Hydrogel Inverse Replicas of Breath Figures Exhibit

Jan 11, 2016 - Analysis of the wetting behavior using the Cassie–Baxter approximation leads to estimation of the contact angle in the superoleophobi...
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Hydrogel Inverse Replicas of Breath Figures Exhibit Superoleophobicity Due to Patterned Surface Roughness Jaspreet Singh Arora,† Joseph C. Cremaldi,† Mary Kathleen Holleran,† Thiruselvam Ponnusamy,† Jibao He,† Noshir S. Pesika,*,†,‡ and Vijay T. John*,†,‡ †

Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States Vector Borne Infectious Diseases Research Center, Tulane University, New Orleans, Louisiana 70112, United States

‡

S Supporting Information *

ABSTRACT: The wetting behavior of a surface depends on both its surface chemistry and the characteristics of surface morphology and topography. Adding structure to a flat hydrophobic or oleophobic surface increases the effective contact angle and thus the hydrophobicity or oleophobicity of the surface, as exemplified by the lotus leaf analogy. We describe a simple strategy to introduce micropatterned roughness on surfaces of soft materials, utilizing the template of hexagonally packed pores of breath figures as molds. The generated inverse replicas represent micron scale patterned beadlike protrusions on hydrogel surfaces. This added roughness imparts superoleophobic properties (contact angle of the order of 150° and greater) to an inherently oleophobic flat hydrogel surface, when submerged. The introduced pattern on the hydrogel surface changes morphology as it swells in water to resemble morphologies remarkably analogous to the compound eye. Analysis of the wetting behavior using the Cassie−Baxter approximation leads to estimation of the contact angle in the superoleophobic regime and in agreement with the experimental value.

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that overhanging “mushroom-like” structures, for example, lead to formation of stable composite interfaces.7,16 The re-entrant surface curvature in such structures imparts exceptional nonwetting behavior to surfaces that are inherently wetting.17 Another class of nonwetting surfaces is slippery liquid induced porous surfaces (SLIPS) obtained by liquid impregnation on rough surfaces.18 In this paper, we describe a simple strategy to obtain a submerged superoleophobic surface through the patterning of hydrogel films. In principle, the hydrophilicity of such polymers that absorb water leads to a degree of oleophobicity with the contact angle of oil on the surface typically greater than 90°.1,4 To obtain contact angles characteristic of natural superoleophobic surfaces (>150°), it is important to couple the surface chemistry of the material with surface roughness. Thus, inherently oleophobic surfaces (contact angle >90°) can be made superoleophobic by imparting roughness to the surface. A brief description of the role of surface roughness in enhancing oleophobicity follows. An oil drop in contact with a submerged rough surface can exist in two states. Wetting the entirety of the roughness achieves a Wenzel state.19 However, if the drop only contacts a fraction of the solid substrate, the Cassie−Baxter state is prevalent.20 The change in contact angle due to the addition of surface features can be described by the simplified Cassie−

INTRODUCTION Surfaces that prevent the spreading of oils are termed as oleophobic surfaces. When an oil drop rests on a superoleophobic surface, it makes a contact angle greater than 150°. In the recent literature, submerged materials with superoleophobic properties have attracted applications in antibioadhesion,1 microfluidics,2 oil−water separation,3,4 and marine anti-biofouling.5 The design of oleophobic surfaces is different from hydrophobic surfaces. Oils have lower surface energies than water and spread easily. From an energy balance perspective, surfaces with low interfacial energy for contact with the surrounding medium are necessary to prevent oil spreading.6,7 Low surface energy fluoro compounds have been used to generate surfaces that are superhydrophobic8 and superoleophobic,6,7 but the chemical inertness of these materials poses an environmental hazard in terms of their bioaccumulative potential.9 In addition to the surface chemistry, the surface topography affects the wettability. There are several examples of highly oleophobic surfaces found in nature, perhaps as an evolutionary response to contamination by oily materials in the natural habitat of these organisms. These include the skins of sharks and filefish,10−12 the bottom surface of the lotus leaf submerged in water,13 and the bronchosomes of leafhoppers,14 and pioneering work has been done to replicate these structures with synthetic materials.12,13 A common aspect of such surfaces is the presence of hierarchical patterns that lead to superoleophobicity.15 The surface curvature is also a factor that affects the contact angle on a surface, and it has been reported © XXXX American Chemical Society

Received: October 17, 2015 Revised: January 8, 2016

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Figure 1. (a) Schematic of an oil drop on a submerged poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel. (b) Adding roughness to the surface increases the contact angle and makes it superoleophobic. (c) The oil drop being in the Cassie−Baxter state and partially wetting the patterned surface.

Baxter state on the breath figure surface because of the air pockets in the pores.36−38 While this would also be true for inverse replicates of breath figures made with hydrophobic polymers, the use of hydrogels for the inverse replica presents an interesting situation where hydration layers on the swollen hydrogel inverse replica create surfaces that are oleophobic, when submerged, with oil drops resting in the Cassie−Baxter state. Our study describes the expansion of the breath figure replica concept to pHEMA hydrogels. pHEMA hydrogels are biocompatible materials used extensively as implants,39 contact lenses,40 drug delivery devices,41 and scaffolds for cell growth.42 Inverse templates of polystyrene breath figures on pHEMA hydrogels form patterned semispherical beadlike protrusions dictated by the pore morphology of the original breath figure molds. In this paper we have investigated the oil contact properties of pHEMA hydrogels, while submerged in water. The hydrogel is inherently oleophobic when submerged in water and transitions to superoleophobic behavior through the addition of protrusions on the surface through the replica molding process. Using the assumption of a Cassie−Baxter state, we also use the geometry of the protrusions to estimate the effective contact angle which shows reasonable agreement with the experimentally determined value.

