Superrepellency of underwater hierarchical structureson Salvinia leafYaolei Xianga, Shenglin Huanga , Tian-Yun Huanga, Ao Dongb , Di Caoa, Hongyuan Lia, Yahui Xuea, Pengyu Lva,and Huiling Duan ( )a,c,1

aState Key Laboratory for Turbulence and Complex Systems, Department of Mechanics and Engineering Science, Beijing Innovation Center for EngineeringScience and Advanced Technology, College of Engineering, Peking University, Beijing 100871, People’s Republic of China; bPeking-Tsinghua Center for LifeSciences, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China; and cCenter for Applied Physicsand Technology, Key Laboratory of High Energy Density Physics, and Inertial Fusion Sciences and Application Collaborative Innovation Center of Ministry ofEducation, Peking University, Beijing 100871, People’s Republic of China

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved December 16, 2019 (received for review January 2, 2019)

Biomimetic superhydrophobic surfaces display many excellentunderwater functionalities, which attribute to the slippery airmattress trapped in the structures on the surface. However, theair mattress is easy to collapse due to various disturbances, lead-ing to the fully wetted Wenzel state, while the water filling themicrostructures is difficult to be repelled to completely recoverthe air mattress even on superhydrophobic surfaces like lotusleaves. Beyond superhydrophobicity, here we find that the float-ing fern, Salvinia molesta, has the superrepellent capability toefficiently replace the water in the microstructures with air androbustly recover the continuous air mattress. The hierarchicalstructures on the leaf surface are demonstrated to be crucial tothe recovery. The interconnected wedge-shaped grooves betweenepidermal cells are key to the spontaneous spreading of air overthe entire leaf governed by a gas wicking effect to form a thinair film, which provides a base for the later growth of the airmattress in thickness synchronously along the hairy structures.Inspired by nature, biomimetic artificial Salvinia surfaces are fab-ricated using 3D printing technology, which successfully achievesa complete recovery of a continuous air mattress to exactly imi-tate the superrepellent capability of Salvinia leaves. This findingwill benefit the design principles of water-repellent materialsand expand their underwater applications, especially in extremeenvironments.

underwater air-mattress recovery | hierarchical structures |Salvinia leaf | biomimetic materials

B iomimetic underwater superhydrophobic surfaces haveattracted considerable attention because of their excellent

properties in engineering application, such as drag reduction (1–4), antibiofouling (5, 6), and anticorrosion (7, 8), which rely onthe existence of a continuous slippery air mattress trapped in themicrostructures of the surface (3, 9, 10). However, many fac-tors, including liquid pressure, fluid flow, and air diffusion, candestroy the the air mattress and lead to the fully wetted structures(11–14). Recently, several studies have explored the recovery ofthe underwater air mattress (15–18), while the understandingof the mechanism of continuous air-mattress recovery remainsincomplete, and the design principles of the surface structuresfor efficient air-mattress recovery are still required. Here wefind that the hierarchical structures on the Salvinia leaves canefficiently and robustly recover the collapsed air mattress byspontaneously trapping the replenished air. Moreover, we revealthe underlying mechanism of the recovery process and fabricatebiomimetic artificial Salvinia surfaces by following the naturedesign principles. The finding here not only reveals the physi-cal mechanism of the efficient and robust air-mattress recovery,but also promotes the practical applications of the slippery airmattress especially in extreme environments.

The floating aquatic fern Salvinia molesta (Fig. 1 A and B) isone of the most famous invasive plants (19, 20). When Salviniais immersed accidentally underwater, the dense hairy structures

on the leaf surface are capable of trapping a thick air mattress(Fig. 1C), which can support its respiration and photosynthesis(21–23). Even if the air mattress collapses due to the unavoidablepressure compression and fluctuations, the capability of Salviniato recover the continuous air mattress will improve its survivalchances in the severe natural environment. Experiments on theair-mattress recovery on Salvinia leaves are implemented below.

