Bioinspired Ultrahigh Water Pinning NanostructuresJaslyn Bee Khuan Law,*,† Andrew Ming Hua Ng,† Ai Yu He,† and Hong Yee Low*,†,‡

†Institute of Materials Research and Engineering , A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602Singapore‡Engineering Product Development, Singapore University of Technology and Design, 20 Dover Drive, 138682 Singapore

*S Supporting Information

ABSTRACT: Rose petal mimetic surfaces with ultrahighwater pinning forces have been fabricated via nanoimprintingprocess onto three different polymer films. Water pinningforces ranging from 104 to 690 μN are obtained on free-standing polycarbonate films with imprinted nanostructures.Through a systematic variation of the surface structures, thisstudy provides experimental evidence that an ultrahigh waterpinning force can be achieved by combining two surfacetopographical designs: (1) conical- or parabolic-shapednanoprotrusions and (2) isotropic and continuous nano-protrusions. These design criteria ensure that a continuoussolid−liquid contact line is achieved and provide a rule-of-thumb to engineer surfaces with tunable water pinning forces. The ultrahigh water pinning film is further demonstrated tomitigate the “coffee ring” effect, a phenomenon associated with nonuniform deposition from a drying solute-laden liquid droplet.

■ INTRODUCTION

In nature, lotus leaf is well-known for its strong water-repellentproperty (“lotus effect”)1 where a water droplet rolls off the leafspontaneously when slightly tilted. In contrast, rose petalexhibits strong water pinning property (“rose petal effect”)where a water droplet does not roll off the petal surface evenwhen it is turned upside down.2 Interestingly, lotus leaf androse petal both exhibit high static water contact angle of ∼150°(superhydrophobicity) but with different contact anglehysteresis.3−5 Water contact angle hysteresisthe differencebetween the advancing contact angle and the receding contactangleis one of the common methods to describe the motionof liquid droplet on a surface.1,4,6 Generally, a superhydropobicsurface also possesses low contact angle hyteresis where a waterdroplet will roll off the surface with a slight tilt (charateristics oflotus leaf).1,3,6 The rose petal surface exhibits high static watercontact angle, characteristic of superhydrophobicity yet strongwater droplet adhesion, and its pinning property has beenattributed to attractive van der Waals and capillary forces.2,4,5

Subsequently, synthetic patterned surfaces of micro- andnanotextures with good water pinning property werereported.4,5,7

In addition to contact angle hyteresis, other descriptions suchas capillary bridges,8 solid−liquid contact line,9−11 and thethree-phase contact lines theories9,11,12 have provided furtherinsights to explain the motion of water droplet on texturedsurfaces. Based on solid−liquid contact line theory, when thecontact line exerts a pinning force that is equal and in oppositedirection to a body force (weight of the droplet), the dropletremains pinned onto the surface. The maximum total pinning

force (F) of the droplet scales with the length of the solid−liquid contact line length, according to the equation10

=F c Lpin sl (1)

where cpin is the line pinning coefficient with units of force perlength (units of surface tension) and Lsl is the total solid−liquidcontact line length. Theories on contact line density have beenderived by Extrand9 to predict the stability of a water dropleton superhydrobic surface textures. In a more recent work, localcontact pinning lines under an advancing or receding dropletwere captured using an environmental scanning electronmicroscope (ESEM), providing imaging evidence of multiplepinning sites on the solid−liquid contacts on textured surfaces.8Many studies have focused on the interest in developing stablesuperhydrophobic surfaces. The theories on contact anglehysteresis and solid−liquid contact line/pinning sites have beendeveloped to predict the stability of water droplets on surfacetextures; namely, when surface textures are discontinuous, awater droplet that sits on such surfaces will be unstable; such adroplet will be constantly rotating and will roll off the surfacespontaneously with a small tilt angle. On the contrary, whensurface structures are continuous, a continuous solid−liquidcontact line results in water droplet pinned onto the surface.These earlier works can be related to the contrasting wettingbehavior of rose petal and lotus leaf.

