ARTICLE pubs.acs.org/IECR
Biomimicked Superhydrophobic Polymeric and Carbon Surfaces Chandra S. Sharma,†,‡ Kumar Abhishek,† Hari Katepalli,† and Ashutosh Sharma*,† † ‡
Department of Chemical Engineering and DST Unit on Nanosciences, Indian Institute of Technology, Kanpur-208016, U.P., India Department of Chemical Engineering, Indian Institute of Technology, Hyderabad, Yeddumailaram-502205, A.P., India ABSTRACT: We report two direct and easy ways of fabricating stable, superhydrophobic polymeric and carbon surfaces directly by biomimicking the patterns found on natural plant leaves by micromolding and nanoimprint lithography. Two distinct classes of naturally occurring microtextures on superhydrophobic leaves were mimicked in this study, which include leaves of Elephant creeper (Argyreia Nervosa) and Nasturtium (Tropaeolum Majus). These show structural superhydrophobicity derived from high aspect ratio hairs and lower aspect ratio microtextures, respectively. Both the textures could be replicated by micromolding in different polymers, polydimethylsiloxane, polystyrene, and an organic resorcinol-formaldehyde (RF) gel. Patterned RF gel surfaces yielded superhydrophobic carbon surfaces upon pyrolysis because RF gel is a polymer precursor to glassy carbon. The nanoimprint lithography could be used for a direct transfer of the lower aspect ratio leaf patterns on the surfaces of various other polymers like poly(ethylene terephthalate) and poly(methyl methacrylate).
’ INTRODUCTION Nonwetting surfaces which have very high water contact angle (WCA), particularly larger than 150°, are termed as superhydrophobic surfaces. Superhydrophobic surfaces have received intense attention because of the wide range of applications in optics1,2,6,8,11 (transparent, nonreflective, and highly reflective surfaces), low friction surfaces,1,4,5,7,8,10,11 and antiadhesion1,3,6,8 12 (architectural glasses, windshields of cars, nonwettable textiles) and antibiofoulant surfaces.6,8 However, there are only few reports13 23 on fabrication of carbon superhydrophobic surfaces although carbon superhydrophobic surfaces have potential applications in carbon based microfluidics13,18 and bio-MEMS.22 Additionally, since carbon is electrically conductive and possesses high surface area,15,24,25 the carbon superhydrophobic surfaces may be useful as electrode material in high-performance fuel cells, as capacitors, and in sensing devices.15,16,23,24 Several methods of preparing superhydrophobic surfaces have been discussed in the literature such as etching,2,5,6,8,26 lithography,5,6,8,9,19,26 self-organization,8 layer-by-layer and colloidal assembly,6,8,15,26 electrochemical deposition,6,8,26 and electrospinning.6,8,23,25,26 The most common way to fabricate carbon superhydrophobic surfaces is either growing, aligning, or arranging carbon nanotubes/nanofibers13,15 22 or by annealing the carbon films.14,23 Recently, we have fabricated nearly superhydrophobic carbon surfaces by electrospinning a negative photoresist which is a precursor of carbon.24,25 In this study, we aim to fabricate the superhydrophobic polymeric and carbon surfaces by taking inspiration from nature. The idea of mimicking the textures found in nature to fabricate the superhydrophobic surface is a recent development.27 35 In the past few years, attempts have been made to directly replicate the natural leaf templates,28,29,31,34,36,37 butterfly wings,30 and water strider legs32,33 to create large area super hydrophobic polymeric surfaces. In a recent study,38 a wide variety of water-repellent plant species showing superhydrophobicity because of micro- and nanoscale surface textures was reported. Another recent study39 to understand the mechanism of water r 2011 American Chemical Society
rolling from superhydrophobic leaves suggested that there are two distinctly different types of water-repellent plant leaves. The first type of leaves, such as Lotus (Nelumbo Nucifera L.) and Nasturtium (Tropaeolum Majus) look macroscopically smooth. They exhibit super hydrophobic by the presence of hierarchical micrometer- and nanometer-sized structural bumps spread uniformly on the leaf surface.38,39 The second type consists of hairy leaves39 such as medicinal creepers like Argyreia Nervosa and Lady’s Mantle. In order to consider both the mechanisms, we have explored biomimicking of both the classes of micropatterned and hairy surface leaves in this study. Hairy surfaces have not been reported to be widely mimicked except for a few recent reports.37,40 In this study, the surface textures on these natural plant