In Situ Optical Observation of Air-Mattress Recovery onSubmerged Salvinia LeafPrior to each experiment, a liquid pressure of 6.8 atm was appliedto collapse the air mattress on submerged Salvinia leaves. A dig-ital camera was then employed to observe the recovery process.Here we used a syringe to inflate air into the wetted structures(Fig. 1 E and F). As soon as the air was inflated, a silvery filmwas visible on the leaf surface due to the reflection of light atthe water–air interface, indicating that a thin air film emerged tocover the entire base of the leaf surface. As the replenished airvolume increased, the air mattress grew thicker and thicker untilthe microstructures were completely filled with air (reflectedby the gradual increase in silvery brightness; Movie S1), whichexhibits a superwater-repellent capability. In addition to airinflation, other air replenishment methods, like regulating thepressure of surrounding water, can also recover the collapsed air

Significance

Instability and collapse of the underwater slippery air mat-tress hinder its applications, after which the air mattresscannot be recovered even on superhydrophobic surfaces likelotus leaves. Beyond superhydrophobicity, we present theunderwater superrepellent capacity of Salvinia leaves, whichcan efficiently and robustly recover the invalid slippery airmattress by trapping the replenished air to replace the waterin the microstructures. The interconnected wedge-shapedgrooves on the base are key to the recovery, which spon-taneously transport the replenished air to the entire surfacegoverned by a gas wicking effect. Using 3D printing tech-nology, biomimetic artificial Salvinia surfaces are fabricated,which successfully achieves the recovery of the air mattress.This finding will greatly extend the underwater applicationsof water-repellant surfaces.

Author contributions: Y. Xiang and H.D. designed research; Y. Xiang, S.H., T.-Y.H., A.D.,D.C., H.L., P.L., and H.D. performed research; Y. Xiang analyzed data; and Y. Xiang, S.H.,T.-Y.H., Y. Xue, P.L., and H.D. wrote the paper.y

The authors declare no competing interest.y

This article is a PNAS Direct Submission.y

Published under the PNAS license.y1 To whom correspondence may be addressed. Email: hlduan@pku.edu.cn.y

This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1900015117/-/DCSupplemental.y

First published January 21, 2020.

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Fig. 1. (A) Photo of a cluster of S. molesta floating on water. (B) One leaf within the cluster. The surface of the leaf is densely covered with hairy structures.(C) Air mattress trapped on the hairy structured surface after fresh immersion underwater, shining in a silvery appearance due to the reflection of light. (D)Collapse of the air mattress induced by a pressurization up to 6.8 atm in a sealed chamber. (E) Schematics of the air replenishment process, where a syringe isused to inflate air into the structures of Salvinia leaves. (F) Air replenishment through a syringe with the needle right above the leaf. With the air inflation,a thin air film forms on the base of the leaf and then the air film grows thicker and eventually fully fills the hairy structures.

mattress (SI Appendix, Fig. S1A). It is worth noting that, beyondsuperhydrophobicity, this superrepellency is still valid under highliquid pressures up to 7 atm in our experiments. The air-mattressrecovery can be even achieved in flow with Reynolds number upto 5,000 (SI Appendix, Fig. S1B). Thus, this superrepellency isrobust and expected to be able to endure extreme conditions.Moreover, the complete recovery of the air mattress is indepen-dent of the location of air inflation (SI Appendix, Fig. S1 C–E),and consequently, tiling a few Salvinia leaves together conducesto a large area of air mattress recovered over all of the leaves (SIAppendix, Fig. S1F). In addition, identical experiments were per-formed on a lotus leaf as control observations. In contrast to theSalvinia leaf, discrete bubbles, instead of a continuous air mat-tress, formed on the lotus leaf (SI Appendix, Fig. S2). As bothSalvinia and lotus leaves are superhydrophobic, the superrepel-lent capability of water on Salvinia leaves is attributed to theunique hierarchical microstructures on the leaf surface, whichare revealed below.

