Article pubs.acs.org/Langmuir
Bioinspired Ultrahigh Water Pinning Nanostructures Jaslyn 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, 117602 Singapore ‡ Engineering Product Development, Singapore University of Technology and Design, 20 Dover Drive, 138682 Singapore S Supporting Information *
ABSTRACT: Rose petal mimetic surfaces with ultrahigh water pinning forces have been fabricated via nanoimprinting process onto three different polymer films. Water pinning forces ranging from 104 to 690 μN are obtained on freestanding polycarbonate films with imprinted nanostructures. Through a systematic variation of the surface structures, this study provides experimental evidence that an ultrahigh water pinning force can be achieved by combining two surface topographical designs: (1) conical- or parabolic-shaped nanoprotrusions and (2) isotropic and continuous nanoprotrusions. These design criteria ensure that a continuous solid−liquid contact line is achieved and provide a rule-ofthumb to engineer surfaces with tunable water pinning forces. The ultrahigh water pinning film is further demonstrated to mitigate the “coffee ring” effect, a phenomenon associated with nonuniform deposition from a drying solute-laden liquid droplet.
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INTRODUCTION In nature, lotus leaf is well-known for its strong water-repellent property (“lotus effect”)1 where a water droplet rolls off the leaf spontaneously when slightly tilted. In contrast, rose petal exhibits strong water pinning property (“rose petal effect”) where a water droplet does not roll off the petal surface even when it is turned upside down.2 Interestingly, lotus leaf and rose petal both exhibit high static water contact angle of ∼150° (superhydrophobicity) but with different contact angle hysteresis.3−5 Water contact angle hysteresisthe difference between the advancing contact angle and the receding contact angleis one of the common methods to describe the motion of liquid droplet on a surface.1,4,6 Generally, a superhydropobic surface also possesses low contact angle hyteresis where a water droplet will roll off the surface with a slight tilt (charateristics of lotus leaf).1,3,6 The rose petal surface exhibits high static water contact angle, characteristic of superhydrophobicity yet strong water droplet adhesion, and its pinning property has been attributed to attractive van der Waals and capillary forces.2,4,5 Subsequently, synthetic patterned surfaces of micro- and nanotextures with good water pinning property were reported.4,5,7 In addition to contact angle hyteresis, other descriptions such as capillary bridges,8 solid−liquid contact line,9−11 and the three-phase contact lines theories9,11,12 have provided further insights to explain the motion of water droplet on textured surfaces. Based on solid−liquid contact line theory, when the contact line exerts a pinning force that is equal and in opposite direction to a body force (weight of the droplet), the droplet remains pinned onto the surface. The maximum total pinning © 2013 American Chemical Society
force (F) of the droplet scales with the length of the solid− liquid contact line length, according to the equation10 F = c pinLsl
(1)
where cpin is the line pinning coefficient with units of force per length (units of surface tension) and Lsl is the total solid−liquid contact line length. Theories on contact line density have been derived by Extrand9 to predict the stability of a water droplet on superhydrobic surface textures. In a more recent work, local contact pinning lines under an advancing or receding droplet were captured using an environmental scanning electron microscope (ESEM), providing imaging evidence of multiple pinning sites on the solid−liquid contacts on textured surfaces.8 Many studies have focused on the interest in developing stable superhydrophobic surfaces. The theories on contact angle hysteresis and solid−liquid contact line/pinning sites have been developed to predict the stability of water droplets on surface textures; namely, when surface textures are discontinuous, a water droplet that sits on such surfaces will be unstable; such a droplet will be constantly rotating and will roll off the surface spontaneously with a small tilt angle. On the contrary, when surface structures are continuous, a continuous solid−liquid contact line results in water droplet pinned onto the surface. These earlier works can be related to the contrasting wetting behavior of rose petal and lotus leaf. Received: September 10, 2013 Revised: November 30, 2013 Published: December 20, 2013 325
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The surface of a typical red rose petal consists of a two-layer hierarchically ordered micro/nanostructures (Figure 1a): the first layer consists of near-isotropic and continuous conicalshaped microprotrusions (20−30 μm) while the second layer consists of nanofolds residing on top of the first layer microprotrusions, presenting it with a hierarchical topography.
The cone-shaped microprotrusions is reported to induce a strong adhesion between the water and the petal surface due to capillary effect.4 In contrast, the lotus leaf surface is covered with random and sporadic distribution of hierarchical microbumps (Figure 1b) which do not favor a continuous solid− liquid contact line; hence, a water droplet rolls off very easily when the surface is slightly tilted.1,4,11 By contrasting the surface morphologies of rose petal and lotus leaf, we postulated two characteristic surface designs to achieve ultrahigh water pinning force: (1) conical-shape nanoprotrusions to impart a high capillary force; (2) isotropic and continuous nanoprotrusions to create a continuous solid−liquid contact line. In order to verify our hypothesis, we designed three groups of nanostructures that represent the characteristics of (1) isotropic and continuous, (2) isotropic but discontinuous, and (3) anisotropic and continuous. Here, “continuous” refers to structures that are fully packed with no “flat” space in between the protrusion, or in other words, the pitch size is equal to the largest width of the structures; while discontinuous refers to structures that are separated by a pitch size larger than the largest width of the structures. Creating synthetic surfaces with water pinning property has broad technological implications ranging from dew collection as water source for residence in arid regions, antidrip function for greenhouse films in agricultural countries, liquid transport and control in microfluidics and spectroscopy, and structural genomics as well as for the reduction of “coffee-ring effect” in printing and coating technologies.13−19 Recent progress in creating water pinning surfaces,2,4,5,7,16 while promising, still suffers from limitations that restrict its wide deployment. First, these structures were created by direct replication from rose petal topography acting as natural template2,4 or from bottomup synthesis methods with laser ablation,16 liquid flame spray,7 or chemical vapor deposition.5 These techniques lack spatial patterning precision, have limited surface pattern variations, and are unsuitable for mass production. In this work, we take advantage of the highly precise yet versatile nanoimprint technique to fabricate a series of well-defined nanostructures on different polymer films. Nanoimprinting20,21 offers the advantages of being highly versatile in terms of the materials that can be imprinted; it ranges from free-standing to spin-coatable thermoplastic film/resin to UV-curable prepolymers. More importantly, this high-resolution technique can be scaled up through a roll-to-roll equipment; hence, the nanostructures developed here can be incorporated into the traditional roll-toroll coating and printing processes.22 Typical nanoimprint process steps are shown in Figure 1c and described in the Experimental Details section.
Figure 1. Bioinspired synthetically fabricated isotropic and continuous nanoprotrusion structures using nanoimprint process. (a) Optical image of a rose petal and SEM images (top and tilt view) of its surface topography showing near-isotropic and continuous microprotrusion and hierarchical structure. (b) Optical image of a lotus leaf and SEM image (tilt view) of its surface topography showing a sporadic distribution of hierarchical microbumps. (c) A typical thermal nanoimprint process flow. (d) SEM image (tilt view) of fabricated nanoprotrusion 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: shape of water droplet, CA: 108 ± 1°). (f) SEM image (tilt view) of fabricated nanoprotrusion structure with “parabolic” profile, 250 nm pitch and height (sample A3) Inset (right): shape of water droplet, CA: 114 ± 3°). (g) Scalable manufacturing of the synthetic film using a semi-R&D mode roll-to-roll nanoimprinter, and the inset on the right shows a large area patterned film (110 mm × 65 mm pattern area) of sample A1.