Received: September 10, 2013Revised: November 30, 2013Published: December 20, 2013

Article

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The surface of a typical red rose petal consists of a two-layerhierarchically ordered micro/nanostructures (Figure 1a): thefirst layer consists of near-isotropic and continuous conical-shaped microprotrusions (20−30 μm) while the second layerconsists of nanofolds residing on top of the first layermicroprotrusions, presenting it with a hierarchical topography.

The cone-shaped microprotrusions is reported to induce astrong adhesion between the water and the petal surface due tocapillary effect.4 In contrast, the lotus leaf surface is coveredwith random and sporadic distribution of hierarchical micro-bumps (Figure 1b) which do not favor a continuous solid−liquid contact line; hence, a water droplet rolls off very easilywhen the surface is slightly tilted.1,4,11 By contrasting thesurface morphologies of rose petal and lotus leaf, we postulatedtwo characteristic surface designs to achieve ultrahigh waterpinning force: (1) conical-shape nanoprotrusions to impart ahigh capillary force; (2) isotropic and continuous nano-protrusions to create a continuous solid−liquid contact line.In order to verify our hypothesis, we designed three groups ofnanostructures that represent the characteristics of (1) isotropicand continuous, (2) isotropic but discontinuous, and (3)anisotropic and continuous. Here, “continuous” refers tostructures that are fully packed with no “flat” space in betweenthe protrusion, or in other words, the pitch size is equal to thelargest width of the structures; while discontinuous refers tostructures that are separated by a pitch size larger than thelargest width of the structures.Creating synthetic surfaces with water pinning property has

broad technological implications ranging from dew collection aswater source for residence in arid regions, antidrip function forgreenhouse films in agricultural countries, liquid transport andcontrol in microfluidics and spectroscopy, and structuralgenomics as well as for the reduction of “coffee-ring effect” inprinting and coating technologies.13−19 Recent progress increating water pinning surfaces,2,4,5,7,16 while promising, stillsuffers from limitations that restrict its wide deployment. First,these structures were created by direct replication from rosepetal topography acting as natural template2,4 or from bottom-up synthesis methods with laser ablation,16 liquid flame spray,7

or chemical vapor deposition.5 These techniques lack spatialpatterning precision, have limited surface pattern variations, andare unsuitable for mass production. In this work, we takeadvantage of the highly precise yet versatile nanoimprinttechnique to fabricate a series of well-defined nanostructures ondifferent polymer films. Nanoimprinting20,21 offers theadvantages of being highly versatile in terms of the materialsthat can be imprinted; it ranges from free-standing to spin-coat-able thermoplastic film/resin to UV-curable prepolymers. Moreimportantly, this high-resolution technique can be scaled upthrough a roll-to-roll equipment; hence, the nanostructuresdeveloped here can be incorporated into the traditional roll-to-roll coating and printing processes.22 Typical nanoimprintprocess steps are shown in Figure 1c and described in theExperimental Details section.

■ EXPERIMENTAL DETAILSFabrication of Synthetic Nanoprotrusion Structures via

Thermal Nanoimprint. Nanoimprint molds made from nickel (Ni)material were used to fabricate three different types of syntheticnanoprotrusion structures separately on polycarbonate film. Themolds (50 mm × 50 mm) were fabricated commercially (NILTechnology). These topographies are controllably patterned onto a0.125 mm thick, commercially available free-standing polymer film,polycarbonate (PC) sheet (Innox), using a nanoimprinter (ObducatAB) via thermal nanoimprint process. A schematic flow showing atypical thermal nanoimprint process is shown in Figure 1c. First, thenickel mold with the desired inverse topography was treated with anantistiction layer (FDTS, (1H,1H,2H,2H)-perfluorodecyltrichlorosi-lane) using a self-assembled monolayer coater (AVC, Sorona) tofacilitate easy demolding. Next, the mold was pressed against the