Microstructures on Salvinia Leaf Surface and the Details ofthe Air-Mattress Recovery at MicroscaleThe Salvinia leaf surface is covered with a dense hairy forest(Fig. 2A) owning three geometrical characteristics: 1) intercon-nected microgrooves on the epidermis, 2) long hair stems, and3) eggbeater-shaped heads. The length of the hairy structures(including the hair stems and the eggbeater-shaped heads) l andthat of the adjacent distance d are hundreds of micrometers,which are much smaller than the capillary length. Microgroovescovered with nanowax crystals (21) are formed at the jointof the neighboring convex epidermal cells and connected overthe entire epidermis (Fig. 2 B and C). A confocal image inFig. 2D shows the cross-sections of the microgrooves whosemorphology can be approximated as a wedge with a half cor-ner angle α = 17.2◦ ± 6.1◦. The measured equilibrium con-tact angle on the surface of the microgrooves θ = 146.1◦ ±7.8◦ (see SI Appendix, Figs. S3 and S4 for details of themicrostructure properties). Note that this large value of con-

tact angle is due to the residual air that remained in the nano-structures to achieve superhydrophobicity (detailed explanationsin SI Appendix).

We zoomed in at the microscale to observe the recovery ofthe air mattress in detail (SI Appendix, Movie S2). Three stagescorresponding to the three geometrical characteristics were cap-tured, as shown in Fig. 2 E–G. First, immediately after inflation(Fig. 2E), air expanded spontaneously through the intercon-nected microgrooves until all microgrooves were fully filled withair. As the inflation was continuously conducted, the replen-ished air spilled from the microgrooves to form a thin air filmcovering the entire base of the leaf surface. The dashed boxin Fig. 2E highlights the process of air expansion along onegroove (point 1) by tracking the silvery reflection on the water–air interface. Second, based on the thin air film covering the leafsurface, the three-phase-contact lines (TCLs) on different hairstems slid vertically upward synchronously along the hair stems(movements of points 2 and 3 on different TCLs in Fig. 2F),which indicates an increase in the thickness of the entire airmattress. Third, due to the nonuniform sizes of the hairy struc-tures, some TCLs arrived early and pinned (21) at the top ofthe hairy structures (point 5 in Fig. 2G), waiting for the otherTCLs to continue to slide along the hairy structures (point 4 inFig. 2G), until the air mattress was completely recovered. Toshow the process of air-mattress recovery on the Salvinia surfacemore clearly, the three stages of recovery are summarized in theschematics in Fig. 2H.

Mechanism of Air-Mattress RecoveryIn principle, the realization of air-mattress recovery requires twoconsecutive steps, i.e., 1) the formation of seed air and 2) thespreading of air within the structures. The process of air-mattressrecovery on Salvinia leaves can be characterized as the verticalgrowth of the continuous seed air film synchronously along thehairy structures, in which the transport of the replenished air inthe wedge-shaped grooves to the entire base of the leaf surfacecreates the seed air film and the hairy structures (including the

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Fig. 2. (A) SEM image of the S. molesta surface. The microstructures of the Salvinia surface consist of three parts, i.e., the eggbeater-shaped heads, thehair stems, and the ellipsoidal epidermal cells covering the base of the leaf. From the leaf edge to the center, the hair length l varies from 500 µm to2,400 µm and the adjacent hair distance d varies from 350 µm to 850 µm. (B) Higher magnification of the yellow box in A. The hairy structures andthe base of Salvinia surface are covered with epidermal cells. Interconnected grooves are formed between the adjacent cells. (C) A 3D confocal imageof the epidermal cells and the microgrooves. (D) Cross-section of microgrooves at the red dotted box in C, showing the profile of the wedge-shapedgrooves with the half corner angle α = 17.2◦ ± 6.1◦. (E) Air spreading along the grooves indicated by the brightness increase of the water–air interfacein the dashed box. (F) Movement of the TCLs indicated by the movement of points 2 and 3 up along the hair stems, leading to the growth of the airmattress. (G) Pinning effect at the eggbeater. The eggbeater pins the early arrived TCL at point 5 to wait for the arrival of the later TCL at point 4, whichensures that the air mattress is completely recovered. (H) Schematics of three stages during air-mattress recovery. The numbers label the same positions asthose in E, F, and G.

hair stems and the eggbeater-shaped heads) provide a frame forthe spreading of air. The mechanism for air-mattress recovery onSalvinia leaves is addressed below.

