ResearchCite this article: Watson GS, Schwarzkopf L,Cribb BW, Myhra S, Gellender M, Watson JA.

2015 Removal mechanisms of dew via

self-propulsion off the gecko skin. J. R. Soc.

Interface 12: 20141396.http://dx.doi.org/10.1098/rsif.2014.1396

Subject Areas:biomaterials, biophysics

Authors for correspondence:Gregory S. Watson

e-mail: gwatson1@usc.edu.au

Electronic supplementary material is available

Removal mechanisms of dew via

fog provide mechanisms to remove these small droplets off the gecko skin

on March 11, 2015http://rsif.royalsocietypublishing.org/Downloaded from at http://dx.doi.org/10.1098/rsif.2014.1396 or

via http://rsif.royalsocietypublishing.org.Jolanta A. Watson

e-mail: jolanta.watson@griffith.edu.auKeywords:lizard, gecko, condensation, nanostructures,

dew, contaminantsReceived: 22 December 2014

Accepted: 16 February 2015rsif.royalsocietypublishing.orgsurface. The formation of small droplets and subsequent removal from theskin may aid in reducing microbial contact (e.g. bacteria, fungi) and limitconducive growth conditions under humid environments. Aswell as providingan inhospitablemicroclimate formicroorganisms, the formation and removal ofsmall droplets may also potentially aid in other areas such as reduction andcleaning of some surface contaminants consisting of single or multipleaggregates of particles.

1. IntroductionGeckos have received considerable interest, generally focusing on the adhesionproperties of the small structures (setae) on their feet (typically Gekko gecko;[17]). The remaining regions of the lizard body have received very little attentionin relation to specific functions which may be associated with the nano- andmicrostructuring [8,9]. This is somewhat surprising as geckos have interestingand intricate microstructuring on the skin regions. Foot microstructuring ingeckos may also be evolutionarily linked to the body skin microstructuring(e.g. substructure of scales) [10].

One prominent feature of this group of lizards is the small hairs, typicallycalled spines, spinules or microspinules spaced 0.20.7 mm apart and up to sev-eral micrometres in height [11,12]. A previous study has shown that these spinescan be water-repellent and suggested that they may serve as a self-cleaning sur-face where rain may carry away contaminants [8]. Maintaining the surface of thegecko skin free from a water film may be beneficial for a number of reasons. Forexample, the growth of many microorganisms is enhanced by increased wateravailability and proliferation may result from such wetting conditions. Indeed,studies have shown that some lizards are susceptible to various external contami-nants that can cause serious skin problems and diseases [13]. High humidityconditions and low temperatures have been shown to act as potential factors inthe development of such reptile bacterial infections [14]. Thus, the wetting behav-iour of the skin (the mechanical barrier and potential portal) plays an importantrole in particle and microbial resistance.

& 2015 The Author(s) Published by the Royal Society. All rights reserved.self-propulsion off the gecko skin

Gregory S. Watson1, Lin Schwarzkopf2, Bronwen W. Cribb3, Sverre Myhra4,Marty Gellender5 and Jolanta A. Watson1

1School of Science and Engineering, University of the Sunshine Coast, Maroochydore DC, Queensland 4558,Australia2School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia3Centre for Microscopy and Microanalysis and School of Biological Sciences, The University of Queensland,St Lucia, Queensland 4072, Australia4The University of Oxford, Begbroke Science Park, Sandy Lane, Yarnton OX5 1PF, UK5Previously Queensland Department of Environment and Heritage Protection, GPO Box 2454, Brisbane,Queensland 4001, Australia

Condensation resulting in the formation of water films or droplets is an un-avoidable process on the cuticle or skin of many organisms. This processgenerally occurs under humid conditions when the temperature drops belowthe dew point. In this study, we have investigated dew conditions on the skinof the gecko Lucasium steindachneri. When condensation occurs, we show thatsmall dew drops, as opposed to a thin film, form on the lizards scales. As thedroplets grow in size and merge, they can undergo self-propulsion offthe skin and in the process can be carried away a sufficient distance to freelyengage with external forces. We show that factors such as gravity, wind and

(4) Importantly, what specific mechanisms are used andwhat factors are involved to aid this process in total (MDPlan) of 4 and 10 were used for higher magnifications

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In this study, we describe the intricate topographicalsurface structure of the body of the gecko (L. steindachneri)and investigate the wetting formation under humid con-ditions and potential pathways by which condensates can betransported from the bodys surface.