Figure 1. Bioinspired synthetically fabricated isotropic and continuousnanoprotrusion structures using nanoimprint process. (a) Opticalimage of a rose petal and SEM images (top and tilt view) of its surfacetopography showing near-isotropic and continuous microprotrusionand hierarchical structure. (b) Optical image of a lotus leaf and SEMimage (tilt view) of its surface topography showing a sporadicdistribution of hierarchical microbumps. (c) A typical thermalnanoimprint process flow. (d) SEM image (tilt view) of fabricatednanoprotrusion with “conical” profile, 300 nm pitch and height(sample A1) (right inset: shape of water droplet, CA: 109 ± 4°). (e)SEM image (tilt view) of fabricated nanoprotrusion structure with“parabolic” profile, 300 nm pitch and height (sample A2) (inset: shapeof water droplet, CA: 108 ± 1°). (f) SEM image (tilt view) offabricated nanoprotrusion structure with “parabolic” profile, 250 nmpitch and height (sample A3) Inset (right): shape of water droplet,CA: 114 ± 3°). (g) Scalable manufacturing of the synthetic film usinga semi-R&D mode roll-to-roll nanoimprinter, and the inset on theright shows a large area patterned film (110 mm × 65 mm patternarea) of sample A1.

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polymer sheet for a duration of imprint time of 300 s at a controlledimprint temperature (180 °C) above the glass transition temperature(Tg) of the polymer (Tg of PC: 150 °C) that allows the softenedviscous polymeric material to completely fill the nanoprotrusion,driven by an external imprint pressure (40 bar) from the system.Following this, the temperature of the nanoimprint system was cooleddown below the Tg of the polymer (80 °C). Upon demolding, thesenanostructures are formed permanently as part of the polymer film.For topographies of nanogratings and isolated nanopillars, thermalnanoimprint process similar to the above is fabricated onto PC film,except that different types of molds were used. The nanogratingsmolds and the isolated nanoholes molds (inverse topography of theimprinted structure) were silicon molds (∼25 mm × 25 mm)fabricated by Institute of Microelectronics. The closed-packednanopillar topography on PC was fabricated employing the samethermal nanoimprint process as above and by using a commerciallyavailable alumina membrane (Anodisc, Whatman).For patterning the nanoprotrusion topography onto poly(methyl

methacrylate) (PMMA) film (Micro resist technology, GmbH), theimprint conditions were the same as that on PC, except that theimprint temperature was at 150 °C (Tg of PMMA = 105 °C). Forpatterning of the structures onto polydimethylsiloxane (PDMS) film,we used conventional PDMS casting method using a ratio of 10:1 ratioof elastomer and curing agent (Sylgard 184, Dow Corning). The roll-

to-roll nanoimprinted large area film shown in Figure 1g is fabricatedusing a custom-built SRS 300 UV roll-to-roll nanoimprinting system(Solves Innovative Technology). Ni mold with similar structure as insample A1 but with inverse topography (patterned area of 120 mm ×70 mm) is used. For more information on the roll-to-rollnanoimprinting processes on polymer film, please refer to ref 22.

Characterization. The morphological characterization of thesamples was examined by using a field emission scanning electronmicroscope (SEM) (JEOL FESEM, JSM 6700F). Prior to SEMimaging, the samples were sputter-coated with 20 nm thick gold (FineCoater, JEOL JFC-1200) to prevent sample charging up and facilitateimaging.

Static water contact angle (CA) measurement were performed usinga contact angle goniometer (Rame-Hart 100). A deionized (DI) waterdroplet (4 μL) was dispensed gently onto the sample surface using anautomatic pipet, and a photograph of the water droplet was takenimmediately with the goniometer camera. CA values were obtainedfrom the integrated software in the goniometer. For each sample,average CA measurement were obtained by measuring five differentlocations on the sample. Also, in order to ensure that water CAcharacterization of the samples was due solely to topographic effectwithout any chemical influence, dummy imprints on blank PC wereperformed on the FDTS-treated mold before the imprint on the actualsample. Thus, any physisorbed FDTS on the mold would be