leaves, Elephant creeper (Argyreia Nervosa) and Nasturtium (Tropaeolum Majus) are first biomimicked by replica molding using different polymers, polydimethylsiloxane (PDMS), polystyrene (PS), and resorcinol-formaldehyde (RF) gel. RF gel introduced by Pekala41 is a especially attractive polymeric precursor for glassy carbon. We show that replication of the microscale features of natural leaves on polymer surfaces generates superhydrophobicity in these polymeric surfaces. Biomimicked RF gel surfaces achieved a large contact angle (>150 °C) which resulted in superhydrophobic carbon surfaces upon pyrolysis. Both the aqueous and organic RF gels are investigated for optimal reproduction of surface texture by micromolding. Additionally, nanoimprint lithography42 (NIL) was also tested to directly replicate the leaf patterns on a variety of polymeric surfaces like poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA), and RF gel. We show that by simply Special Issue: Ananth Issue Received: February 23, 2011 Accepted: July 12, 2011 Revised: July 2, 2011 Published: July 12, 2011 13012
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Figure 1. Schematic representation of the preparation of a negative PDMS replica from the original leaf.
biomimicking different polymer surfaces including RF gel, one can fabricate a large number of polymeric and carbon superhydrophobic surfaces over large areas (∼square centimeters) in a direct and simple way.
’ EXPERIMENTAL SECTION Materials. Two different classes of plant leaves with distinct surface textures, Argyreia Nervosa and Tropaeolum Majus, were used in this study for biomimicking their surfaces to fabricate superhydrophobic polymer and carbon surfaces. Sylgard 184 (a two part polydimethylsiloxane (PDMS) elastomer) consisting of an oligomer and a cross-linking agent was obtained from Dow Chemicals. Resorcinol (99% purity), formaldehyde (37% w/v; stabilized by 11 14 wt % methanol), hydrochloric acid (36%), potassium carbonate (99.0% purity), acetone (99.5%), and HPLC grade chloroform were obtained from Qualigens Fine Chemicals, India. While acetone was used as a diluent in the preparation of organic RF sol, ultra pure milli-Q water was used for the same purpose in making aqueous RF sol. In addition to these, poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) sheets were supplied by Obducat AB, Sweden, and used directly without any further processing for nanoimprint lithography (Obducat AB, Sweden). Preparation of Negative PDMS Replica of Leaf. Figure 1 shows the schematic representation of the experimental procedure followed to prepare a negative replica of the leaf surface in flexible cross-linked PDMS. First the leaf was cut into square pieces of 1.5 cm 1.5 cm, length by breadth (Figure 1a), which was attached to a silanized hydrophobic glass plate (Figure 1b) using double sided tape. Prepolymer solution of Sylgard 184 elastomer mixed with the cross-linking agent (20:1 by weight) was then poured over this leaf mold (Figure 1b) and deaerated thoroughly to remove any dissolved and trapped gas bubble remaining adhered to the textured, hairy surface of the leaf. The prepolymer liquid was then cured at 120 °C for 6 h with a silanized glass plate supported by spacers of thickness 2 mm, placed on top of it (Figure 1c). In order to ensure that the hairs remained almost vertical, the curing was done with the hairy surface
of the leaf facing downward as shown in Figure 1c. The crosslinked sample with the leaf embedded inside it (Figure 1d) was immersed in chloroform for about 2 h to swell the cross-linked PDMS block. At a relatively large extent of swelling in chloroform, the leaf gets completely detached from the swollen sample of the PDMS leaving behind a negative replica in the form of holes on the PDMS surface as shown in Figure 1e. The PDMS negative replica was then slow dried in a closed container at room temperature. Preparation of Superhydrophobic Polymer Surfaces. The negative replica of the leaf surface generated on PDMS as described above was further used to replicate the leaf patterns on the RF gel and the polystyrene (PS) surface. To make the positive replica of the leaf in RF gel, RF sol was prepared in two different media: aqueous and organic. First, resorcinol (R) and formaldehyde (F) were mixed and continuously stirred for about 15 min to get the clear solution. For organic RF sol, hydrochloric acid used as an acidic catalyst was added to acetone separately. In the case of aqueous RF sol, potassium carbonate used as a basic catalyst was mixed with water used as a diluent and