2. Experimental procedure2.1. Gecko capture and preparationBox-patterned geckos (L. steindachneri) were captured at nightby hand, from the Mingela Ranges (S 20 0800600 E 146 5203200),Queensland (QLD), Australia. The Mingela Ranges are semi-arid, with a long-term (50 year) median rainfall of 493 mm,and 62.5 rain days per year on average [19]. Only healthy,adult lizardswere returned to the laboratory, held inplastic con-tainers with a heat source for thermoregulation, paper towel assubstrate (changed weekly), a small tree branch and water adlibitum. They were fed domestic European crickets (Achetadomestica) three times a week. Geckos were allowed to shedtwice, so that any oils were completely absent from their skinbefore experiments were conducted. Lizards were euthanizedto ensure the skins surface was intact, and undamaged.Lizard skins were surgically separated by scalpel, cut into smal-ler sections (15 15 mm2) and attached to glass slides fordroplet experiments.

2.2. Characterization of surface micro- andnanostructure

Scanning electron microscope (SEM) imaging was performedon a small section of lizard skin (approx. 3 5 mm2) mountedWe have investigated a ground-dwelling gecko (Lucasiumsteindachneri) that lives in semi-arid habitats, where contact viarain is limited. For these geckos, rain may be totally absentbetween skin-shedding events; however, these lizards live in anenvironment inwhichhumidairand lowovernight temperaturescan potentially result in condensed liquid water (dew). Thus, thebehaviourof the condensate on the lizard skin is apossible sourceof water contamination and thus the focus of our study.

Recently, it has been shown on some natural and artificialsuperhydrophobic surfaces that when small condensing waterdroplets merge, as they grow in size, then the combined singledroplet is self-propelled off the surface [1517]. This processarises from a change in surface energy from the transformation(coalescence) event [15,17]. Interestingly, this process has alsobeen shown to aid in removal of small contaminating particlesfromsurfaces and is thus apotentialmechanism for self-cleaning[17]. To date, the only reported examples of the self-propulsionprocess on an animal surface has been on insect wings [17,18].Obvious questions that arise from these studies include

(1) To what extent does this self-propulsion process occur innature?

(2) Are surfaces with more intricate and complex hierarchi-cal structures than simple insect cuticles used for suchfunctionality?

(3) Does the process aid in species survival?(effectively up to approx. 300). Contamination experimentswere carried out using silica (KOBO MSS-500/20averagediameter approx. 20 mm) and polymethylmethacrylate (BangsLaboratories BB03Nmean diameter 83 mm) beads. Thesewere seeded on the skin by distributing a thin coating (singleand clumped) of particles onto a clean glass slide and thengently tapping the inverted slide over the skin.

3. Results and discussion3.1. Topographical characterization of the gecko skinAdult box patterned geckos L. steindachneri (figure 1a) typicallyachieve a length of approximately 55 mm with a mass ofapproximately 2.5 g and live in a range of areas includingsemi-arid regions and are typically ground-dwelling [20]. Theoptical images of the dorsal regions (figure 1a,b) show signifi-cant pigmentation which may serve as camouflage for thelizard. Previous studies have investigated the oberhautchen(outer thin layer) of numerous lizard skins and determinedthat they are composed of keratin [2125]. X-ray photon elec-tron spectroscopy data of the gecko skin in our study alsosuggest keratin is a component of the dorsal and abdominalregions where detailed scans showed binding energies ofsulfur and nitrogen consistent with an organic environment.

The micro- and nanostructure on the dorsal and ventralscales was examined at higher resolutions using a SEM. Onthe dorsum, the scales were up to 200 mm in diameter with asimilar centre-to-centre spacing and height, typically over50 mm, and on the abdomen they were approximately 250 mmin diameter with a similar spacing (figure 1b,c). The areasbetween the scales comprise regions where the skin is heavilyfolded (figure 1d). The structuring on the scales at the micro-and nanolevel consists of spinules (hairs) and a basal layer.Scales have spinules with lengths of approximately 500 nm toon an aluminium stub with carbon-impregnated double-sidedadhesive, sputter coated with 710 nm of platinum, thenimaged using a JEOL 7001 field emission SEM at 8 kV.

2.3. Condensation experimentsThe sectioned skin samples were mounted on a cooled copperelement maintained at a temperature below the dew point(16208C). The surface temperature of the skin was monitoredwith a dual-temperature meter HT-L13 with thermocouples(type K) and an infrared thermometer MT300. Typical con-ditions of dew formation were achieved in the laboratory withhumidity of 7585% and ambient temperatures of 24278C.