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EXPERIMENTAL DETAILS
Fabrication of Synthetic Nanoprotrusion Structures via Thermal Nanoimprint. Nanoimprint molds made from nickel (Ni) material were used to fabricate three different types of synthetic nanoprotrusion structures separately on polycarbonate film. The molds (50 mm × 50 mm) were fabricated commercially (NIL Technology). These topographies are controllably patterned onto a 0.125 mm thick, commercially available free-standing polymer film, polycarbonate (PC) sheet (Innox), using a nanoimprinter (Obducat AB) via thermal nanoimprint process. A schematic flow showing a typical thermal nanoimprint process is shown in Figure 1c. First, the nickel mold with the desired inverse topography was treated with an antistiction layer (FDTS, (1H,1H,2H,2H)-perfluorodecyltrichlorosilane) using a self-assembled monolayer coater (AVC, Sorona) to facilitate easy demolding. Next, the mold was pressed against the 326
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Figure 2. Images demonstrating the water pinning ability of the synthetically fabricated nanoprotrusion topography and comparison of the quantified water pinning forces exhibited by various topographies patterned on transparent free-standing PC film. (a, b) Photographs of the fabricated nanoprotrusion patterned PC film (sample A2) showing water droplet staying pinned onto the surface when the sample is tilted vertically (90° of inclination) and upside down (180° of inclination). Inset in (b) shows the corresponding shape and pinning ability of the water droplet when the sample is tilted upside down. (c) Photograph showing an array of water droplet staying pinned onto the nanoprotrusion patterned PC film when tilted nearly upside down. (d) Plot of water pinning forces exhibited by different nanotopographies fabricated on PC film. Inset on top right shows a schematic 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. to-roll nanoimprinted large area film shown in Figure 1g is fabricated using a custom-built SRS 300 UV roll-to-roll nanoimprinting system (Solves Innovative Technology). Ni mold with similar structure as in sample A1 but with inverse topography (patterned area of 120 mm × 70 mm) is used. For more information on the roll-to-roll nanoimprinting processes on polymer film, please refer to ref 22. Characterization. The morphological characterization of the samples was examined by using a field emission scanning electron microscope (SEM) (JEOL FESEM, JSM 6700F). Prior to SEM imaging, the samples were sputter-coated with 20 nm thick gold (Fine Coater, JEOL JFC-1200) to prevent sample charging up and facilitate imaging. Static water contact angle (CA) measurement were performed using a contact angle goniometer (Rame-Hart 100). A deionized (DI) water droplet (4 μL) was dispensed gently onto the sample surface using an automatic pipet, and a photograph of the water droplet was taken immediately with the goniometer camera. CA values were obtained from the integrated software in the goniometer. For each sample, average CA measurement were obtained by measuring five different locations on the sample. Also, in order to ensure that water CA characterization of the samples was due solely to topographic effect without any chemical influence, dummy imprints on blank PC were performed on the FDTS-treated mold before the imprint on the actual sample. Thus, any physisorbed FDTS on the mold would be
polymer sheet for a duration of imprint time of 300 s at a controlled imprint temperature (180 °C) above the glass transition temperature (Tg) of the polymer (Tg of PC: 150 °C) that allows the softened viscous 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 cooled down below the Tg of the polymer (80 °C). Upon demolding, these nanostructures are formed permanently as part of the polymer film. For topographies of nanogratings and isolated nanopillars, thermal nanoimprint process similar to the above is fabricated onto PC film, except that different types of molds were used. The nanogratings molds and the isolated nanoholes molds (inverse topography of the imprinted structure) were silicon molds (∼25 mm × 25 mm) fabricated by Institute of Microelectronics. The closed-packed nanopillar topography on PC was fabricated employing the same thermal nanoimprint process as above and by using a commercially available alumina membrane (Anodisc, Whatman). For patterning the nanoprotrusion topography onto poly(methyl methacrylate) (PMMA) film (Micro resist technology, GmbH), the imprint conditions were the same as that on PC, except that the imprint temperature was at 150 °C (Tg of PMMA = 105 °C). For patterning of the structures onto polydimethylsiloxane (PDMS) film, we used conventional PDMS casting method using a ratio of 10:1 ratio of elastomer and curing agent (Sylgard 184, Dow Corning). The roll327
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transferred to the dummy imprints and would not interfere with the CA and water pinning result. Droplet Characterization for the “Coffee-Ring Effect”. The characterization of the different surfaces on the “coffee-ring effect” was achieved using Fluorescein molecule sodium salt as a dye marker (F6377 Sigma-Aldrich). A solution of 0.05 mg/mL of fluorescein molecule sodium salt and deionized water was mixed as a stock solution at a volume fraction of 5 × 10−4. Droplets (0.5 μL: chosen to fit a droplet within the field of view of the microscope) were deposited onto the surfaces of the nanoimprinted PC, nonpatterned PC film, and a nonpatterned PC film with silanization. The films were allowed to dry in a large enclosure with controlled ambient temperature and humidity. Microscopy images of the dried droplet was captured using a Leica microscope (DM 5000B).
The three variations of the nanoprotrusion structures exhibit similar level of water pinning force ranging from 680 to 690 μN (Figure 2d); these values outperform many state-of-the-art synthetic structures (15−230 μN),12,15,16,24 the water pinning force reported on the natural rose petal (63.8 μN),11 and independent measurement of the natural red rose petal with microprotrusion topography shown in Figure 1a (99 ± 2 μN). In the context of our first postulation that nanoprotrusions can impart a greater pinning force than microprotrusions, we compare the solid−liquid contact line length between a nanoprotrusion versus a microprotrusion of the same shape. Figure 3 shows a schematic illustration of the solid−liquid
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RESULTS AND DISCUSSION The first group of nanostructures represents the isotropic and continuous surface topography. Three types of isotropic and continuous topographies are hexagonal array of nanoprotrusions that mimic the rose petal’s microprotrusionwith a variation 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 pitch and height of 300 nm (Figure 1e: sample A2); (iii) a “parabolic” profile with a pitch and height of 250 nm (Figure 1f: sample A3). A “conical” profile has a sharper tip compared to the “parabolic” profile. Figure 1g shows a larger nanoimprinted film of sample A1, obtained through roll-to-roll nanoimprinting (110 mm × 65 mm patterned area). For interest in roll-to-roll nanoimprinting process, please refer to ref 20. 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 the nanoprotrusion patterned PC film (sample A2) at 90° and 180° of inclination (see Supporting Information Movie M1). In contrast, the same volume of water droplet easily rolled off a pristine (nonpatterned) PC film when the film was slightly tilted (see Supporting Information Movie M2). To quantify the water pinning behavior, one can either measure the contact angle hysteresis or determine the pinning force. Contact angle hysteresis measurements provide results that are often relative as they are dependent on the different methods and the droplet volumes used, which varies across different works.7 Instead, we determine the pinning force experienced by the liquid on the surfacefirst introduced by Fumidge23 in 1962 and subsequently used by other research groups.16,24,25 Water pinning force (F) on the surface of the fabricated sample is determined from the equation F = mg sin α
Figure 3. Schematic illustration of the continuity of the solid−liquid contact line (dotted line) on the topview of four different types of geometries: (a) isotropic, continuous microprotrusion; (b) isotropic, continuous nanoprotrusion; (c) isotropic, discontinuous nanopillar; (d) anisotropic, continuous nanograting.