then stirred continuously for 30 min. The two solutions were then mixed and stirred continuously for 15 min. The resorcinol to formaldehyde (R/F) and resorcinol to diluent (R/D) molar ratios were kept constant to be 0.50 and 0.037, respectively, in both the cases. It is to be noted here that water present in the formaldehyde solution is not taken into consideration while calculating the dilution ratio in the case of aqueous RF sol. The resorcinol to catalyst molar ratio (R/C) was 10 and 25 for organic and aqueous RF sol, respectively. Before pouring RF sol on the PDMS negative replica, PDMS template was swelled in acetone (Figure 2a). The dry negative PDMS template could not be used directly because acetone, which is also the primary solvent of RF sol, swelled the dry PDMS negative template as well. However, as swelling of the crosslinked PDMS block by acetone is a slow process, in situ swelling by acetone present in RF sol alone was not sufficient. Therefore, the PDMS negative template was first soaked completely in acetone by keeping it immersed for 30 min, after which the RF solution was poured on the swelled PDMS pattern kept in a container (Figure 2b). This container was deaerated completely and kept at room temperature until the completion of 13013
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Figure 2. Schematic representation of the preparation of superhydrophobic polymer and carbon surfaces from the negative replica made in PDMS.
the gelation process of the RF sol. After the sol converted into a solid state gel, the PDMS stamp was gently peeled away followed by drying at 50 °C for 24 h. This step was important to avoid cracking of the gel, which is undesirable for isotropic shrinkage.43,44 With the aid of this gentle drying protocol, the RF gel shrank isotropically43,44 to yield a patterned superhydrophobic RF gel surface as shown in Figure 2c. In the case of polystyrene, the PS solution was prepared in toluene in the ratio 1:3 (w/w). This solution was poured on the swelled PDMS template similar to that of RF sol. In this case, the PDMS negative template was swelled in toluene (Figure 2e), which is also the solvent for PS. The swelled PDMS negative template poured with PS solution (Figure 2f) was then deaerated and kept for initial drying in a closed container at room temperature. Later, the samples were put in an oven at 60 °C for 24 h for ensuring a complete removal of the solvent. The dried PS film (thickness ∼50 μm) captured the pattern on the surface of the PDMS (Figure 2g). Finally the dried PS film was gently peeled off from the complementary negative PDMS template as shown in Figure 2h. Transferring the Leaf Patterns Using NIL. The original plant leaves were used directly as NIL molds. Leaf patterns were imprinted on two different polymeric materials PET and PMMA at the temperature (105 °C) which is slightly above their glass transition temperature. The applied pressure in the process was varied from 10 to 30 bar. Further, these PET and PMMA surfaces with the negative replica of the leaf surface were used as templates to prepare positive replica in RF gel by replica molding as discussed in the previous section. Preparation of Superhydrophobic Carbon Surfaces. After biomimicking the leaf patterns onto the RF gel surfaces, these surfaces were subjected to pyrolysis at 900 °C under inert atmosphere to prepare textured carbon surfaces (Figure 2d). The heating rate was fixed to be 5 °C/min while maintaining the constant nitrogen flow rate (0.2 L/min) throughout the process. After pyrolysis, samples were cooled to room temperature under the nitrogen atmosphere. Characterization. Field emission scanning electron microscopy (SUPRA 40VP, Gemini, Zeiss) was used to characterize
the surface morphologies. All samples were sputter coated with a thin layer of Au Pd to reduce the surface charging of the samples during SEM analysis. An optical profilometer (Wyko NT 9100, Veeco instruments Inc.) was used to map the 3-D topography of the surfaces. Wettability of the surfaces was characterized by measuring the static contact angle and the contact angle hysteresis using contact angle goniometer (Rame-Hart). In all measurements, 5 μL (∼3 mm spherical drop diameter) water droplets were used. A confocal micro-Raman microscope (CRM 200, WiTec, Germany with λ = 543 nm) was used to record the Raman spectra in order to confirm the yield of glassy carbon upon pyrolysis. The functional groups on the carbon replica surface were characterized by Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) (Bruker Optik, Gmbh, Germany).