2.4. Droplet cleaning mechanism observationsDroplet formation and dynamics were captured using a Nikon(V1) digital camera (video mode at 400 and 1200 frames persecond (fps), 640 240 and 320 120 pixel resolution, respect-ively, frame rates and acquisition times of 2.50 and 0.833 ms perframe and video play-back at 30 fps or 15 fps). Videos (4064)were taken observing condensation (duration ranged from 5to 15 s), with another 336 videos observing small water dropletsimpacting with the gecko skin surface (dispensed and dis-persed by spray bottle). The camera housing was attached toa Canon macro lens (EF-S 60 mm ultrasonic) for wide-field,low-magnification observations, whereas a custom-made opti-cal microscope equipped with Olympus microscope lenses

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(e)

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200 mmover 4 mm(figure 1e). The spinules are spherically cappedwith aradius of curvature less than 30 nm. The density of spinules ishigh: over 400 per 10 10 mm. The structuring below the spi-nules comprises a network of intersecting ribs forming ahoneycomb-like basal layer (figure 1e). Lenticular sense organsare also visible on the scale edges (figure 1d). This micro-and nanostructuring is of sufficient density and roughness toindicate that the surface may have anti-wetting properties.

3.2. Wettability and dew formation under naturalcondensation conditions

The intricate patterning of the gecko spinule array topogra-phy facilitates a hydrophobic interaction with water. Staticwater contact angles confirm this and are generally withinthe range of 1511558 (figure 2a) on the dorsal and abdomi-nal regions of the gecko, which is at the higher end ofreported values on lizard skin [8,9] and superhydrophobicin nature. Hysteresis values were comparable to those ofthe lotus leaf. There are a number of theories to express thesuperhydrophobic condition, all of which have certainassumptions and limitations [2630]. Cassie & Baxter [27]express the superhydrophobic state in terms of a numberof interfaces: a liquidair interface with the ambient

30 mm

Figure 1. Images reveal the gecko and the multi-level micro- and nanostructuring on(b) Optical image shows the microstructure of the outer skin on the dorsum of the gecscales in a relatively close-packed hexagonal patterning. (d ) Topographical SEM image oflizard, and (e) microstructure on the dorsum showing a micro/nanostructuring consisti300 mmenvironment surrounding the droplet and a surface underthe droplet involving solidair, solidliquid and liquidairinterfaces. Equation (3.1) expresses the contact angle (uC)formed with a rough surface:

cos uC f1 cos u1 f2 (3:1)

where f1 is the total area of solid under the drop per unit pro-jected area under the drop, f2 is defined in an analogous wayof viewing the liquidair component and u1 is the contactangle on a smooth surface of the same material.

Equation (3.1) necessitates the surface to have the requiredroughness to allow air in topographically favoured regionssuch as troughs and surface depressions. Thus, topographieswhich increase the airwater interface and minimize thesolidliquid contact area may lead to higher contact angles.While the use of the original CassieBaxter equation (equation(3.1)) is often preferable to other variations of the expression, itdoes require knowledge of the extent of penetration of theliquid into the depressions of the surface [30]. If the contactangle is known, however, then equation (3.1) can be used to pre-dict as a first approximation the extent of penetrationof the liquidinto the troughs of a rough surface. From our measured geo-metric values and approximating the spinule terminus as aspherical cap (area 2prh, where h is the central distance from

2 mm

its body. (a) Photograph of the box-patterned gecko Lucasium steindachneri.ko and (c) abdominal region. These regions consisted primarily of dome-shapedthe epidermal fold (scales) and areas between scales on the dorsal region of theng of spinules and a base layer patterning. (Online version in colour.)

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1 mmthe top of the sphere to the point atwhich liquidwill invade) andu1 as 1158 andusing a droplet invasion of 12.5 nmwill calculate apredicted contact angle of approximately 1758. The experimen-tally determined value of 1511558 suggests that an area ofapproximately 40 times this value is required to bring the contactangle within the measured range. Thus, the droplet may invadeto agreaterextent. Inclinedor slightly bent spinules near the apexmay significantly increase the contact area with resting bodieson the surface. In addition,owing to thedome-shapedscale topo-graphyand folds on the skin surface,water dropletsmay interactwith the spinule shafts to varied degrees. Indeed, the spinulesmay undergo bending (especially near the tips), which mayalso increase contact area. Ifwemake avery basic approximationof the spinules as cylindrical spring structures and consider onlythe narrower end regions of the tapered shafts as the structuralelements, then we can view a tip displacement by modificationof a cantilever bending equation [31]

Dsp 4L3Fdrop

3pNspRs4E(3:2)

whereL is the length (400 nmused in our casewhere a significantfraction of the bending will presumably occur), E is the elastic

(d )