Figure 2. Images demonstrating the water pinning ability of the synthetically fabricated nanoprotrusion topography and comparison of the quantifiedwater pinning forces exhibited by various topographies patterned on transparent free-standing PC film. (a, b) Photographs of the fabricatednanoprotrusion patterned PC film (sample A2) showing water droplet staying pinned onto the surface when the sample is tilted vertically (90° ofinclination) and upside down (180° of inclination). Inset in (b) shows the corresponding shape and pinning ability of the water droplet when thesample is tilted upside down. (c) Photograph showing an array of water droplet staying pinned onto the nanoprotrusion patterned PC film whentilted nearly upside down. (d) Plot of water pinning forces exhibited by different nanotopographies fabricated on PC film. Inset on top right shows aschematic of the water droplet on a tilted plane and the related parameters to determine the pinning force (F = pinning force, α = angle of tilt, mg =weight of the liquid); scale bar = 500 nm.

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transferred to the dummy imprints and would not interfere with theCA and water pinning result.Droplet Characterization for the “Coffee-Ring Effect”. The

characterization of the different surfaces on the “coffee-ring effect” wasachieved using Fluorescein molecule sodium salt as a dye marker(F6377 Sigma-Aldrich). A solution of 0.05 mg/mL of fluoresceinmolecule sodium salt and deionized water was mixed as a stocksolution at a volume fraction of 5 × 10−4. Droplets (0.5 μL: chosen tofit a droplet within the field of view of the microscope) were depositedonto the surfaces of the nanoimprinted PC, nonpatterned PC film, anda nonpatterned PC film with silanization. The films were allowed todry in a large enclosure with controlled ambient temperature andhumidity. Microscopy images of the dried droplet was captured using aLeica microscope (DM 5000B).

■ RESULTS AND DISCUSSION

The first group of nanostructures represents the isotropic andcontinuous surface topography. Three types of isotropic andcontinuous topographies are hexagonal array of nanoprotru-sions that mimic the rose petal’s microprotrusionwith avariation on the nanoprotrusion profiles, pitch, and height,namely (i) a “conical” profile with a pitch and height of 300 nm(Figure 1d: sample A1); (ii) a “parabolic” profile with a pitchand height of 300 nm (Figure 1e: sample A2); (iii) a“parabolic” profile with a pitch and height of 250 nm (Figure1f: sample A3). A “conical” profile has a sharper tip comparedto the “parabolic” profile. Figure 1g shows a larger nano-imprinted film of sample A1, obtained through roll-to-rollnanoimprinting (110 mm × 65 mm patterned area). Forinterest in roll-to-roll nanoimprinting process, please refer to ref20.The group of surface topography with isotropic and

continuous nanoprotrusions all exhibits water pinning property.Figures 2a−c show that a water droplet pinned onto thenanoprotrusion patterned PC film (sample A2) at 90° and 180°of inclination (see Supporting Information Movie M1). Incontrast, the same volume of water droplet easily rolled off apristine (nonpatterned) PC film when the film was slightlytilted (see Supporting Information Movie M2). To quantify thewater pinning behavior, one can either measure the contactangle hysteresis or determine the pinning force. Contact anglehysteresis measurements provide results that are often relativeas they are dependent on the different methods and the dropletvolumes used, which varies across different works.7 Instead, wedetermine the pinning force experienced by the liquid on thesurfacefirst introduced by Fumidge23 in 1962 and sub-sequently used by other research groups.16,24,25 Water pinningforce (F) on the surface of the fabricated sample is determinedfrom the equation

α=F mg sin (2)

where α is the tilting angle at the onset of water rolling off thesurface, m is the mass of the water droplet, and g is theacceleration of gravity (alternatively, mg = weight of the waterdroplet in grams), which is schematically illustrated in the insetof Figure 2d. To quantify the water pinning force, one caneither measure the tilting angle with a fixed volume (or weight)of water droplet or vary the weight of the water droplet at afixed tilting angle. We used the latter method to determine thewater pinning force. In our experiment, we fixed the angle of tiltat 90° and increased the volume of the water droplet (in stepsof 1 μL) until the weight of the water droplet reached a criticalvalue (i.e., critical weight) to make the water droplet roll off.