’ RESULTS AND DISCUSSION Figure 3 summarizes the results on the fabrication of superhydrophobic polymeric and carbon surfaces using the dense hairy leaf of Argyreia Nervosa. The water droplet rolls off easily from the back side of the leaf surface because of the presence of dense microhairs. These hairs remain lying down horizontally on the leaf surface with a unidirectional growth toward the hanging bottom portion of the leaf. As shown in Figure 3a, the average diameter of the hair of the natural leaf was 15.8 ( 4.7 μm with a very high aspect ratio (>30). The aspect ratio of the hair was obtained by measuring the average length and diameter of 20 hairs in six different fields of view through SEM images. As mentioned in the Experimental Section, a special protocol was required while curing and peeling of the PDMS template from the original leaf in order to fabricate a faithful negative replica of the high aspect ratio hairy surface. For example, experiments were also done by placing the hairy leaf facing upward while curing, instead of facing downward. It was observed that in the former case, the holes formed on the PDMS negative replica were a little flattened and were often blocked midway so as not to allow a full replication of the hairy surface in the polymer. This may 13014
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Figure 3. SEM images of (a) original Argyreia Nervosa leaf; (b) negative PDMS replica showing holes; (c d) PS and organic RF gel derived positive carbon replica showing dense hairy surfaces.
have been because of the weight of the polymer leading to hair distortion or bunching. Swelling of the PDMS template in chloroform helped in a regulated self-removal of the leaf mold, thus giving a negative PDMS template with deep slender holes (Figure 3b). This negative PDMS template was further used as a mold for replicating the hairy surface in other polymers like PS. SEM images of the dense hairy PS surface (Figure 3c) confirms the effectiveness of this approach for biomimicking of the hairy leaf patterns. The PS dense hairs were observed to be randomly oriented and standing at a small angle of inclination. Similarly, RF sol was used to mimick the dense hairy leaf pattern of Argyreia Nervosa successfully from the negative PDMS template. As RF sol is the polymer precursor of carbon, later this dense hairy RF surface was pyrolyzed to yield a hairy carbon surface as shown in Figure 3d. As RF gel shrunk during drying and pyrolysis,43,44 the average diameter of hairs in the carbon surfaces was reduced to 7.6 ( 3.1 μm with an aspect ratio nearly 20. The length of hairs in the positive replica either in the PS or in the RF gel decreased also because of partial breaking of hairs while peeling out the polymer hairy surface from the PDMS negative template. However, it can easily be observed from Figure 3 that the process of replicating the dense hairy leaf to its corresponding PDMS negative template and subsequently into PS and organic RF gel positive template is fairly efficient and reproducible. To establish the robustness of this approach, we further mimicked another kind of leaf, Tropaeolum Majus, which shows the superhydrophobic nature because of the presence of a particular type of regular array of microtextures and also due to presence of hydrophobic wax. Figure 4a shows the original leaf pattern while Figure 4b shows the negative replica of this leaf structure in PDMS. PS positive replica as shown in Figure 4c shows the replication of surface microbumps as found in the original leaf. Similarly, RF gel surface could also be patterned with similar surface features followed by pyrolysis to generate artificial