contact line of a droplet sitting on the surface with different types of geometries. The micro- and nanoprotrusion dimensions per unit area are compared using the contact line density (CLD) calculation:9 CLD =
total length of solid − liquid contact length (m) total apparent area (m 2) (3)
Here, we assume that a water droplet sits on a three-phase contact line (solid−liquid−air) and does not penetrate fully into these textured surfaces. Lai et al.12 reported air pockets exist 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 textured surfaces with contact angles as low as 90°. Over a total apparent area of 1 mm2, the nanoprotrusion (i.e., using sample A1) with a diameter of 300 nm has an estimated contact line density of 1.05 × 107/m. In comparison, a microprotrusion with a diameter of 20 μm (from the microprotrusion of a rose petal seen in Figure 1a) has an estimated contact line density of 1.57 × 10 5 /m (see Supporting Information SI-2 for CLD calculation). A higher CLD leads to a higher water pinning force. Hence, the synthetic nanoprotrusion designs exhibit higher 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 water pinning force, we designed and fabricated two groups of nanostructures with opposing topography characteristics, namely (i) isotropic but discontinuous and (ii) anisotropic and continuous. Two types of isotropic and discontinuous nanostructures are the random nanopillars (200 nm diameter, 400 nm pitch, 400 nm height) and the ordered nanopillars (250 nm diameter, 400 nm pitch and 250 nm height). As shown in
(2)
where α is the tilting angle at the onset of water rolling off the surface, m is the mass of the water droplet, and g is the acceleration of gravity (alternatively, mg = weight of the water droplet in grams), which is schematically illustrated in the inset of Figure 2d. To quantify the water pinning force, one can either measure the tilting angle with a fixed volume (or weight) of water droplet or vary the weight of the water droplet at a fixed tilting angle. We used the latter method to determine the water pinning force. In our experiment, we fixed the angle of tilt at 90° and increased the volume of the water droplet (in steps of 1 μL) until the weight of the water droplet reached a critical value (i.e., critical weight) to make the water droplet roll off. 328
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Figure 2d, both the random and orderly separated nanopillars exhibit reduced water pinning force compared to the isotropic and continuous nanoprotrusions. The anisotropic and continuous structure is a nanograting structure of 250 nm width, 500 nm pitch, and 250 nm height. For this structure, the water pinning forces were measured for the directions parallel and orthogonal to the grating direction. For both measurements on the nanograting, the water pinning forces are the lowest among all the nanostructures (Figure 2d). The marginally higher pinning force on the nanograting when the tilting angle is perpendicular to the longitudinal direction of the grating is due to their physical geometry barrier, a phenomenon well reported.26 Supporting Information SI-1 reports the measured quantitative critical weight of the water droplet on these topographies and their corresponding water pinning forces. Comparing the estimated contact line densities across the three groups of nanostructuresthe isotropic and continuous nanoprotrusion (sample A1, Figure 3b), the isotropic and discontinuous 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 of the contact line density (CLD) descends in the order of isotropic 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 three groups of nanostructures decreases in the same order: isotropic and continuous nanoprotrusion (F: 682 ± 16 μN) > isotropic and isolated nanopillars (F: 484 ± 9 μN) > anisotropic and continuous nanogratings (F: 104 ± 11 μN). The bioinspired isotropic and continuous nanoprotrusion presents a continuous solid−liquid contact line (Figure 3b) that maximizes the overall contact line length to enable a water droplet to pin strongly onto its surface. In contrast, discontinuous and anisotropic surface structures such as the nanopillars (Figure 3c), and the nanogratings (Figure 3d) present a discontinuous solid−liquid contact line, which reduces its overall contact line length and hence does not favor water pinning. Water droplet on a surface with discontinuous solid−liquid contact line is unstable and will have low adhesion to the surface. All three isotropic and continuous nanoprotrusions (samples A1, A2, and A3) show an increased static water contact angle compared to the nonpatterned PC film. The static contact angles for A1, A2, and A3 are 109 ± 4°, 108 ± 1°, and 114 ± 3°, respectively, while the static water contact angle for nonpatterned PC film is 88 ± 1°. Unlike the natural rose petal, the nanoimprinted PC films are not considered superhydrophobic. Superhydrophobicity in rose petal is a result of the hierarchical layer structure.4 We further performed experiments on two other polymer films with different pristine static water contact angle: poly(methyl methacrylate), PMMA, and polydimethylsiloxane, PDMS. PMMA and PDMS films were imprinted with the same nanoprotrusion topography as sample A2. Comparing PC, PMMA, and PDMS with the same nanoprotrusions, all three films exhibit similar high water pinning forces despite the large differences in their pristine hydrophobicity (see Table 1). In solution-based deposition, nonuniform film associated with nonuniform evaporation of the solvent often result as a higher concentration of solute deposited on the outer front of an evaporating dropleta phenomenon often referred as the
Table 1. Summary of Measured Water Contact Angles and Water Pinning Forces for Nonpatterned and Nanoimprinted PC, PMMA, and PDMS with the Same Nanoprotrusion Topography as Sample A2 sample nanoprotrusion topography patterned on PMMA nanoprotrusion topography patterned on PC nanoprotrusion topography patterned on PDMS
measured pristine material static water contact angle (deg)
measured pinning force (μN)
70 ± 1
684 ± 15
88 ± 1
672 ± 13
115 ± 2
660 ± 16
“coffee-ring effect”.18,27 A surface with a continuous solid− liquid contact line may be favorable for controlling a uniform deposition from a solution. We examined the evaporation profile of a water droplet containing fluorescein molecule on pristine (nonpatterned) PC film and PC film with the rose petal mimetic nanoprotrusion surface (sample A2). Figures 4a
Figure 4. Microscopic images of a water droplet containing fluorescein molecules (0.5 μL) that has been fully dried on different types of topography on PC film. (a) Ring-like stain observed on the nonpatterned PC film which has water CA ∼ 88°. (b) Uniform deposit on the nanoprotrusion-patterned PC film (sample A2) which has water CA ∼ 108°. (c) Ring-like stain observed on the nonpatterned silanized PC film with a water CA ∼ 106°, act as control sample with similar hydrophobicity as the nanoprotrusion sample in (b). The difference in background color of the figures is due to the difference in topography of the film. 0.5 μL is chosen to fit the complete droplet within the field of view of the microscope setup. Scale bar = 0.25 mm.
and 4b show microscopic images of dried droplet stain on pristine PC and the nanostructured PC film, respectively. The ring effect is clearly seen on the nonpatterned PC film. On the contrary, the droplet on the nanoprotrusion surface is a uniformly colored circular spot. The ring-like “coffee-ring effect” on a drying solute-laden droplet has been known to be influenced by the advective flow in the droplet due to the difference in evaporation rate at the droplet center versus the edge.18,27 Liquid evaporation rate controlled by the evaporation flux27,28 depends on the cross-sectional shape (or the contact angle) of the droplet. In a control experiment, we used a nonpatterned PC film that has been silanized to achieve a similar water contact angle (measured CA ∼ 106 ± 1°) as the nanoprotrusion patterned PC film (sample A2, measured CA ∼ 108 ± 1°) and performed the same conditions of droplet evaporation experiment. When the droplet was completely dried as shown by the microscopic image in Figure 4c, a similar ring-like stain was still observed on the silanized PC film. Hence, the mitigation of the “coffee-ring effect” in the 329
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pristine nonpatterned PC film. This material is available free of charge via the Internet at http://pubs.acs.org.
nanostructured PC film is primarily due to the presence of the nanostructures. The ring stain occurring on the nonpatterned and silanized nonpatterned PC film is expected. Unlike a solute-free liquid droplet on a nonpatterned surface, where the droplet diameter decreases during evaporation because there is no pinning,7 a solute-laden liquid becomes self-pinned with the solute particles acting as weak pinning sites at the circumference of the droplet during the initial stage of the evaporation process.29 Then pinning of the contact line at the circumference of the droplet and the inherent spatially nonuniform evaporation rate of the liquid droplet cause a net advective flow outward from the droplet center, transferring the solute to the edge of the droplet while the process repeats itself until the droplet is completely dried.19,27 On the other hand, the use of an isotropic, and continuous ordered nanoprotrusion topography to reduce the “coffee-ring effect” is interesting. Substrate roughness has been known as a factor contributing to the “coffee-ring effect”.18,29 A recent report has demonstrated the use of silver-coated disordered nanocones on silicon (Si) substrate for the suppression of this effect.30 The silver-coated Si nanocones have similar geometry as our nanoprotrusion. Interestingly, the silver-coated Si nanocone design was reported to have water contact angle of 105°, which is close to the water contact angle of our PC nanoprotrusions. Such nanostructure conferring the surface with good water pinning ability presents a continuous pinning site and furthermore acts as a geometry barrier that pins the solutes as they flow outward from the droplet center during the advective flow process.