The three variations of the nanoprotrusion structures exhibitsimilar level of water pinning force ranging from 680 to 690 μN(Figure 2d); these values outperform many state-of-the-artsynthetic structures (15−230 μN),12,15,16,24 the water pinningforce reported on the natural rose petal (63.8 μN),11 andindependent measurement of the natural red rose petal withmicroprotrusion topography shown in Figure 1a (99 ± 2 μN).In the context of our first postulation that nanoprotrusions canimpart a greater pinning force than microprotrusions, wecompare the solid−liquid contact line length between ananoprotrusion versus a microprotrusion of the same shape.Figure 3 shows a schematic illustration of the solid−liquid

contact line of a droplet sitting on the surface with differenttypes of geometries. The micro- and nanoprotrusiondimensions per unit area are compared using the contact linedensity (CLD) calculation:9

=−

CLDtotal length of solid liquid contact length (m)

total apparent area (m )2

(3)

Here, we assume that a water droplet sits on a three-phasecontact line (solid−liquid−air) and does not penetrate fullyinto these textured surfaces. Lai et al.12 reported air pocketsexist for closed-system (i.e., continuous) as well as open-system(i.e., discontinuous) nanostuctures. Furthermore, Paxson et al.8

provided imaging evidence of air trapped under texturedsurfaces with contact angles as low as 90°. Over a total apparentarea of 1 mm2, the nanoprotrusion (i.e., using sample A1) witha diameter of 300 nm has an estimated contact line density of1.05 × 107/m. In comparison, a microprotrusion with adiameter of 20 μm (from the microprotrusion of a rose petalseen in Figure 1a) has an estimated contact line density of 1.57× 105/m (see Supporting Information SI-2 for CLDcalculation). A higher CLD leads to a higher water pinningforce. Hence, the synthetic nanoprotrusion designs exhibithigher water pinning force compared to the natural rose petal.Next, to verify our hypothesis that an isotropic and

continuous geometry is important to achieve a high waterpinning force, we designed and fabricated two groups ofnanostructures with opposing topography characteristics,namely (i) isotropic but discontinuous and (ii) anisotropicand continuous. Two types of isotropic and discontinuousnanostructures are the random nanopillars (200 nm diameter,400 nm pitch, 400 nm height) and the ordered nanopillars (250nm diameter, 400 nm pitch and 250 nm height). As shown in

Figure 3. Schematic illustration of the continuity of the solid−liquidcontact line (dotted line) on the topview of four different types ofgeometries: (a) isotropic, continuous microprotrusion; (b) isotropic,continuous nanoprotrusion; (c) isotropic, discontinuous nanopillar;(d) anisotropic, continuous nanograting.

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Figure 2d, both the random and orderly separated nanopillarsexhibit reduced water pinning force compared to the isotropicand continuous nanoprotrusions. The anisotropic and con-tinuous structure is a nanograting structure of 250 nm width,500 nm pitch, and 250 nm height. For this structure, the waterpinning forces were measured for the directions parallel andorthogonal to the grating direction. For both measurements onthe nanograting, the water pinning forces are the lowest amongall the nanostructures (Figure 2d). The marginally higherpinning force on the nanograting when the tilting angle isperpendicular to the longitudinal direction of the grating is dueto their physical geometry barrier, a phenomenon wellreported.26 Supporting Information SI-1 reports the measuredquantitative critical weight of the water droplet on thesetopographies and their corresponding water pinning forces.Comparing the estimated contact line densities across the

three groups of nanostructuresthe isotropic and continuousnanoprotrusion (sample A1, Figure 3b), the isotropic anddiscontinuous nanopillars (250 nm diameter, 400 nm pitch,Figure 3c), and the anisotropic and continuous nanogratings(250 nm line and 500 nm pitch, Figure 3d)the magnitude ofthe contact line density (CLD) descends in the order ofisotropic and continuous nanoprotrusion (CLD: 1.05 × 107/m)> isotropic and discontinuous nanopillars (CLD: 4.91 × 106/m> anisotropic and continuous nanogratings (CLD: 4.00 × 106/m) (see Supporting Information SI-2 for CLD calculation).The magnitudes of the measured pinning forces for these threegroups of nanostructures decreases in the same order: isotropicand continuous nanoprotrusion (F: 682 ± 16 μN) > isotropicand isolated nanopillars (F: 484 ± 9 μN) > anisotropic andcontinuous nanogratings (F: 104 ± 11 μN).The bioinspired isotropic and continuous nanoprotrusion