A stable full expansion of air in wedge-shaped grooves is thepremise to the recovery of the air mattress as the formation ofthe seed air will significantly influence the spreading process. Athermodynamic free-energy model (24) was used to analyze thestability of the air column in a wedge-shaped groove by introduc-ing perturbations on the water–air interface (see Fig. 3A for theschematics and SI Appendix, SI Text for the detailed theory). Theresults show that the air column is stable for any perturbation(“full expansion” in Fig. 3B) when the half corner angle of thewedge (α) and the contact angle (θ) satisfy

α+(π− θ)<π/2. [1]

When Eq. 1 is not satisfied, the air column will lose its sta-bility and dissociate into pieces (“semiexpansion” in Fig. 3B).Moreover, Eq. 1 demonstrates a gas wicking effect which is aninverse process of the classical interior-corner wetting phenom-ena depicted by the Concus–Finn (CF) condition (i.e., α + θ <π/2 and “no expansion” in Fig. 3B) (25–27). Therefore, threeregimes can be determined to describe the stability of the aircolumn, i.e., full expansion (Eq. 1), no expansion (the CF con-dition), and semiexpansion (the unstable states in between), asshown in the phase diagram in Fig. 3C. Artificial wedge-shapedgrooves with different corner angles and contact angles fabri-cated by 3D printing were used to verify the theory (Fig. 3E–G), which shows a good agreement with the phase diagram

(see SI Appendix, Figs. S5 and S6 for the properties of the arti-ficial wedge-shaped grooves). For the Salvinia leaf with α =17.2◦ ± 6.1◦ and θ = 146.1◦ ± 7.8◦, the experimental datacorresponded to the mode of full expansion (Fig. 3C), whichreveals the mechanism of the stable expansion of air in theinterconnected microgrooves.

To further verify the importance of full expansion of air inwedge-shaped grooves to the air-mattress recovery on the realSalvinia leaves, fresh leaves were dehydrated with a critical pointdryer, which can maintain both microstructures and hydropho-bicity unchanged, followed by a modification of the wettabilityusing oxygen plasma treatment. As the surface hydrophobicitydecreases (indicated by the increase of the plasma treatmenttime in Fig. 3D), the recovery capability of a continuous air mat-tress decreases (Fig. 3 H–J), which is reflected by the volumedecrease of the inflated air in the microstructures in Fig. 3D.Confocal microscopy is employed to observe the local detailsof the water–air interface morphology in different inflated con-ditions, also demonstrating the three modes of air expansionalong the wedge-shaped grooves (Fig. 3 K–M). The experimentalresults give strong evidence that full expansion of air (Fig. 3K)leads to a complete recovery of the air mattress (Fig. 3H),whereas semiexpansion (Fig. 3L) and no expansion (Fig. 3M)result in the semirecovery (Fig. 3I) and unrecovered condi-tions (Fig. 3J), respectively, which verifies the significance ofthe air expansion through the wedge-shaped grooves on the air-mattress recovery. Essentially speaking, the full expansion of airis achieved by the gas wicking effect, which ensures an efficientand spontaneous transport of air through the interconnected

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Fig. 3. (A) Schematics of the cross-section of the air column in the wedge-shaped groove, where the two straight lines represent the wedge corner, the arcrepresents the water–air interface, α is the half wedge corner angle, θ is Young’s contact angle, r is the radius of the arc, and ϕ is the azimuthal coordinate.(B) Schematics of air full expansion, semiexpansion, and no expansion along the wedge-shaped groove. (C) Phase diagram of the air expansion modes. Thedots correspond to the experimental results in E, F, and G. (D) The air volume that can be inflated into the microstructures as a function of the duration ofplasma treatment on the dried Salvinia surface. The dots are the experimental results from H, I, and J. (E–G) The 3D confocal images of the three modes of airexpansion along the wedge-shaped groove corresponding to those in B. The blue color indicates the bulk water and the red color represents the interfaces.(H–J) Effect of wettability on the air-mattress recovery. The recovery capability of the air mattress decreases as the decrease of surface hydrophobicity whichis tuned by the increase of the duration of plasma treatment t. t = 0, 21, and 30 s in H, I, and J, respectively. (K–M) The 3D confocal snapshots and 2Dcross-sections of the local morphology of the water–air interface, showing the three modes of air expansion along the wedge-shaped grooves in H, I, and J,respectively.

wedge-shaped grooves and makes full use of the replenishedair to rapidly cover the entire base of the surface to form acontinuous air film.

