second, when
Characteristic bump height ϵ and spacing s of experimentally used nonwettable surfaces
Finally, we can determine the critical impact velocity,
Drop Fragmentation.
For
Decrease in Contact Time.
We measured the contact distance of a spreading drop, d, defined as the minimum distance between the drop center and the contact line on either the outer rim or a nucleated hole. Also, the contact time,
(A) Temporal evolution of dimensionless contact distance
For
Discussion
We showed that an impacting drop with high We (
It is known that the exposure to rain can lower the body temperature of birds (34) and destabilize flying insects (35). The decrease in the drop-contact time limits the heat and momentum transfer onto organisms (4⇓–6). Our quantitative measurements on various biological surfaces may unravel how birds lower hypothermia risks and how insects maintain flight stability as well as preserve regional heterothermy during rainfall. In addition, we observed that an impacting drop is shattered into smaller satellite droplets carrying plant pathogenic spores (SI Appendix, section H). This sheds light on an additional spore dispersal mechanism besides the recently discovered vortex-induced dispersal mechanism (36).
Materials and Methods
Preparation of Biological Specimens.
Bird feathers, insect wings, and katsura leaves were used because of their superhydrophobic property with hierarchical structures (17, 20, 21, 37, 38). For bird feathers, the carcass of northern gannet (Morus bassanus) was obtained from the Smithsonian Museum of Natural History. Insect samples were collected from the Cornell University Insect Collection, which had been gently killed in atmospheric conditions. Here, we used dragonfly (Anax), cecropia moth (Hyalophora cecropia), zebra swallowtail butterfly (Protographium marcellus), and cracker butterflies (Hamadryas). Katsura leaves (Cercidiphyllum japonicum) were obtained from near Tower Road, Ithaca, NY (
Preparation of Artificial Superhydrophobic Surfaces.
We coated smooth glass substrates using a commercial nanoparticle spray (NeverWet; Rust-Oleum), which renders the glass substrate superhydrophobic with hierarchical structures (13). The spray coating consists of two different steps: step 1 and step 2. Step 1 and step 2 provide a layer of tiny particles creating rough physical morphology and a layer of hydrophobic adhesives with fewer particles, respectively. By selectively combining steps 1 and 2, we acquired two different superhydrophobic surfaces with varying roughness. Type I and type II artificial surfaces are created by applying either steps 1 and 2 or only step 2. Type III and type IV artificial surfaces are fabricated via a conventional soft lithography process, where we use polydimethylsiloxane (PDMS) as an elastomeric stamp. Square-shaped microbumps of
Physical Property of Microbumps.
A microscope was used to determine the physical property of microbumps on various surfaces. The microscopic images at different locations were taken, and the mean and SD values were calculated. The characteristic bump height, ϵ, and spacing, s, are listed in Table 1. The detailed methods of characterizing the surface morphology are described in SI Appendix, section I.
Drop Impact Experiments.
A water drop is released at a certain height, falls under gravity, and then impacts a dry substrate. By choosing a syringe needle size and the drop-releasing height, the radius R and the impact velocity U of a water drop can be varied from 1.2 to 2.0 mm and from 0.7 to 6.6 m/s, respectively. The dynamics of an impacting drop are videotaped by two synchronized high-speed cameras (Fastcam SA-Z and Mini AX100) with a resolution of 1,024 × 672
Data Availability.
All data and Matlab scripts are available on the Open Science Framework (DOI 10.17605/OSF.IO/6RD8K).
Acknowledgments
We thank Hyunggon Park, Hope A. Gruszewski, and David G. Schmale III at Virginia Polytechnic Institute and State University for supplying and recording the drop dynamics on an infected wheat leaf. We also thank Kamel Fezza and Tao Sun at Argonne National Laboratory for helping us to set up and use the X-ray phase contrast imaging equipment. This work was supported by National Science Foundation (NSF) Grant CBET-1604424 and US Department of Agriculture Award 2018-67013-28063. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. This work was performed in part at the Cornell NanoScale Facility, a National Nanotechnology Coordinated Infrastructure member supported by NSF Grant NNCI-1542081.
Footnotes
- ↵1To whom correspondence may be addressed. Email: sunnyjsh{at}cornell.edu.
Author contributions: S.K., E.E., J.J.D., and S.J. designed research; S.K., Z.W., and S.J. performed research; S.K., Z.W., and S.J. analyzed data; S.K., Z.W., and S.J. wrote the paper; and E.E. prepared micropatterned surfaces.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Data deposition: The data reported in this paper have been deposited in the Open Science Framework (OSF) database, DOI 10.17605/OSF.IO/6RD8K.
See online for related content such as Commentaries.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2002924117/-/DCSupplemental.
- Copyright © 2020 the Author(s). Published by PNAS.
This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
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