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Corresponding Authors
*E-mail [email protected] (J.B.K.L.). *E-mail [email protected] (H.Y.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from SERC (A*STAR) Public, Private Partnership funding to conduct this research. The authors acknowledge Kevin Khaw, Lee Yeong Yuh, and Karen Chong of IMRE for the help and usage of the video microscopy facilities.
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REFERENCES
(1) Koch, K.; Barthlott, W. Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials. Philos. Trans. R. Soc., A 2009, 367, 1487−1509. (2) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114−4119. (3) Koch, K.; Bhushan, B.; Jung, Y. C.; Barthlott, W. Fabrication of artificial lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion. Soft Matter 2009, 5, 1386− 1393. (4) Xi, J.; Jiang, L. Biomimic superhydrophobic surface with high adhesive forces. Ind. Eng. Chem. Res. 2008, 47, 6354−6357. (5) Dawood, M. K.; Zheng, H.; Liew, T. H.; Leong, K. C.; Foo, Y. L.; Rajagopalan, R.; Khan, S. A.; Choi, W. K. Mimicking both petal and lotus effects on a single silicon substrate by tuning the wettability of nanostructured surfaces. Langmuir 2011, 27, 4126−4133. (6) Bhushan, B.; Jung, Y. C.; Koch, K. Self-cleaning efficiency of artificial superhydrophobic surfaces. Langmuir 2009, 25, 3240−3248. (7) Teisala, H.; Tuominen, M.; Aromaa, M.; Stepien, M.; Makela, J. M.; Saarinen, J. J.; Toivakka, M.; Kuusipalo, J. Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces. Langmuir 2012, 28, 3138−3145. (8) Paxson, A. T.; Varanasi, K. K. Self-similarity of contact line depinning from textured surfaces. Nat. Commun. 2013, 4, 1492. (9) Extrand, C. W. Designing for optimum liquid repellency. Langmuir 2006, 22, 1711−1714. (10) Walker, S. W.; Shapiro, B.; Nochetto, R. H. Electrowetting with contact line pinning: Computational modeling and comparisons with experiments. Phys. Fluids 2009, 21. (11) Oner, D.; McCarthy, T. J. Ultrahydrophobic surfaces. Effects of topography length scales on wettability. Langmuir 2000, 16, 7777− 7782. (12) Lai, Y.; Gao, X.; Zhuang, H.; Huang, J.; Lin, C.; Jiang, L. Designing superhydrophobic porous nanostructures with tunable water adhesion. Adv. Mater. 2009, 21, 3799−3803. (13) Tian, X.; Chen, Y.; Zheng, Y.; Bai, H.; Jiang, L. Controlling water capture of bioinspired fibers with hump structures. Adv. Mater. 2011, 23, 5486. (14) Katsoulas, N.; Bartzanas, T.; Kitta, E. Effects of anti-drip polyethylene covering films on microclimate and crop production. In International Symposium on Advanced Technologies and Management Towards Sustainable Greenhouse Ecosystems: Greensys2011; Kittas, C., Katsoulas, N., Bartzanas, T., Eds.; 2012; Vol. 952, pp 209−215. (15) Wu, D.; Wu, S.-Z.; Chen, Q.-D.; Zhang, Y.-L.; Yao, J.; Yao, X.; Niu, L.-G.; Wang, J.-N.; Jiang, L.; Sun, H.-B. Curvature-driven reversible in situ switching between pinned and roll-down superhydrophobic states for water droplet transportation. Adv. Mater. 2011, 23, 545.
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CONCLUSIONS In conclusion, polymer films with exceptionally high water pinning forces have been achieved through nanoimprinted surface structures, without the incorporation of any chemical treatment. Through a series of systematic surface structure variation, a design cue for tuning water pinning force on polymeric films has been demonstrated. Nanoprotrusions with conical or parabolic-shaped structure, arranged isotropically and continuously across the surface provide the highest water pinning forces. The nanostructures are designed to significantly increase the solid−liquid contact line length of a water droplet on its surface, thereby achieving a high water-pinning force. The exceptionally high water pinning film is further shown to effectively mitigate nonuniform coating commonly referred to as the “coffee-ring effect”. The study has taken the advantage of the highly versatile and scalable nanoimprint technology: specifically, a variety of surface topographies can be fabricated on a variety of polymeric materials. The roll-to-roll nanoimprinted synthetic film shown in this study further opens up the likelihood for industrial adoption.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
* Supporting Information S
SI-1: a table of the quantitative critical weight of the water droplet and the corresponding water pinning force measured on different topographical features fabricated on PC film; SI-2: contact line density (CLD) calculation of the four different geometries shown in Figure 3; movie M1: video showing a water droplet (40 μL) staying pinned onto a PC film surface with patterned synthetic nanoprotrusion topography, even when the surface is tilted upside down; movie M2: video showing a water droplet (40 μL) easily rolls off the surface of a 330
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(16) McLauchlin, M. L.; Yang, D.; Aella, P.; Garcia, A. A.; Picraux, S. T.; Hayes, M. A. Evaporative properties and pinning strength of laserablated, hydrophilic sites on lotus-leaf-like, nanostructured surfaces. Langmuir 2007, 23, 4871−4877. (17) Berejnov, V.; Thorne, R. E. Effect of transient pinning on stability of drops sitting on an inclined plane. Phys. Rev. E 2007, 75, 066308. (18) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 2011, 476, 308−311. (19) Soltman, D.; Subramanian, V. Inkjet-printed line morphologies and temperature control of the coffee ring effect. Langmuir 2008, 24, 2224−2231. (20) Dumond, J. J.; Low, H. Y. Recent developments and design challenges in continuous roller micro- and nanoimprinting. J. Vac. Sci. Technol. B 2012, 30. (21) Schift, H. Nanoimprint lithography: An old story in modern times? A review. J. Vac. Sci. Technol. B 2008, 26, 458−480. (22) Dumond, J. J.; Mahabadi, K. A.; Yee, Y. S.; Tan, C.; Fuh, J. Y. H.; Lee, H. P.; Low, H. Y. High resolution UV roll-to-roll nanoimprinting of resin moulds and subsequent replication via thermal nanoimprint lithography. Nanotechnology 2012, 23. (23) Furmidge, C. G. Studies at phase interfaces. 1. Sliding of liquid drops on solid surfaces and a theory for spray retention. J. Colloid Sci. 1962, 17, 309. (24) Latthe, S. S.; Dhere, S. L.; Kappenstein, C.; Imai, H.; Ganesan, V.; Rao, A. V.; Wagh, P. B.; Gupta, S. C. Sliding behavior of water drops on sol-gel derived hydrophobic silica films. Appl. Surf. Sci. 2010, 256, 3259−3264. (25) Wong, T.-S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443− 447. (26) Li, W.; Fang, G.; Lij, Y.; Qiao, G. Anisotropic wetting behavior arising from superhydrophobic surfaces: Parallel grooved structure. J. Phys. Chem. B 2008, 112, 7234−7243. (27) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827−829. (28) Peiss, C. N. Evaporation of small water drops maintained at constant volume. J. Appl. Phys. 1989, 65, 5235−5237. (29) Deegan, R. D. Pattern formation in drying drops. Phys. Rev. E 2000, 61, 475−485. (30) Coppe, J.-P.; Xu, Z.; Chen, Y.; Liu, G. L. Metallic nanocone array photonic substrate for high-uniformity surface deposition and optical detection of small molecules. Nanotechnology 2011, 22.