300 mm

Figure 2. Images of various sized static water droplets interacting with the gecko skinof water (approx. 3 mm in diameter) on the dorsal region of the gecko Lucasium st(above 1508). (bd ) Condensation formed early in the morning under natural envirdistribution, density and size range. Parts (b,c) were taken at a slight angle from thecondensation was observed during a period where temperatures were 21348C and 8up to 94%). (e) Condensation drops autonomously propelling off the skin (geckoveelectronic supplementary material, movie S1, which shows that the propelled dro( f ) Self-propulsion of water droplets from the abdominal region of the gecko skin. Dsupplementary material, movie S2). (Online version in colour.)(e)

( f )65 mm

600 mm

60 mmmodulus of the material (1.5 GPa), Fdrop is the total dropletforce (a value of the radius, Rs, of 30 nm is used, reflectingthe lower region of the cylindrical element (spinule)). For awater droplet of 2 mm diameter (approx. 4 ml) resting on theskin and assuming an equally distributed force contact on theend of around 100 000 spinules (based on spinule density anddroplet contact area determined bymicroscopic imagingdirectlyabove the droplet), a deflection of over 5 nmwould take place foreach individual spinule near the tip.

It should be noted that the above description makes anumber of assumptions and ignores numerous other par-ameters/factors that may cause the deviation of the contactangle from predictions, such as the non-homogeneity of thesurface, defects, chemical variations and the natural bendingitself as it appears in figure 1e. Nevertheless, the above-mentioned illustrative example provides some informativepredictions for water interactions with surface projections ofthe scale as exhibited by the gecko and presents a useful com-parative behaviour with smaller droplet interactions andother organisms.

The spinule topography on the gecko is different from thestructure found on many other lizards [3235]. Some otherorganisms, however, do have a dense spinule (hair) array

300 mm

and snapshots of droplets self-propelling off the gecko skin. (a) Small dropleteindachneri. The droplet maintains a near-spherical shape and contact angleonmental conditions on the dorsal region of the gecko skin showing dropletplane of the surface. Part (d ) was obtained along the surface plane. Natural238C (average day-time and night-time temperatures, respectively: humidityscence) with the surface oriented horizontally (see also the accompanyingplets clearly leave the surface and move under the influence of gravity).roplets in the range of 1080 mm propel off the surface (see also electronic

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on March 11, 2015http://rsif.royalsocietypublishing.org/Downloaded from on their surface. Many insects, for example, have hairs on theircuticle and especially wing surfaces [36] such as those found onlacewings, craneflies and termites [18,3740]. In these cases,however, the hairs on the wings are significantly longer andspaced over an order of magnitude further apart (over 10 mmin length and spacing from5 to 15 mmapart). This is also typicalof many other insect species where hairs are separated by tensof micrometres. The much larger spacing and lower density ofhairs on insects provides a shedding mechanism to removewater quickly so as to keep wings free from liquid, thus main-taining functionality. The nanostructures (e.g. nano-grooves,nano-channels) on the hairs of many insects have been shownto aid in anti-wetting [37,38]. The gecko, while not havingsuch channel structuring on the hairs, does however exhibit amuch higher density of hairs. Thus, it appears that the highhair density on the gecko skin is sufficient tominimize the inter-action timewith water in various forms without the aid of finerstructuring. Small-scale structuring found on insect surfaces atthe submicrometre dimensions, of a similar lateral scale to thespinule spacing as seen on the gecko, has been associatedwith cleaning/self-cleaning of the cuticle surfaces [39,41]. Com-paratively, largewater droplets such as the example in figure 2acan become mobile with minor tilting of the surface (a fewdegrees) and thus are easily displaced from their equilibriumstate. This not only facilitates water removal from the surface,but also facilitates the extrication of collected contaminantsfrom the skin.

In more arid regions, a significant period may pass with-out rain and thus the most likely form of water contaminationwill arise from atmospheric condensation on the lizard skin.When we exposed gecko skin samples to natural environ-mental condensation conditions, dew formed on the surfacein the form of small droplets as opposed to a continuousfilm (figure 2bd ). The dew droplet population that formedon the gecko skin consisted of many small densely packedspherically shaped, micrometre-sized droplets. The surface-bound droplets on the skin were typically in the range of10100 mm with each scale region often housing multipledroplets. From examination of figure 2bd, it is clear thatthe density of dew droplets (related to the spacing of nuclea-tion sites) and the growth of droplets from the condensationprocess will facilitate merging of two or multiple clusters ofsmall droplets.