Based on the thin air film formed by the wedge-shapedgrooves, the hairy structures will provide a frame for the air mat-tress to grow in thickness. As the hairy structures intersect thewater–air interface, the resistance distributes along the circum-ference of the hairy structures in the vertical direction (F post

slide

in SI Appendix, Fig. S7). When the air pressure in the air mat-tress pair overcomes the sum of hydrostatic pressure pwater andthe vertical resistance F post

slide, the TCLs will start to slide verti-cally along the hairy structures. On the one hand, if the sizesof the hairy structures are ideally uniform, the entire water–airinterface will rise uniformly until the air mattress is completelyrecovered. On the other hand, if the sizes of the hairy structuresare nonuniform, some TCLs will arrive early and pin at the topof the shorter hairy structures to wait for the other TCLs to con-tinue to slide along the longer hairy structures vertically untilthe air mattress is completely recovered (SI Appendix, Fig. S7),in which the maximum pinning force F top

pin,max at the top of thehairy structures should be larger than the sliding resistance F post

slide

to maintain a stable water–air interface. The pinning effect ofthe eggbeater-shaped heads can greatly increase F top

pin,max, whichenhances the robustness of the air-mattress recovery. Therefore,no matter whether the hairy structures are uniform or not, theair-mattress recovery will proceed smoothly, indicating that therequirement for the configuration and distribution of the hairystructures is flexible (see SI Appendix, Fig. S8 for the air-mattress

recovery on the other two species of Salvinia with different hairystructures). The hairy structures provide only a frame for theair mattress to grow, while the air-mattress recovery is domi-nated by the features of the wedge-shaped grooves on the baseof Salvinia leaves. In contrast, if the air-mattress recovery isgoverned by the mechanism of regulating the hairy structures,a geometric constraint of hairy structures is requisite (15). Ourinvestigations, however, demonstrate that the mechanism of air-mattress recovery on Salvinia leaves allows the hairy structuresto grow beyond the geometric limitation (proved by the exper-imental results in SI Appendix, Figs. S9 and S10), which greatlyextends the geometric design of microstructures for air-mattressrecovery.

Air-Mattress Recovery on Artificial Salvinia Surface: Lessonsfrom Salvinia LeafInspired by the nature of Salvinia , we fabricated artificialSalvinia surfaces using 3D printing technology (Fig. 4A) (28).The artificial surface imitates the main features of the uniquehierarchical structures on Salvinia leaves, i.e., 1) the wedge-shaped grooves on the base for the formation of seed air filmand 2) the hairy structures for the spreading of air. The base ofthe artificial Salvinia surface was covered with half-cylindriformbulges to mimic the convex epidermal cells. Wedge-shapedgrooves were formed between the adjacent cells, with α = 10◦

and θ = 156◦ ± 5.2◦, which satisfied the full expansion condi-tion (Fig. 3C). The artificial hairy structures are fabricated tosupport the air mattress growing in thickness (Fig. 4A), which

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Fig. 4. (A) SEM image of the 3D printed artificial Salvinia surface, duplicating the three main geometrical characteristics of the microstructures on Salvinialeaf, i.e., wedge-shaped grooves, hair stems, and eggbeater-shaped hair heads. Inset shows the details. (B) The 2D confocal images sequentially showingthe air-mattress recovery by air inflation on the biomimetic Salvinia surface. The microscope was focused on the plane of the top of eggbeater heads.The blue color indicates water and the black color indicates that water is replaced by air. (C) The 3D confocal images sequentially showing the recoveryprocess on the biomimetic Salvinia surface, including the initial state of air-mattress collapse, the air spreading along the wedge-shaped grooves, and thecomplete recovery of the air mattress. (D) SEM image of the control specimen with the same structures as those in A but without microgrooves on the base.Inset shows the details. (E) The 2D confocal images sequentially showing the formation of bubbles on the control specimen in contrast to a continuous airmattress on the biomimetic Salvinia surface. Inset shows the focal plane on the base of the specimen, which confirms the tip of the needle is placed deepinto the microstructures to guarantee the direct contact of the inflated air with the specimen surface.