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Bioinspired Ultrahigh Water Pinning Nanostructures Jaslyn 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, 117602 Singapore ‡ Engineering Product Development, Singapore University of Technology and Design, 20 Dover Drive, 138682 Singapore S Supporting Information *
ABSTRACT: Rose petal mimetic surfaces with ultrahigh water pinning forces have been fabricated via nanoimprinting process onto three different polymer films. Water pinning forces ranging from 104 to 690 μN are obtained on freestanding polycarbonate films with imprinted nanostructures. Through a systematic variation of the surface structures, this study provides experimental evidence that an ultrahigh water pinning force can be achieved by combining two surface topographical designs: (1) conical- or parabolic-shaped nanoprotrusions and (2) isotropic and continuous nanoprotrusions. These design criteria ensure that a continuous solid−liquid contact line is achieved and provide a rule-ofthumb to engineer surfaces with tunable water pinning forces. The ultrahigh water pinning film is further demonstrated to mitigate the “coffee ring” effect, a phenomenon associated with nonuniform deposition from a drying solute-laden liquid droplet.
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INTRODUCTION In nature, lotus leaf is well-known for its strong water-repellent property (“lotus effect”)1 where a water droplet rolls off the leaf spontaneously when slightly tilted. In contrast, rose petal exhibits strong water pinning property (“rose petal effect”) where a water droplet does not roll off the petal surface even when it is turned upside down.2 Interestingly, lotus leaf and rose petal both exhibit high static water contact angle of ∼150° (superhydrophobicity) but with different contact angle hysteresis.3−5 Water contact angle hysteresisthe difference between the advancing contact angle and the receding contact angleis one of the common methods to describe the motion of liquid droplet on a surface.1,4,6 Generally, a superhydropobic surface also possesses low contact angle hyteresis where a water droplet will roll off the surface with a slight tilt (charateristics of lotus leaf).1,3,6 The rose petal surface exhibits high static water contact angle, characteristic of superhydrophobicity yet strong water droplet adhesion, and its pinning property has been attributed to attractive van der Waals and capillary forces.2,4,5 Subsequently, synthetic patterned surfaces of micro- and nanotextures with good water pinning property were reported.4,5,7 In addition to contact angle hyteresis, other descriptions such as capillary bridges,8 solid−liquid contact line,9−11 and the three-phase contact lines theories9,11,12 have provided further insights to explain the motion of water droplet on textured surfaces. Based on solid−liquid contact line theory, when the contact line exerts a pinning force that is equal and in opposite direction to a body force (weight of the droplet), the droplet remains pinned onto the surface. The maximum total pinning © 2013 American Chemical Society
force (F) of the droplet scales with the length of the solid− liquid contact line length, according to the equation10 F = c pinLsl
(1)
where cpin is the line pinning coefficient with units of force per length (units of surface tension) and Lsl is the total solid−liquid contact line length. Theories on contact line density have been derived by Extrand9 to predict the stability of a water droplet on superhydrobic surface textures. In a more recent work, local contact pinning lines under an advancing or receding droplet were captured using an environmental scanning electron microscope (ESEM), providing imaging evidence of multiple pinning sites on the solid−liquid contacts on textured surfaces.8 Many studies have focused on the interest in developing stable superhydrophobic surfaces. The theories on contact angle hysteresis and solid−liquid contact line/pinning sites have been developed to predict the stability of water droplets on surface textures; namely, when surface textures are discontinuous, a water droplet that sits on such surfaces will be unstable; such a droplet will be constantly rotating and will roll off the surface spontaneously with a small tilt angle. On the contrary, when surface structures are continuous, a continuous solid−liquid contact line results in water droplet pinned onto the surface. These earlier works can be related to the contrasting wetting behavior of rose petal and lotus leaf. Received: September 10, 2013 Revised: November 30, 2013 Published: December 20, 2013 325
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The surface of a typical red rose petal consists of a two-layer hierarchically ordered micro/nanostructures (Figure 1a): the first layer consists of near-isotropic and continuous conicalshaped microprotrusions (20−30 μm) while the second layer consists of nanofolds residing on top of the first layer microprotrusions, presenting it with a hierarchical topography.
The cone-shaped microprotrusions is reported to induce a strong adhesion between the water and the petal surface due to capillary effect.4 In contrast, the lotus leaf surface is covered with random and sporadic distribution of hierarchical microbumps (Figure 1b) which do not favor a continuous solid− liquid contact line; hence, a water droplet rolls off very easily when the surface is slightly tilted.1,4,11 By contrasting the surface morphologies of rose petal and lotus leaf, we postulated two characteristic surface designs to achieve ultrahigh water pinning force: (1) conical-shape nanoprotrusions to impart a high capillary force; (2) isotropic and continuous nanoprotrusions to create a continuous solid−liquid contact line. In order to verify our hypothesis, we designed three groups of nanostructures that represent the characteristics of (1) isotropic and continuous, (2) isotropic but discontinuous, and (3) anisotropic and continuous. Here, “continuous” refers to structures that are fully packed with no “flat” space in between the protrusion, or in other words, the pitch size is equal to the largest width of the structures; while discontinuous refers to structures that are separated by a pitch size larger than the largest width of the structures. Creating synthetic surfaces with water pinning property has broad technological implications ranging from dew collection as water source for residence in arid regions, antidrip function for greenhouse films in agricultural countries, liquid transport and control in microfluidics and spectroscopy, and structural genomics as well as for the reduction of “coffee-ring effect” in printing and coating technologies.13−19 Recent progress in creating water pinning surfaces,2,4,5,7,16 while promising, still suffers from limitations that restrict its wide deployment. First, these structures were created by direct replication from rose petal topography acting as natural template2,4 or from bottomup synthesis methods with laser ablation,16 liquid flame spray,7 or chemical vapor deposition.5 These techniques lack spatial patterning precision, have limited surface pattern variations, and are unsuitable for mass production. In this work, we take advantage of the highly precise yet versatile nanoimprint technique to fabricate a series of well-defined nanostructures on different polymer films. Nanoimprinting20,21 offers the advantages of being highly versatile in terms of the materials that can be imprinted; it ranges from free-standing to spin-coatable thermoplastic film/resin to UV-curable prepolymers. More importantly, this high-resolution technique can be scaled up through a roll-to-roll equipment; hence, the nanostructures developed here can be incorporated into the traditional roll-toroll coating and printing processes.22 Typical nanoimprint process steps are shown in Figure 1c and described in the Experimental Details section.
Figure 1. Bioinspired synthetically fabricated isotropic and continuous nanoprotrusion structures using nanoimprint process. (a) Optical image of a rose petal and SEM images (top and tilt view) of its surface topography showing near-isotropic and continuous microprotrusion and hierarchical structure. (b) Optical image of a lotus leaf and SEM image (tilt view) of its surface topography showing a sporadic distribution of hierarchical microbumps. (c) A typical thermal nanoimprint process flow. (d) SEM image (tilt view) of fabricated nanoprotrusion 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: shape of water droplet, CA: 108 ± 1°). (f) SEM image (tilt view) of fabricated nanoprotrusion structure with “parabolic” profile, 250 nm pitch and height (sample A3) Inset (right): shape of water droplet, CA: 114 ± 3°). (g) Scalable manufacturing of the synthetic film using a semi-R&D mode roll-to-roll nanoimprinter, and the inset on the right shows a large area patterned film (110 mm × 65 mm pattern area) of sample A1.