3.3. Self-propulsion of condensing droplets fromcoalescence

To assesswhether naturallyoccurring condensed droplets, suchas those shown in figure 2, when merging will form larger dro-plets and remain pinned to the skins surface, or alternatively beself-propelled off the surface from changes in energy duringcoalescence, we conducted condensation experiments. By cool-ing the gecko skin below the dew point, surface condensationwas induced forming droplets in the same size range as thoseshown in figure 2bd. As they grew, some merged and thenself-propelled from the skin surface. Figure 2e shows a snapshotin time of micro-droplets propelling autonomously off the skin(see the accompanying electronic supplementary material,movie S1). The fully merged propelled droplets were typicallyapproximately 40120 mm in diameter.

A recent paper [16] has shown that on a fabricated surfaceas the subcooling value increases (termed supersaturation bythe authors) the micro/nanostructures fail to maintain thejumping-induced removal mechanism owing to an increasein the number of nucleation sites. This leads to flooding anda loss of superhydrophobicity and the formation of highlypinned Wenzel droplet morphologies. In our case, humidityand temperature changes, while potentially large, generallyoccur over a relatively long period of time in the naturalenvironment of the gecko, and thus flooding seems unlikelyto occur. Observation of droplet numbers as shown infigure 2 supports this premise; however, how the geckostructuring performs in relation to fabricated surfaces in thisarea is of interest and worthy of exploration in future studies.

To investigate the range of droplet sizes involved, videos athigher magnification were collected of self-propulsion on boththe dorsal and abdominal skin of the gecko. A sequence ofimages from the abdominal region of the gecko skinwhere dro-plets of varying size (from 10 mm to over 80 mm in diameter)merge and are self-propelled from the surface is seen infigure 2f (see also electronic supplementarymaterial, movie S2).

This mechanism demonstrates that small water droplets canbe projected from the skin surface with no external forces. If thelizard, however, is orientated in a horizontal position withrespect to gravity as demonstrated in figure 2e, then the self-propelled droplet may eventually return to the skin under theinfluence of gravity. It may remain there if no other forces areacting on it and the droplet propulsion was orthogonal(normal) to the overall plane of the lizards skin. However, theultimate destination of propelled droplets will depend onnumerous factors (e.g. lizards in their natural environment willadopt several orientations and will also be exposed to otherenvironmental influences and forces). With this in mind, weexplored a number of skin surface orientations and observedthe overall dynamics of thedroplet-propelledprocess indifferentsituations.Wehave identified a numberofmechanismsthatmayaid in complete transfer of droplets from the surface of the skin.

3.3.1. Direct self-propulsion of droplets off the skin surface viathe droplet merging process (single jumping event)

For illustrative purposes, the droplet propulsion process can beviewed in a very simplified manner by considering changes inthe droplet surface energies. If we consider the case where twosmall water droplets (not necessarily of equal size) coalesce on asuperhydrophobic surface to form a larger droplet, the maxi-mum height, Hmax, that can be attained by a droplet can bedetermined by integrating the velocity of a droplet over itstime-of-flight resulting in equation (3.3):

Hmax rwRm3raCDln 1 9raCDfw

R2mr2wg(1 f)2

(1 f3)2=3 1

( )" #(3:3)

where ra,w are the densities of air and water, respectively; Rm isthe radius of the merged single droplet; CD is the drag coeffi-cient of the droplet in air; fw is the water surface tension; g isthe gravitational acceleration (9.8 m s22); and f is the ratio ofdrop diameters.

Equation (3.3) sets an upper limit for themaximumpossibleheight that can be reached assuming all the released surfaceenergy is converted into kinetic energyof the droplet and ignor-ing viscous forces and adhesional forces of the droplet to thesurface. It also assumes that the droplet is propelled verticallyand that the drag coefficient remains constant as the dropletdecelerates. Interestingly, the maximum height calculatedfrom equation (3.3) varies little over the range of smaller dropletdiameters. The height achieved, however, is significantly

0.47

urface ang lateron in c

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Equation (3.3) predicts a maximum height approaching20 mm; however, the majority of droplets were propelled0.52 mm (figure 2e and electronic supplementary material,movie S1). Amore comprehensive view of the propulsion pro-cess has been presented by Peng et al. [42]. They view the initialtotal kinetic energy of the coalesced droplet as Eitk DEs 2Evis 2 Eh2 Ew 2 Ecah, where Eitk is the initial total kineticenergy of the coalesced droplet, DEs is the surface energyreleased by the droplet coalescence, Evis is the viscous dissipa-

0 ms 3.34 ms 6.67 ms

8.34 ms4.17 ms0.83 ms

9.17 ms5 ms1.67 ms

10.83 ms5.83 ms2.5 ms

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Figure 3. (a) Lateral sweeping motion of self-propelled droplets along the splementary material, movie S4). (b) Large mobile self-propelled droplet movismaller droplets (electronic supplementary material, movie S5). (Online versition in the droplet, Eh is the gravitational energy changeduring droplet coalescence and Ew is the work of adhesion.Ecah is the energy consumed overcoming the contact anglehysteresis.