presents a continuous solid−liquid contact line (Figure 3b) thatmaximizes the overall contact line length to enable a waterdroplet to pin strongly onto its surface. In contrast,discontinuous and anisotropic surface structures such as thenanopillars (Figure 3c), and the nanogratings (Figure 3d)present a discontinuous solid−liquid contact line, whichreduces its overall contact line length and hence does notfavor water pinning. Water droplet on a surface withdiscontinuous solid−liquid contact line is unstable and willhave low adhesion to the surface.All three isotropic and continuous nanoprotrusions (samples

A1, A2, and A3) show an increased static water contact anglecompared to the nonpatterned PC film. The static contactangles for A1, A2, and A3 are 109 ± 4°, 108 ± 1°, and 114 ±3°, respectively, while the static water contact angle fornonpatterned PC film is 88 ± 1°. Unlike the natural rose petal,the nanoimprinted PC films are not considered super-hydrophobic. Superhydrophobicity in rose petal is a result ofthe hierarchical layer structure.4 We further performedexperiments on two other polymer films with different pristinestatic water contact angle: poly(methyl methacrylate), PMMA,and polydimethylsiloxane, PDMS. PMMA and PDMS filmswere imprinted with the same nanoprotrusion topography assample A2. Comparing PC, PMMA, and PDMS with the samenanoprotrusions, all three films exhibit similar high waterpinning forces despite the large differences in their pristinehydrophobicity (see Table 1).In solution-based deposition, nonuniform film associated

with nonuniform evaporation of the solvent often result as ahigher concentration of solute deposited on the outer front ofan evaporating dropleta phenomenon often referred as the

“coffee-ring effect”.18,27 A surface with a continuous solid−liquid contact line may be favorable for controlling a uniformdeposition from a solution. We examined the evaporationprofile of a water droplet containing fluorescein molecule onpristine (nonpatterned) PC film and PC film with the rosepetal mimetic nanoprotrusion surface (sample A2). Figures 4a

and 4b show microscopic images of dried droplet stain onpristine PC and the nanostructured PC film, respectively. Thering effect is clearly seen on the nonpatterned PC film. On thecontrary, the droplet on the nanoprotrusion surface is auniformly colored circular spot. The ring-like “coffee-ringeffect” on a drying solute-laden droplet has been known to beinfluenced by the advective flow in the droplet due to thedifference in evaporation rate at the droplet center versus theedge.18,27 Liquid evaporation rate controlled by the evaporationflux27,28 depends on the cross-sectional shape (or the contactangle) of the droplet. In a control experiment, we used anonpatterned PC film that has been silanized to achieve asimilar water contact angle (measured CA ∼ 106 ± 1°) as thenanoprotrusion patterned PC film (sample A2, measured CA ∼108 ± 1°) and performed the same conditions of dropletevaporation experiment. When the droplet was completelydried as shown by the microscopic image in Figure 4c, a similarring-like stain was still observed on the silanized PC film.Hence, the mitigation of the “coffee-ring effect” in the

Table 1. Summary of Measured Water Contact Angles andWater Pinning Forces for Nonpatterned and NanoimprintedPC, PMMA, and PDMS with the Same NanoprotrusionTopography as Sample A2

sample

measured pristine materialstatic water contact angle

(deg)

measuredpinning force

(μN)