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EXPERIMENTAL DETAILS
Fabrication of Synthetic Nanoprotrusion Structures via Thermal Nanoimprint. Nanoimprint molds made from nickel (Ni) material were used to fabricate three different types of synthetic nanoprotrusion structures separately on polycarbonate film. The molds (50 mm × 50 mm) were fabricated commercially (NIL Technology). These topographies are controllably patterned onto a 0.125 mm thick, commercially available free-standing polymer film, polycarbonate (PC) sheet (Innox), using a nanoimprinter (Obducat AB) via thermal nanoimprint process. A schematic flow showing a typical thermal nanoimprint process is shown in Figure 1c. First, the nickel mold with the desired inverse topography was treated with an antistiction layer (FDTS, (1H,1H,2H,2H)-perfluorodecyltrichlorosilane) using a self-assembled monolayer coater (AVC, Sorona) to facilitate easy demolding. Next, the mold was pressed against the 326
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Figure 2. Images demonstrating the water pinning ability of the synthetically fabricated nanoprotrusion topography and comparison of the quantified water pinning forces exhibited by various topographies patterned on transparent free-standing PC film. (a, b) Photographs of the fabricated nanoprotrusion patterned PC film (sample A2) showing water droplet staying pinned onto the surface when the sample is tilted vertically (90° of inclination) and upside down (180° of inclination). Inset in (b) shows the corresponding shape and pinning ability of the water droplet when the sample is tilted upside down. (c) Photograph showing an array of water droplet staying pinned onto the nanoprotrusion patterned PC film when tilted nearly upside down. (d) Plot of water pinning forces exhibited by different nanotopographies fabricated on PC film. Inset on top right shows a schematic 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. to-roll nanoimprinted large area film shown in Figure 1g is fabricated using a custom-built SRS 300 UV roll-to-roll nanoimprinting system (Solves Innovative Technology). Ni mold with similar structure as in sample A1 but with inverse topography (patterned area of 120 mm × 70 mm) is used. For more information on the roll-to-roll nanoimprinting processes on polymer film, please refer to ref 22. Characterization. The morphological characterization of the samples was examined by using a field emission scanning electron microscope (SEM) (JEOL FESEM, JSM 6700F). Prior to SEM imaging, the samples were sputter-coated with 20 nm thick gold (Fine Coater, JEOL JFC-1200) to prevent sample charging up and facilitate imaging. Static water contact angle (CA) measurement were performed using a contact angle goniometer (Rame-Hart 100). A deionized (DI) water droplet (4 μL) was dispensed gently onto the sample surface using an automatic pipet, and a photograph of the water droplet was taken immediately with the goniometer camera. CA values were obtained from the integrated software in the goniometer. For each sample, average CA measurement were obtained by measuring five different locations on the sample. Also, in order to ensure that water CA characterization of the samples was due solely to topographic effect without any chemical influence, dummy imprints on blank PC were performed on the FDTS-treated mold before the imprint on the actual sample. Thus, any physisorbed FDTS on the mold would be
polymer sheet for a duration of imprint time of 300 s at a controlled imprint temperature (180 °C) above the glass transition temperature (Tg) of the polymer (Tg of PC: 150 °C) that allows the softened viscous 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 cooled down below the Tg of the polymer (80 °C). Upon demolding, these nanostructures are formed permanently as part of the polymer film. For topographies of nanogratings and isolated nanopillars, thermal nanoimprint process similar to the above is fabricated onto PC film, except that different types of molds were used. The nanogratings molds and the isolated nanoholes molds (inverse topography of the imprinted structure) were silicon molds (∼25 mm × 25 mm) fabricated by Institute of Microelectronics. The closed-packed nanopillar topography on PC was fabricated employing the same thermal nanoimprint process as above and by using a commercially available alumina membrane (Anodisc, Whatman). For patterning the nanoprotrusion topography onto poly(methyl methacrylate) (PMMA) film (Micro resist technology, GmbH), the imprint conditions were the same as that on PC, except that the imprint temperature was at 150 °C (Tg of PMMA = 105 °C). For patterning of the structures onto polydimethylsiloxane (PDMS) film, we used conventional PDMS casting method using a ratio of 10:1 ratio of elastomer and curing agent (Sylgard 184, Dow Corning). The roll327
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transferred to the dummy imprints and would not interfere with the CA and water pinning result. Droplet Characterization for the “Coffee-Ring Effect”. The characterization of the different surfaces on the “coffee-ring effect” was achieved using Fluorescein molecule sodium salt as a dye marker (F6377 Sigma-Aldrich). A solution of 0.05 mg/mL of fluorescein molecule sodium salt and deionized water was mixed as a stock solution at a volume fraction of 5 × 10−4. Droplets (0.5 μL: chosen to fit a droplet within the field of view of the microscope) were deposited onto the surfaces of the nanoimprinted PC, nonpatterned PC film, and a nonpatterned PC film with silanization. The films were allowed to dry in a large enclosure with controlled ambient temperature and humidity. Microscopy images of the dried droplet was captured using a Leica microscope (DM 5000B).
The three variations of the nanoprotrusion structures exhibit similar level of water pinning force ranging from 680 to 690 μN (Figure 2d); these values outperform many state-of-the-art synthetic structures (15−230 μN),12,15,16,24 the water pinning force reported on the natural rose petal (63.8 μN),11 and independent measurement of the natural red rose petal with microprotrusion topography shown in Figure 1a (99 ± 2 μN). In the context of our first postulation that nanoprotrusions can impart a greater pinning force than microprotrusions, we compare the solid−liquid contact line length between a nanoprotrusion versus a microprotrusion of the same shape. Figure 3 shows a schematic illustration of the solid−liquid
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RESULTS AND DISCUSSION The first group of nanostructures represents the isotropic and continuous surface topography. Three types of isotropic and continuous topographies are hexagonal array of nanoprotrusions that mimic the rose petal’s microprotrusionwith a variation 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 pitch and height of 300 nm (Figure 1e: sample A2); (iii) a “parabolic” profile with a pitch and height of 250 nm (Figure 1f: sample A3). A “conical” profile has a sharper tip compared to the “parabolic” profile. Figure 1g shows a larger nanoimprinted film of sample A1, obtained through roll-to-roll nanoimprinting (110 mm × 65 mm patterned area). For interest in roll-to-roll nanoimprinting process, please refer to ref 20. 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 the nanoprotrusion patterned PC film (sample A2) at 90° and 180° of inclination (see Supporting Information Movie M1). In contrast, the same volume of water droplet easily rolled off a pristine (nonpatterned) PC film when the film was slightly tilted (see Supporting Information Movie M2). To quantify the water pinning behavior, one can either measure the contact angle hysteresis or determine the pinning force. Contact angle hysteresis measurements provide results that are often relative as they are dependent on the different methods and the droplet volumes used, which varies across different works.7 Instead, we determine the pinning force experienced by the liquid on the surfacefirst introduced by Fumidge23 in 1962 and subsequently used by other research groups.16,24,25 Water pinning force (F) on the surface of the fabricated sample is determined from the equation F = mg sin α
Figure 3. Schematic illustration of the continuity of the solid−liquid contact line (dotted line) on the topview of four different types of geometries: (a) isotropic, continuous microprotrusion; (b) isotropic, continuous nanoprotrusion; (c) isotropic, discontinuous nanopillar; (d) anisotropic, continuous nanograting.