The authors state that approximately 25% of the energyreleased by the droplet coalescence is converted to the effec-tive kinetic energy in the vertical motion of the coalesceddroplet jumping from the surface. Thus, the higher propelledheights predicted in equation (3.3) can be attributed to theomission of other factors such as viscous dissipation. Thedroplet propulsion process was also measured at 1200 fps,allowing droplet speed to be measured accurately. Thetypical initial velocities of droplets were below 0.4 m s21.

The merging of two or more droplets can cause self-propulsion off a surface (electronic supplementary material,movies S1S2). If the gecko is orientated in a vertical pos-ition, then the single propulsion of a droplet can result incomplete removal from the surface. It is evident that if thegecko is inclined at near-vertical angles (approaching 908),then propelled droplets will be able to traverse large dis-tances and are more likely to be totally removed from thesurface (see electronic supplementary material, figure S1and movie S3surface inclined at approx. 708). If the lizardis orientated in a horizontal or near-horizontal stance, thenthe underlying body and side regions of the gecko will alsobe amenable to this mechanism. The small size of the geckoand the rounded cross-sectional profile of the lizard bodywill also facilitate complete removal via a single propulsionof a water droplet. Once droplets are self-propelled fromthe skin in these situations gravity can assist in completeremoval of the droplets. Thus, no other external force isrequired for droplet removal. As the distance that a dropletcan be propelled is significant in relation to the smallest lat-eral dimensions of this lizard then this mechanism can, inprinciple, remove droplets from anywhere on the surface ofthe skin.

3.3.2. Self-propelled impacting droplets which laterally sweepthe surface and facilitate removal of surface-boundcondensationremoval facilitated by sufficiently high

0 ms

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5.83 ms

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rising from coalescence of multiple droplets of varying sizes (electronic sup-ally with a high kinetic energy sufficient to maintain direction and scavengeolour.)kinetic energy of impacting dropletsMerging droplets are typically propelled in near-vertical direc-tions in relation to the skin surface (for example, electronicsupplementary material, movie S1). Some proportion of dro-plets, however, is propelled at angles facilitating motionalong the plane of the skins surface rather than directly outof plane. This lateral sweeping of droplets across the surfaceis often facilitated by the coalescence of multiple droplets ofvarying sizes (figure 3a and electronic supplementarymaterial,movie S4). This may potentially be aided by the skin topogra-phy where droplets that merge are initially resting on scalesand thus are on an inclined surface (figure 2f ). Sufficientlylarge droplets merging can be driven along the surface withsufficient momentum to remove themselves from the surfaceand in the process collect smaller droplets along the way asshown in figure 3a,b (see also electronic supplementarymaterial, movie S5). The processmay be viewed as an extendedform of the lotus effect (impacting and rolling droplets), wherethe initiating step is via self-propulsion(s) of condensation asopposed to a falling or rolling droplet from rain. Thus, theremoval process may take the form of transformation of the

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(a)self-propulsion process to that resembling a rolling processalong the surface as in the lotus effect.

3.3.3. Impacting droplets facilitate self-propulsion off the skinsurface (impacting droplets from fog and small self-propelled droplets)

The situationwhere small falling water droplets impact station-ary surface-bound droplets is shown in figure 4a,b. A fallingdroplet of 20 mmdiameterwith avelocityof 0.013 m s21 impact-ing a stationary surface-bound droplet of approximately 70 mmis illustrated in figure 4a and also in the electronic supplemen-tary material, movie S6. Figure 4b shows a very small droplet(highlighted by the arrow) travelling at a slow velocity,making contact with two stationary surface droplets ofapproximately 100 and 70 mm in close proximity to each other,resulting in self-propulsion off the surface (see also electronicsupplementary material, movie S7).