patterned leaf structures in the carbon surface. Reduction in the feature-size is because of the gel shrinkage upon drying (Figure 4d). A high magnification inset image in Figure 4d clearly shows the good quality of transferred patterns on the organic RF gel carbon surface which is similar to that of the actual leaf patterns. As discussed above, the RF sol used to transfer the hairy and microtextured leaf patterns successfully is an organic solvent based gel. We further tested an aqueous RF sol also for micromolding, which produced the results shown in Figure 5. The density and fidelity of hairs transferred in the aqueous RF sol based carbon surface was rather poor (Figure 5a) as compared to the surfaces obtained by the organic sol. Similarly in case of Tropaeolum Majus leaf, fine micrometer and submicrometer sized features found on the leaf surface were not completely replicated in aqueous RF sol based carbon surfaces (Figure 5b). As the surface tension of organic solvent (acetone in our case) is much less than water, wettability of organic RF gel is better than aqueous RF gel. This is supported by the contact angle measurement of these two different surfaces. The aqueous RF gel derived carbon surface is more hydrophilic (WCA = 60°) as compared to the organic RF sol derived carbon surface (WCA = 83°). Therefore, fine features with submicrometer size and high aspect ratio are transferred much better in the organic RF gel. Micro-Raman spectroscopy was used to confirm the yield and nature of the carbon surfaces produced upon pyrolysis. As shown in Figure 6, there were two broad peaks centered around 1340 and 1590 cm 1 for organic RF gel derived carbon. These two peaks correspond to well-known first order bands known as D (defect band) and G (graphite band), respectively.24,45 It indicates that a fraction of disordered amorphous sp2 C C bonding is present in the resulting carbon.24,45 In the case of aqueous RF gel derived carbon, there is a slight shift to the first order D band which was observed at 1350 cm 1. However, the G band was observed at 1590 cm 1 itself. The ratio of intensity of the two first-order D and G bands as denoted by ID/IG decreased 13015
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Figure 4. SEM images of (a) original Tropaeolum Majus leaf; (b) negative PDMS replica of part a; (c,d) PS and organic RF gel derived positive carbon replica of part a. A high-magnification image of the RF gel derived carbon patterned surface (part d) is shown as the inset image.
Figure 5. SEM images of positive replica of (a) hairy Argyreia Nervosa leaf and (b) Tropaeolum Majus leaf in aqueous RF sol derived carbon.
Figure 6. Raman spectrum of carbon surfaces derived from aqueous and organic RF gel.
from 1.04 for aqueous RF gel derived carbon to 0.89 for organic RF gel based carbon. This indicates45 that organic RF gel derived
carbon is more crystalline in nature than the aqueous RF gel derived carbon. As observed above, the replica molding with polymer solutions and organic RF sol works well in faithfully replicating the high-aspect ratio features of leaves on the polymeric surfaces. Subsequent pyrolysis of RF gel produces the same features in glassy carbon surfaces. In what follows, we discuss the results obtained on a direct transfer of patterns by the nanoimprint lithography technique (NIL). NIL is a simple, high-throughput, and cost-effective patterning method.42 It also allowed us to diversify the approach of biomimicking the natural leaf patterns in a different set of polymers (PET and PMMA). As pointed out in the Experimental Section, original leaves were directly used as mold. In order to replicate the leaf patterns, we optimized the pressure applied during the process while maintaining the temperature fixed at 105 °C, which is slightly above the glass transition temperatures of both PET and PMMA that were used. In the case of the dense hairy leaf of Argyreia Nervosa, NIL patterning was not successful because the soft hairs were either broken or completely flattened on even an application of mild pressures without leaving any 13016
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Figure 7. SEM images of Tropaeolum Majus leaf patterns using NIL on (a c) PMMA and (e g) PET surfaces. Images shown in parts a and e are at 10 bar; parts b and f are at 20 bar; while parts c and g are at 30 bar, respectively. SEM images in parts d and h are obtained on organic RF gel derived carbon surfaces by replica molding using NIL patterned PMMA and PET stamps at 20 bar as shown in the SEM images in parts b and f. In all the cases shown, the NIL temperature was 105 °C.