includes the hair stem and the eggbeater-shaped head, with thehairy structures height la = 69 µm and the distance between twoneighboring hairy structures da = 66 µm. Confocal microscopywas used to observe the recovery of the air mattress inflated bya microinjection system (Movie S3). The complete recovery of acontinuous air mattress was successfully realized to exactly imi-tate the superrepellency of Salvinia leaves, as shown in Fig. 4 Band C from 2D and 3D perspectives, respectively. In addition, acontrol specimen was fabricated with the same structures as thatof the biomimetic Salvinia surface but without microgrooves onthe base (Fig. 4D). As expected, only bubbles formed instead ofa continuous air mattress under the same experimental condi-tions (Fig. 4E). Moreover, experiments on artificial specimenswith different hairy structures were also performed to furtherdemonstrate that the requirement of the air-mattress recoveryfor the configuration and distribution of the hairy structures isflexible (SI Appendix, Figs. S9–S11).

ConclusionWe present the underwater superrepellent capacity of Salvinialeaves, which can efficiently and robustly reactivate the invalidslippery air mattress on underwater surfaces by trapping thereplenished air to replace the water in the surface structures. Theefficiency of the air-mattress recovery is reflected by the spon-taneous transport of air in the wedge-shaped grooves governedby the gas wicking effect, which ensures a rapid formation of athin air film covering the whole leaf surface. The robustness ofthe air-mattress recovery is reflected by two aspects. First, thefull expansion of air in the wedge-shaped grooves ensures the aircolumn to remain stable in infinite length for any perturbation,which indicates a robust recovery of the air mattress. Second, thepinning effect of the eggbeater-shaped heads can enhance thestability of the water–air interface, which improves the robust-ness of the air-mattress recovery. Due to the robustness, the

air-mattress recovery on Salvinia leaves can be achieved on alarge area and remain valid in extreme environments, such ashigh pressure, fluctuating waves, and even fast flows. Last butnot least, following the design principle of hierarchical struc-tures of Salvinia leaves, a biomimetic artificial surface that isfabricated using 3D printing technology successfully imitatesthe superrepellent capability of Salvinia leaves to completelyrecover a continuous air mattress. The finding here not onlyreveals the underlying mechanisms of the water-repellent prop-erty of Salvinia leaves, but also promotes the wide application ofwater-repellent materials in underwater applications.

Materials and MethodsThe description of the materials and methods used in this study is avail-able in SI Appendix. The morphologies of natural Salvinia leaves andartificial Salvinia surfaces were measured by a scanning electron micro-scope and a confocal microscope. On natural Salvinia leaves, two methodswere performed to recover the collapsed air mattress, i.e., air inflationand depressurization, and the recovery process was observed by a digi-tal microscope with variable magnification. On artificial Salvinia surfaces,a micropipette was employed to inflate air into the wetted microstructuresto recover the air mattress, and the recovery process was observed by theconfocal microscope.

Data Availability. All data are included in the main text and SI Appendix.

ACKNOWLEDGMENTS. This work was supported by the National NaturalScience Foundation of China under Grants 91848201, 11521202, 11988102,11872004, 11802004, and 11702003 and the Young Elite Scientists Sponsor-ship Program by the China Association for Science and Technology underGrant 2017QNRC001. We thank Li Zhang and Lijia Qu for assistance withCritical Point Dryer; Yulong Li, Yiqiong Liu, and Yan Zhang for assistancewith Micromanipulator; Xiangyu Wang and Wei Shi for assistance with dataprocessing; Kai Zhang, Xin Yi, and Xiying Li for their helpful discussions; andWanyin Cui from Nanoscribe GmbH (China) and the Advanced Micro/Nano-manufacturing Laboratory at Beijing Innovation Center for EngineeringScience and Advanced Technology for technical support.

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