5 ms 5.83 ms

8.34 ms 9.17 ms

Figure 4 Small water droplets merging with surface drops and self-propelling off thvelocity of 0.013 m s21 impacts with a stationary condensed droplet on the surface athe surface at a speed of over seven times greater than the impacting droplet (electrlighted by the arrow) contacting two stationary condensed droplets on the surface whiprocess and are self-propelled off the skin (electronic supplementary material, movi200 mm1.67 ms

4.17 ms

4.17 ms 22.5 ms

0 ms 21.67 ms(b)Fog conditions occur regularly in areas inhabited by thisparticular gecko (observation from collection and weatherdata for regions of habitat (Bureau of Meteorology [19])) andoften coincide when high humidity conditions take place. Fogas well as previously self-propelled droplets may collide withthe skin and merge with existing dew droplets, resulting in anewly merged droplet self-propelling off the surface. The pro-cess may be aided by kinetic energy of the impacting mobiledroplet; however, for slow moving small droplets the surfaceenergy changes are the primary driving force for propulsionof the final droplet. The change in surface energy from coalesc-ence in figure 4a results in a self-propelled droplet approaching0.1 m s21. Small, mobile impacting droplets may also facilitateself-propulsion by adding to the volume of stationary dropletsforcing them to merge with neighbouring droplets (figure 4band electronic supplementary material, movie S7). Propulsionarising from small impacting droplets may also be responsiblefor future coalescence and subsequent propulsion events.

6.67 ms

15.83 ms 24.17 ms

12.5 ms 23.34 ms

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e surface of the gecko skin. (a) A small impacting droplet of 20 mm with and the resulting change in surface energy propels the coalesced droplet aboveonic supplementary material, movie S6). (b) A small impacting droplet (high-ch are in very close proximity. The two surface-bound droplets merge from thee S7).

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III

Figure 5. Schematic summary of the mechanisms that can potentially totally rthe skin surface (near vertical projection). (II) Multiple coalescence of droplets dalong the surfacecan result in lotus-type collection. (III) Impacting drople(IV) Wind-assisted removal. (Online version in colour.)

0 ms 3.33 ms (b)(a)3.3.4. Wind-assisted removal of self-propelled dropletsA significant population of propelled droplets have sufficientmomentum to be transported outside the skinair boundarylayer (figure 2e), allowing them to be exposed to externalenvironmental forces (such as light breezes). Importantly,the self-propulsion releases the droplet from the surfaceairinterface and associated adhesive forces and exposes it toforces that could potentially transport the drop significant dis-tances away. As the droplets are extremely small, their masswill be sufficiently low that very slight external wind forcesare able to fully remove the droplet from the skin once theyhave self-propelled above the surface. For a 50 mm diameterwater condensate, a wind force of less than 1 nN is requiredto oppose the gravitational force. For a 200 mm droplet, theforce increases to approximately 40 nN. Electronic supple-mentary material, movie S8, illustrates the susceptibility ofdroplets to removal if exposed to light breezes over the surface.Droplets can easily be transported several centimetres awayfrom the point of origin. Wind forces may also initiate self-propulsion of droplets by transporting small droplets laterallyon the surface forcing coalescence.

4.17 ms0.83 ms

23.33 ms1.67 ms

24.17 ms2.5 ms

70 mm

Figure 6. (a) A condensed water droplet of approximately 100 mm in diameter encapsulaand its contaminant are self-propelled off the surface. (b) Before and after (self-propulsioIIIIV

mall water droplets off the gecko skin. (I) Direct self-propulsion of droplets offlong the plane of the surface (sweeping) with the droplet scavenging laterallyfog or other falling droplets facilitating self-propulsion off the skin surface.A summary of the mechanisms discussed above is shownschematically in figure 5, all of which can totally removewater droplets from the skin.

3.3.5. Other potential benefits of self-propulsion of condenseddroplets

While the formation of high-contact-angle small micrometre-sized droplets and subsequent self-propulsion provides alandscape under dew conditions which can potentially limitthe growth of microorganisms such as bacteria and fungion the lizard skin, does the phenomenon offer any otherpossible attributes for the organism? It should be noted thatthe self-propulsion of water droplets on artificial superhydro-phobic surfaces has shown to be a way of controlling heattransfer [16] and this may be relevant as some water-repellentreptile surfaces may contribute to thermoregulation.

The self-propelled droplet-assisted mechanisms as seen infigure 5 may also possibly aid in removal of some contaminantsfromthegecko skin.Wehavemadeapreliminary investigation tosee if contaminants (e.g. silicon and polymethylmethacrylate

100 mm

ting a silica particle (highlighted by the arrow) of 15 mm in diameter. The dropletn) frames show droplets which house silica particles. (Online version in colour.)

mm

mm

being

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(b)

(a)

Figure 7. (a) Before and after snapshots of a small clump of PMMA particlesparticles) could be removed by condensation and self-propulsionprocesses. Figure 6a shows that a small silica particle encapsu-lated in a droplet can be removed via the droplet propulsionmechanism. Figure 6b shows groups of droplets (two and threedroplets which house silica beads) demonstrating self-prolusion.