Figure 8. 3-D surface topography of the (a) natural Tropaeolum Majus leaf and (b) biomimicked RF gel derived carbon replica obtained by the room temperature NIL patterned PET stamp at 20 bar using the original leaf as a mold.
imprint on the polymer surfaces. However, microtextured, nonhairy leaf imprints could be obtained. Figure 7a c shows the negative replica of the Tropaeolum Majus leaf surface on PMMA at different applied pressure 10, 20, and 30 bar, respectively. Similarly, the PET surface could be patterned as shown in Figure 7e g by varying the pressure to 10, 20, and 30 bar, respectively. From the SEM images, one can observe that the leaf patterns were better replicated in the samples which were patterned at an optimal pressure of around 20 bar (Figure 7b, f). Therefore, these samples (PMMA and PET) were further used to generate the positive replica on organic RF gel surfaces by the replica molding technique as discussed earlier. These results are shown as Figure 7d,h for PMMA and PET, respectively. Optical profiling was employed to further assess the fidelity of pattern transfer by NIL followed by molding and pyrolysis. A scan size of 174 μm 233 μm was used to obtain a sufficiently large area for the surface height map. 3-D surface topography of the RF gel derived carbon surface (Figure 8b) is compared to the original Tropaeolum Majus leaf topography (Figure 8a). It is observed that the surface features found with the profiler correlated with the SEM images as shown in Figure 7, although the surface height of the features is reduced because of the RF gel shrinkage during drying as discussed earlier. Wettability of Biomimicked Polymeric and Carbon Surfaces. Wetting behavior on a variety of the biomimicked
polymeric and carbon surfaces reported in this work was characterized by measuring the sessile-drop equilibrium water contact angle (WCA) and contact angle hysteresis (CAH) by tilting the contact angle goniometer stage. Contact angle hysteresis is defined as the difference between the maximum advancing contact angle and the minimum receding contact angle at the moment when a drop begins to slide. This difference arises because of the surface roughness and heterogeneity.46,47 Contact angle hysteresis determines the propensity of the water droplet to stick to a surface.46,47 Figure 9 shows the images of water droplets on some textured leaves and polymeric and carbon surfaces. Figure 9a c shows the water contact angle on the natural dense hairy surface of the Argyreia Nervosa leaf, artificial hairy PS surface, and biomimicked hairy RF gel derived carbon surface, respectively. WCA on the natural leaf hairy surface (Figure 9a) was measured to be 155° ((1.6°) with a CAH of 6.7°. It was observed that the superhydrophobic nature of the dense hairy leaf surface was retained on the artificial PS and RF gel derived carbon replicas of the hairy surface. As shown in Figure 9b,c, WCA was measured to be 154.7° ((2.1°) and 152.3° ((3.4°) on hairy PS and RF gel derived carbon surfaces, respectively. Although, there was a slight increase in the CAH for these two cases, the CAH was found to be 12.1° and 14.7° for hairy PS and RF gel derived carbon surfaces, respectively. 13017
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Figure 10. Sliding angle on different superhydrophobic surfaces.
Figure 9. Images of a water droplet on (a) a Argyreia Nervosa leaf; (b,c) PS and organic RF gel derived carbon replica of the Argyreia Nervosa leaf, respectively; (d) a Tropaeolum Majus leaf; (e,f) PS and organic RF gel derived carbon replica of the Tropaeolum Majus leaf, respectively; (g) NIL induced patterned PET surface at 20 bar; (h) NIL induced patterned PMMA surface at 20 bar; and (i) RF gel derived carbon replica obtained by room temperature NIL patterned PET stamp at 20 bar using the original leaf as a mold.