nanoprotrusiontopography patterned onPMMA

70 ± 1 684 ± 15

nanoprotrusiontopography patterned onPC

88 ± 1 672 ± 13

nanoprotrusiontopography patterned onPDMS

115 ± 2 660 ± 16

Figure 4.Microscopic images of a water droplet containing fluoresceinmolecules (0.5 μL) that has been fully dried on different types oftopography on PC film. (a) Ring-like stain observed on thenonpatterned PC film which has water CA ∼ 88°. (b) Uniformdeposit on the nanoprotrusion-patterned PC film (sample A2) whichhas water CA ∼ 108°. (c) Ring-like stain observed on thenonpatterned silanized PC film with a water CA ∼ 106°, act ascontrol sample with similar hydrophobicity as the nanoprotrusionsample in (b). The difference in background color of the figures is dueto the difference in topography of the film. 0.5 μL is chosen to fit thecomplete droplet within the field of view of the microscope setup.Scale bar = 0.25 mm.

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nanostructured PC film is primarily due to the presence of thenanostructures.The ring stain occurring on the nonpatterned and silanized

nonpatterned PC film is expected. Unlike a solute-free liquiddroplet on a nonpatterned surface, where the droplet diameterdecreases during evaporation because there is no pinning,7 asolute-laden liquid becomes self-pinned with the solute particlesacting as weak pinning sites at the circumference of the dropletduring the initial stage of the evaporation process.29 Thenpinning of the contact line at the circumference of the dropletand the inherent spatially nonuniform evaporation rate of theliquid droplet cause a net advective flow outward from thedroplet center, transferring the solute to the edge of the dropletwhile the process repeats itself until the droplet is completelydried.19,27 On the other hand, the use of an isotropic, andcontinuous ordered nanoprotrusion topography to reduce the“coffee-ring effect” is interesting. Substrate roughness has beenknown as a factor contributing to the “coffee-ring effect”.18,29 Arecent report has demonstrated the use of silver-coateddisordered nanocones on silicon (Si) substrate for thesuppression of this effect.30 The silver-coated Si nanoconeshave similar geometry as our nanoprotrusion. Interestingly, thesilver-coated Si nanocone design was reported to have watercontact angle of 105°, which is close to the water contact angleof our PC nanoprotrusions. Such nanostructure conferring thesurface with good water pinning ability presents a continuouspinning site and furthermore acts as a geometry barrier thatpins the solutes as they flow outward from the droplet centerduring the advective flow process.

■ CONCLUSIONS

In conclusion, polymer films with exceptionally high waterpinning forces have been achieved through nanoimprintedsurface structures, without the incorporation of any chemicaltreatment. Through a series of systematic surface structurevariation, a design cue for tuning water pinning force onpolymeric films has been demonstrated. Nanoprotrusions withconical or parabolic-shaped structure, arranged isotropically andcontinuously across the surface provide the highest waterpinning forces. The nanostructures are designed to significantlyincrease the solid−liquid contact line length of a water dropleton its surface, thereby achieving a high water-pinning force.The exceptionally high water pinning film is further shown toeffectively mitigate nonuniform coating commonly referred toas the “coffee-ring effect”. The study has taken the advantage ofthe highly versatile and scalable nanoimprint technology:specifically, a variety of surface topographies can be fabricatedon a variety of polymeric materials. The roll-to-roll nano-imprinted synthetic film shown in this study further opens upthe likelihood for industrial adoption.

■ ASSOCIATED CONTENT

*S Supporting InformationSI-1: a table of the quantitative critical weight of the waterdroplet and the corresponding water pinning force measuredon different topographical features fabricated on PC film; SI-2:contact line density (CLD) calculation of the four differentgeometries shown in Figure 3; movie M1: video showing awater droplet (40 μL) staying pinned onto a PC film surfacewith patterned synthetic nanoprotrusion topography, evenwhen the surface is tilted upside down; movie M2: videoshowing a water droplet (40 μL) easily rolls off the surface of a

pristine nonpatterned PC film. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail lawj@imre.a-star.edu.sg (J.B.K.L.).*E-mail hongyee_low@sutd.edu.sg (H.Y.L.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge financial support from SERC(A*STAR) Public, Private Partnership funding to conductthis research. The authors acknowledge Kevin Khaw, LeeYeong Yuh, and Karen Chong of IMRE for the help and usageof the video microscopy facilities.

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