The gecko skin surface was also seeded with polymethyl-methacrylate beads (PMMAmean diameter of 83 mm with acontact angle of 608) and exposed to condensation conditions.Large particles and particle clumps were transported alongthe surface with only small amounts of condensed water sur-rounding them (electronic supplementary material, movie S9).Particle clumps rearrange owing to meniscus forces, and theenergy was released in the form of jumping conglomerates ofparticles (figure 7a,b and electronic supplementary material,movie S10).

While a previous study has shown a single jump of hydro-philic glass beads from a superhydrophobic wing surface [17],we have found that small clumps of these less hydrophilicPMMA particles (in heavily contaminated regions) tendedto undergo sequential aggregation with a series of self-propulsions and canbe finallypropelled alongoroff the surface.The aggregate seen in figure 7bmakes intermittent contact withthe surface during the propulsion event by bouncing across thesurface. Interestingly, PMMA clumps comprising only a fewparticles can be propelled from the skin surface (figure 7a). Aswell as nucleation occurring on the skin surface, figure 7ademonstrates that nucleation of droplets can occur on individ-ual particles. It is apparent from figure 7a that only smallcondensed water drops or films relative to the particle size can

along the surface until coming to rest nested up against another large clump (eleccleaned on the surface from numerous droplets/clumps of particles. (Online versionremoved off the surface. (b) A large clump of PMMA particles self-propelling1 mm

(c)facilitate self-propulsion off the surface. The lateral sweepingoff of particles/droplets along the plane of the skin surface isshown in figure 7c (see also electronic supplementary material,movie S11). In this case, multiple droplets/clumps containingnumerous particles can be transported along the surface leavingthe skin free from contamination.

A few recent studies have investigated self-cleaning of thegecko foot [43,44] and suggested that adhesive forces attractinga dirt particle to the substrate and hyperextension are two signifi-cant factors for cleaning. Of note in our study are the similaritiesas well as the significant differences in foot structuring and spi-nule structure (radius of curvature, multiple projections, etc.;see visual comparison in electronic supplementary material,figure S2).

4. ConclusionThe dense array of spinules on the gecko skin provides an archi-tecture suited for removal of large water droplets. It alsoprovides an ideal construction for the self-propulsion of smallcondensed droplets. Dew on some surfaces may promote thegrowth of pathogens such as fungi and bacteria and mayenable spores to develop. In humid environments, this tempor-ary water formation or thin film may result in proliferationresulting in larger populations of microorganisms. As the dewon the superhydrophobic gecko skin is a temporary phenom-enon (constant self-propulsion of droplets and small-sizeddroplets leading to quick evaporation in sunlight/heated

tronic supplementary material, movie S10). (c) A relatively large area beingin colour.)

conditions), this may limit or minimize pathogen attachmentt sroonitlelf-swith

acvecu

removal of just a few particles with minimal water/dropletnceeffeifu

wrmecarcUim

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evolution of scale surfaces in Xantusiid lizards.

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29. Gao L, McCarthy TJ. 2007 How Wenzel and Cassiewere wrong. Langmuir. 23, 37623765. (doi:10.and growth. It is alsoworthmentioning thaplets will lead to a surface inhibiting a watethe skin. We have identified four major premployed in assisting removal of small cExternal environmental forces such as gravent air flow) and impacting small droplaterally projected droplets can assist the seanism for full removal from the surface. Thedroplets from multiple drops merging is anas it does not need to fully rely on gravity asalong the surface.

We have also shown that the skin surfcleaning by these mechanisms, which hacarry away small contaminants. Of parti

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Removal mechanisms of dew via self-propulsion off the gecko skinIntroductionExperimental procedureGecko capture and preparationCharacterization of surface micro- and nanostructureCondensation experimentsDroplet cleaning mechanism observations

Results and discussionTopographical characterization of the gecko skinWettability and dew formation under natural condensation conditionsSelf-propulsion of condensing droplets from coalescenceDirect self-propulsion of droplets off the skin surface via the droplet merging process (single jumping event)Self-propelled impacting droplets which laterally sweep the surface and facilitate removal of surface-bound condensationremoval facilitated by sufficiently high kinetic energy of impacting dropletsImpacting droplets facilitate self-propulsion off the skin surface (impacting droplets from fog and small self-propelled droplets)Wind-assisted removal of self-propelled dropletsOther potential benefits of self-propulsion of condensed droplets

ConclusionEthics statementAcknowledgementsReferences