As discussed earlier, Tropaeolum Majus leaf also shows superhydrophobicity because of the hierarchical structure formed by the presence of nanostructured wax tubules on the microtextured surface. The water contact angle on the natural leaf (Figure 9d) was measured to be 162.1° ((1.8°) with a CAH of 4.8°. The positive PS replica of this surface also retains the superhydrophobic nature as shown in Figure 9e. The water contact angle on the PS replica sans any further treatment was found to be 154.5° ((1.6°) with a CAH of 18.7°. In the case of the RF gel derived carbon replica of the Tropaeolum Majus leaf surface, the water contact angle was measured to be 142° ((2.7°) with a CAH of 16.1° (Figure 9f). The wax tubules found on the natural leaf surface were dissolved after a prolonged solvent exposure. This may be a reason for a somewhat smaller contact angle and a larger CAH in the polymer and carbon replica of the Tropaeolum Majus leaf compared to the case of the natural leaf. In fact, truly superhydrophobic surfaces without an additional treatment with hydrophobic coatings such as fluorocarbons are rather rare. Figure 9g i summarizes the water contact angles on various polymeric and carbon surfaces which are patterned by NIL using natural leaves as templates. WCA on smooth PET and PMMA film was observed to be 80.1° ((2.7°) and 73° ((1.2°), respectively. Replication of leaf patterns on these polymer surfaces (Figure 7b,f) imparted more hydrophobicity as evidenced by the water contact angles. Figure 9g,h shows that WCA on the patterned replica of the Tropaeolum Majus leaf on PET and PMMA at 20 bar applied pressure increases to 110.9° ((1.4°) and 108.2° ((1.7°), respectively. Further, RF gel derived carbon replica prepared from the negative PET patterned template at 20 bar (Figure 7h) shows the ultrahydrophobic nature. WCA in this case was measured to be 132.7° ((3.9°) as shown in Figure 9i. However, we observe that water contact angle on NIL processed
polymer replica of leaves (∼110.9° and ∼108.2°) was much less as compared to polymer replica obtained by micromolding (∼154.5°). This significant difference in water contact angle is mainly attributed to the absence of hierarchical structure formed by wax tubules in the original leaf as these wax tubules did not withstand a high temperature and pressue NIL process. In order to completely understand the wettability behavior of the biomimicked polymeric and carbon superhydrophobic surfaces reported here, we have further measured the sliding angle. Sliding angle is defined as the critical angle by which the water droplet starts to slide on tilting the plane of any superhydrophobic surface.48 As shown in Figure 10, the sliding angle on an artificial hairy PS and RF gel derived carbon surface was found to be 18° and 12°, respectively, as compared to a natural hairy Argyreia Nervosa leaf for which the sliding angle was 8°. Such a state of superhydrophobicity where the sliding angle and CAH are small and the water droplet gets rolled off from the surface on tilting the surface little is known as Cassie’s State47,49 and also termed as the “Lotus effect” in the literature.47,48 However, in the case of biomimicked PS and RF gel derived carbon surfaces from Tropaeolum Majus leaf, the values of the sliding angles were measured to be 32° and 43°, respectively, which were very high as compared to the original leaf (12°). As discussed earlier for CAH, the higher value of the sliding angle in artificial microtextured leaves may be because of the absence of hydrophobic wax which is one of the main reasons for the superhydrophobic nature of the Tropaeolum Majus leaf. This state of superhydrophobicity in which CAH and sliding angle are relatively high is known as Wenzel’s state.47,50 The chemical functionality of the organic RF gel derived carbon surfaces was also studied by FTIR in the attenuated total reflection mode to further investigate the role of surface texturing on hydrophobicity. Figure 11 shows FTIR-ATR spectra of organic RF gel derived carbon surfaces. The reference database for interpretation of peaks has been taken from FDM reference spectra databases. Only strong peaks were observed in the range of 2290 2390 cm 1 that show the presence of the hydroxyl functional group. This attributes to the weak hydrophilic behavior of the organic RF gel derived carbon film (WCA ∼83°). Thus, we observe here that surface morphology plays a significant role in deciding the superhydrophobicity of any surface. The high aspect ratio dense hairy structures and microtextured 13018
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’ REFERENCES
Figure 11. FTIR-ATR spectra of organic RF gel derived carbon surface.
surfaces generated different states of superhydrophobicity in different set of polymers. Interestingly, the polymers chosen in this work (PS, RF Gel, PET, and PMMA) to fabricate the superhydrophobic surfaces by biomimicking the natural leaf patterns are mildly hydrophilic (