contact line of a droplet sitting on the surface with different types of geometries. The micro- and nanoprotrusion dimensions per unit area are compared using the contact line density (CLD) calculation:9 CLD =
total length of solid − liquid contact length (m) total apparent area (m 2) (3)
Here, we assume that a water droplet sits on a three-phase contact line (solid−liquid−air) and does not penetrate fully into these textured surfaces. Lai et al.12 reported air pockets exist 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 textured surfaces with contact angles as low as 90°. Over a total apparent area of 1 mm2, the nanoprotrusion (i.e., using sample A1) with a diameter of 300 nm has an estimated contact line density of 1.05 × 107/m. In comparison, a microprotrusion with a diameter of 20 μm (from the microprotrusion of a rose petal seen in Figure 1a) has an estimated contact line density of 1.57 × 10 5 /m (see Supporting Information SI-2 for CLD calculation). A higher CLD leads to a higher water pinning force. Hence, the synthetic nanoprotrusion designs exhibit higher 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 water pinning force, we designed and fabricated two groups of nanostructures with opposing topography characteristics, namely (i) isotropic but discontinuous and (ii) anisotropic and continuous. Two types of isotropic and discontinuous nanostructures are the random nanopillars (200 nm diameter, 400 nm pitch, 400 nm height) and the ordered nanopillars (250 nm diameter, 400 nm pitch and 250 nm height). As shown in
(2)
where α is the tilting angle at the onset of water rolling off the surface, m is the mass of the water droplet, and g is the acceleration of gravity (alternatively, mg = weight of the water droplet in grams), which is schematically illustrated in the inset of Figure 2d. To quantify the water pinning force, one can either measure the tilting angle with a fixed volume (or weight) of water droplet or vary the weight of the water droplet at a fixed tilting angle. We used the latter method to determine the water pinning force. In our experiment, we fixed the angle of tilt at 90° and increased the volume of the water droplet (in steps of 1 μL) until the weight of the water droplet reached a critical value (i.e., critical weight) to make the water droplet roll off. 328
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Figure 2d, both the random and orderly separated nanopillars exhibit reduced water pinning force compared to the isotropic and continuous nanoprotrusions. The anisotropic and continuous structure is a nanograting structure of 250 nm width, 500 nm pitch, and 250 nm height. For this structure, the water pinning forces were measured for the directions parallel and orthogonal to the grating direction. For both measurements on the nanograting, the water pinning forces are the lowest among all the nanostructures (Figure 2d). The marginally higher pinning force on the nanograting when the tilting angle is perpendicular to the longitudinal direction of the grating is due to their physical geometry barrier, a phenomenon well reported.26 Supporting Information SI-1 reports the measured quantitative critical weight of the water droplet on these topographies and their corresponding water pinning forces. Comparing the estimated contact line densities across the three groups of nanostructuresthe isotropic and continuous nanoprotrusion (sample A1, Figure 3b), the isotropic and discontinuous 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 of the contact line density (CLD) descends in the order of isotropic 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 three groups of nanostructures decreases in the same order: isotropic and continuous nanoprotrusion (F: 682 ± 16 μN) > isotropic and isolated nanopillars (F: 484 ± 9 μN) > anisotropic and continuous nanogratings (F: 104 ± 11 μN). The bioinspired isotropic and continuous nanoprotrusion presents a continuous solid−liquid contact line (Figure 3b) that maximizes the overall contact line length to enable a water droplet to pin strongly onto its surface. In contrast, discontinuous and anisotropic surface structures such as the nanopillars (Figure 3c), and the nanogratings (Figure 3d) present a discontinuous solid−liquid contact line, which reduces its overall contact line length and hence does not favor water pinning. Water droplet on a surface with discontinuous solid−liquid contact line is unstable and will have low adhesion to the surface. All three isotropic and continuous nanoprotrusions (samples A1, A2, and A3) show an increased static water contact angle compared to the nonpatterned PC film. The static contact angles for A1, A2, and A3 are 109 ± 4°, 108 ± 1°, and 114 ± 3°, respectively, while the static water contact angle for nonpatterned PC film is 88 ± 1°. Unlike the natural rose petal, the nanoimprinted PC films are not considered superhydrophobic. Superhydrophobicity in rose petal is a result of the hierarchical layer structure.4 We further performed experiments on two other polymer films with different pristine static water contact angle: poly(methyl methacrylate), PMMA, and polydimethylsiloxane, PDMS. PMMA and PDMS films were imprinted with the same nanoprotrusion topography as sample A2. Comparing PC, PMMA, and PDMS with the same nanoprotrusions, all three films exhibit similar high water pinning forces despite the large differences in their pristine hydrophobicity (see Table 1). In solution-based deposition, nonuniform film associated with nonuniform evaporation of the solvent often result as a higher concentration of solute deposited on the outer front of an evaporating dropleta phenomenon often referred as the
Table 1. Summary of Measured Water Contact Angles and Water Pinning Forces for Nonpatterned and Nanoimprinted PC, PMMA, and PDMS with the Same Nanoprotrusion Topography as Sample A2 sample nanoprotrusion topography patterned on PMMA nanoprotrusion topography patterned on PC nanoprotrusion topography patterned on PDMS
measured pristine material static water contact angle (deg)
measured pinning force (μN)
70 ± 1
684 ± 15
88 ± 1
672 ± 13
115 ± 2
660 ± 16
“coffee-ring effect”.18,27 A surface with a continuous solid− liquid contact line may be favorable for controlling a uniform deposition from a solution. We examined the evaporation profile of a water droplet containing fluorescein molecule on pristine (nonpatterned) PC film and PC film with the rose petal mimetic nanoprotrusion surface (sample A2). Figures 4a
Figure 4. Microscopic images of a water droplet containing fluorescein molecules (0.5 μL) that has been fully dried on different types of topography on PC film. (a) Ring-like stain observed on the nonpatterned PC film which has water CA ∼ 88°. (b) Uniform deposit on the nanoprotrusion-patterned PC film (sample A2) which has water CA ∼ 108°. (c) Ring-like stain observed on the nonpatterned silanized PC film with a water CA ∼ 106°, act as control sample with similar hydrophobicity as the nanoprotrusion sample in (b). The difference in background color of the figures is due to the difference in topography of the film. 0.5 μL is chosen to fit the complete droplet within the field of view of the microscope setup. Scale bar = 0.25 mm.
and 4b show microscopic images of dried droplet stain on pristine PC and the nanostructured PC film, respectively. The ring effect is clearly seen on the nonpatterned PC film. On the contrary, the droplet on the nanoprotrusion surface is a uniformly colored circular spot. The ring-like “coffee-ring effect” on a drying solute-laden droplet has been known to be influenced by the advective flow in the droplet due to the difference in evaporation rate at the droplet center versus the edge.18,27 Liquid evaporation rate controlled by the evaporation flux27,28 depends on the cross-sectional shape (or the contact angle) of the droplet. In a control experiment, we used a nonpatterned PC film that has been silanized to achieve a similar water contact angle (measured CA ∼ 106 ± 1°) as the nanoprotrusion patterned PC film (sample A2, measured CA ∼ 108 ± 1°) and performed the same conditions of droplet evaporation experiment. When the droplet was completely dried as shown by the microscopic image in Figure 4c, a similar ring-like stain was still observed on the silanized PC film. Hence, the mitigation of the “coffee-ring effect” in the 329
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pristine nonpatterned PC film. This material is available free of charge via the Internet at http://pubs.acs.org.
nanostructured PC film is primarily due to the presence of the nanostructures. The ring stain occurring on the nonpatterned and silanized nonpatterned PC film is expected. Unlike a solute-free liquid droplet on a nonpatterned surface, where the droplet diameter decreases during evaporation because there is no pinning,7 a solute-laden liquid becomes self-pinned with the solute particles acting as weak pinning sites at the circumference of the droplet during the initial stage of the evaporation process.29 Then pinning of the contact line at the circumference of the droplet and the inherent spatially nonuniform evaporation rate of the liquid droplet cause a net advective flow outward from the droplet center, transferring the solute to the edge of the droplet while the process repeats itself until the droplet is completely dried.19,27 On the other hand, the use of an isotropic, and continuous ordered nanoprotrusion topography to reduce the “coffee-ring effect” is interesting. Substrate roughness has been known as a factor contributing to the “coffee-ring effect”.18,29 A recent report has demonstrated the use of silver-coated disordered nanocones on silicon (Si) substrate for the suppression of this effect.30 The silver-coated Si nanocones have similar geometry as our nanoprotrusion. Interestingly, the silver-coated Si nanocone design was reported to have water contact angle of 105°, which is close to the water contact angle of our PC nanoprotrusions. Such nanostructure conferring the surface with good water pinning ability presents a continuous pinning site and furthermore acts as a geometry barrier that pins the solutes as they flow outward from the droplet center during the advective flow process.
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Corresponding Authors
*E-mail [email protected] (J.B.K.L.). *E-mail [email protected] (H.Y.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from SERC (A*STAR) Public, Private Partnership funding to conduct this research. The authors acknowledge Kevin Khaw, Lee Yeong Yuh, and Karen Chong of IMRE for the help and usage of the video microscopy facilities.
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REFERENCES
(1) Koch, K.; Barthlott, W. Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials. Philos. Trans. R. Soc., A 2009, 367, 1487−1509. (2) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114−4119. (3) Koch, K.; Bhushan, B.; Jung, Y. C.; Barthlott, W. Fabrication of artificial lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion. Soft Matter 2009, 5, 1386− 1393. (4) Xi, J.; Jiang, L. Biomimic superhydrophobic surface with high adhesive forces. Ind. Eng. Chem. Res. 2008, 47, 6354−6357. (5) Dawood, M. K.; Zheng, H.; Liew, T. H.; Leong, K. C.; Foo, Y. L.; Rajagopalan, R.; Khan, S. A.; Choi, W. K. Mimicking both petal and lotus effects on a single silicon substrate by tuning the wettability of nanostructured surfaces. Langmuir 2011, 27, 4126−4133. (6) Bhushan, B.; Jung, Y. C.; Koch, K. Self-cleaning efficiency of artificial superhydrophobic surfaces. Langmuir 2009, 25, 3240−3248. (7) Teisala, H.; Tuominen, M.; Aromaa, M.; Stepien, M.; Makela, J. M.; Saarinen, J. J.; Toivakka, M.; Kuusipalo, J. Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces. Langmuir 2012, 28, 3138−3145. (8) Paxson, A. T.; Varanasi, K. K. Self-similarity of contact line depinning from textured surfaces. Nat. Commun. 2013, 4, 1492. (9) Extrand, C. W. Designing for optimum liquid repellency. Langmuir 2006, 22, 1711−1714. (10) Walker, S. W.; Shapiro, B.; Nochetto, R. H. Electrowetting with contact line pinning: Computational modeling and comparisons with experiments. Phys. Fluids 2009, 21. (11) Oner, D.; McCarthy, T. J. Ultrahydrophobic surfaces. Effects of topography length scales on wettability. Langmuir 2000, 16, 7777− 7782. (12) Lai, Y.; Gao, X.; Zhuang, H.; Huang, J.; Lin, C.; Jiang, L. Designing superhydrophobic porous nanostructures with tunable water adhesion. Adv. Mater. 2009, 21, 3799−3803. (13) Tian, X.; Chen, Y.; Zheng, Y.; Bai, H.; Jiang, L. Controlling water capture of bioinspired fibers with hump structures. Adv. Mater. 2011, 23, 5486. (14) Katsoulas, N.; Bartzanas, T.; Kitta, E. Effects of anti-drip polyethylene covering films on microclimate and crop production. In International Symposium on Advanced Technologies and Management Towards Sustainable Greenhouse Ecosystems: Greensys2011; Kittas, C., Katsoulas, N., Bartzanas, T., Eds.; 2012; Vol. 952, pp 209−215. (15) Wu, D.; Wu, S.-Z.; Chen, Q.-D.; Zhang, Y.-L.; Yao, J.; Yao, X.; Niu, L.-G.; Wang, J.-N.; Jiang, L.; Sun, H.-B. Curvature-driven reversible in situ switching between pinned and roll-down superhydrophobic states for water droplet transportation. Adv. Mater. 2011, 23, 545.
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CONCLUSIONS In conclusion, polymer films with exceptionally high water pinning forces have been achieved through nanoimprinted surface structures, without the incorporation of any chemical treatment. Through a series of systematic surface structure variation, a design cue for tuning water pinning force on polymeric films has been demonstrated. Nanoprotrusions with conical or parabolic-shaped structure, arranged isotropically and continuously across the surface provide the highest water pinning forces. The nanostructures are designed to significantly increase the solid−liquid contact line length of a water droplet on its surface, thereby achieving a high water-pinning force. The exceptionally high water pinning film is further shown to effectively mitigate nonuniform coating commonly referred to as the “coffee-ring effect”. The study has taken the advantage of the highly versatile and scalable nanoimprint technology: specifically, a variety of surface topographies can be fabricated on a variety of polymeric materials. The roll-to-roll nanoimprinted synthetic film shown in this study further opens up the likelihood for industrial adoption.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
* Supporting Information S
SI-1: a table of the quantitative critical weight of the water droplet and the corresponding water pinning force measured on different topographical features fabricated on PC film; SI-2: contact line density (CLD) calculation of the four different geometries shown in Figure 3; movie M1: video showing a water droplet (40 μL) staying pinned onto a PC film surface with patterned synthetic nanoprotrusion topography, even when the surface is tilted upside down; movie M2: video showing a water droplet (40 μL) easily rolls off the surface of a 330
dx.doi.org/10.1021/la4034996 | Langmuir 2014, 30, 325−331
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Article
(16) McLauchlin, M. L.; Yang, D.; Aella, P.; Garcia, A. A.; Picraux, S. T.; Hayes, M. A. Evaporative properties and pinning strength of laserablated, hydrophilic sites on lotus-leaf-like, nanostructured surfaces. Langmuir 2007, 23, 4871−4877. (17) Berejnov, V.; Thorne, R. E. Effect of transient pinning on stability of drops sitting on an inclined plane. Phys. Rev. E 2007, 75, 066308. (18) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 2011, 476, 308−311. (19) Soltman, D.; Subramanian, V. Inkjet-printed line morphologies and temperature control of the coffee ring effect. Langmuir 2008, 24, 2224−2231. (20) Dumond, J. J.; Low, H. Y. Recent developments and design challenges in continuous roller micro- and nanoimprinting. J. Vac. Sci. Technol. B 2012, 30. (21) Schift, H. Nanoimprint lithography: An old story in modern times? A review. J. Vac. Sci. Technol. B 2008, 26, 458−480. (22) Dumond, J. J.; Mahabadi, K. A.; Yee, Y. S.; Tan, C.; Fuh, J. Y. H.; Lee, H. P.; Low, H. Y. High resolution UV roll-to-roll nanoimprinting of resin moulds and subsequent replication via thermal nanoimprint lithography. Nanotechnology 2012, 23. (23) Furmidge, C. G. Studies at phase interfaces. 1. Sliding of liquid drops on solid surfaces and a theory for spray retention. J. Colloid Sci. 1962, 17, 309. (24) Latthe, S. S.; Dhere, S. L.; Kappenstein, C.; Imai, H.; Ganesan, V.; Rao, A. V.; Wagh, P. B.; Gupta, S. C. Sliding behavior of water drops on sol-gel derived hydrophobic silica films. Appl. Surf. Sci. 2010, 256, 3259−3264. (25) Wong, T.-S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443− 447. (26) Li, W.; Fang, G.; Lij, Y.; Qiao, G. Anisotropic wetting behavior arising from superhydrophobic surfaces: Parallel grooved structure. J. Phys. Chem. B 2008, 112, 7234−7243. (27) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827−829. (28) Peiss, C. N. Evaporation of small water drops maintained at constant volume. J. Appl. Phys. 1989, 65, 5235−5237. (29) Deegan, R. D. Pattern formation in drying drops. Phys. Rev. E 2000, 61, 475−485. (30) Coppe, J.-P.; Xu, Z.; Chen, Y.; Liu, G. L. Metallic nanocone array photonic substrate for high-uniformity surface deposition and optical detection of small molecules. Nanotechnology 2011, 22.
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