Baxter equation for an oil drop submerged in water with partial wetting of the solid substrate (Figure 1).21 cos θ* = r *f cos θ − (1 − f )

(1)

where θ* is the apparent contact angle of a rough surface, θ is the contact angle of the flat surface of the same material, r* is the roughness factor of the wet surface, and f is the areal fraction of the drop in contact with the solid. A detailed explanation of eq 1 is provided in the Supporting Information section (S1). As illustrated in Figure 1, the fraction f arises because of the water pockets between the oil drop and the partially wetted submerged solid. The roughness factor r* is defined as the ratio of the actual surface area of contact of the oil drop to the projected area of contact. Equation 1 indicates that when θ > 90°, adding roughness to the surface increases the contact angle (θ* > θ).21 The water pockets between the oil drop and the surface lead to superoleophobicity and are analogous to the air pockets present when a water droplet partially wets a superhydrophobic patterned surface.20 Such water pockets have been postulated as leading to superoleophobicity by Xu and co-workers,22 who have used a system of clay reinforced layered PNIPAM/chitosan hydrogels (to model seashell nacre) that exhibit surface roughness when the composite is submerged in the high ionic strength environment of seawater. In this study we demonstrate an easily implementable method to introduce roughness onto the surface of a hydrogel and thus enhance its oleophobicity. This is done using inverse replicas of honeycomb structures known as breath figures.23,24 Breath figure polymer films are porous polymer films with an array of hexagonally packed surface pores25 and have found applications as optical band gaps,26 drug delivery devices,27,28 vesicle capturing surfaces,29 and templates.23,30 These structures are generated by the condensation of water droplets from humid air onto the surface of an evaporating polymer solution, where the water droplets do not coalesce due to Marangoni thermal stresses.25,31 The film attains the structure of hexagonally arrayed pores upon complete evaporation of the solvent and the water droplets.25 Breath figures have been fabricated from a number of polymers including polystyrene,29 poly(lactic-co-glycolic acid),32 poly(ε-caprolactone),33 polyimides,34 and poly(methyl methacrylate).35 Intrinsically, breath figures exhibit hydrophobicity when made from hydrophobic polymers. In these systems, a water drop rests in the Cassie−

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MATERIALS AND METHODS

Materials. Poly(DL-lactide-co-glycolide) (PLGA 50:50) polymer (Resomer RG 504 Mw = 56 000 and inherent viscosity = 0.56 dL/g) was purchased from Boehringer Ingelheim Chemicals Inc. Methylene chloride (organic solvent, ACS grade) was obtained from Fisher Scientific, USA. Dichloroethane (DCE), carbon disulfide (CS2), poly(ethylene glycol) (PEG) (MW 3350), 2-hydroxyethyl methacrylate (HEMA), N,N′-methylenebis(acrylamide) (MBA), N,N,N′,N′tetramethylethylenediamine (TEMED), and ammonium persulfate (APS) were purchased from Sigma/Aldrich (St Louis, MO). Polystyrene (PS) (monocarboxy terminated, approximately MW 50 000 and polydispersity index of 1.07) was obtained from Scientific Polymers Inc., Ontario, NY. All chemicals were used as received, without further purification. Preparation of Breath Figure Polymer Films. A spin-coater (model WS-400-6NPP-LITE, Laurell Technologies Corporation, North Wales, PA) was used to prepare the breath figure polymer film. Poly(ethylene glycol) (PEG) and poly(lactic-co-glycolic acid) (PLGA) polymers were used at a weight ratio of 1:9, respectively. Both polymers were dissolved in dichloromethane to a concentration of 15% (w/v). Alternatively, 4% (w/v) of polystyrene in CS2 was also B

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Figure 2. (top) Mechanism of formation of the breath figure films. (bottom) The fabrication of a micropatterned pHEMA hydrogel surface using a breath figure polymer film as a sacrificial template. The hydrogel precursor solution was cast and allowed to infiltrate into the pores. Once the hydrogel solution completely cured overnight, the beadlike patterned hydrogels were obtained by dissolving the breath figure films in methylene chloride. used to fabricate breath figures. A 22 × 22 mm glass coverslip, used as the substrate, was placed on the fragment adapter inside the spincoating chamber. The chamber’s fragment adapter was kept under vacuum to hold the substrate while it spun. The spin-coater’s chamber was connected to a flow of humid air created by bubbling air through distilled water. The relative humidity was maintained at about 70% as measured by a hygrometer (Fisher Scientific). Approximately 0.4 mL of polymer solution was dropped onto the substrate, and the spinning process was rapidly accelerated to 2500 rpm (approximately 13 s) and maintained at this speed for an additional 30 s. During the spin-coating process, the solvent evaporated to form a translucent porous film. The breath figure films were further dried at room temperature for 24 h. Preparation of the Inverse Template on the pHEMA Hydrogel. pHEMA hydrogels were synthesized following the procedure described by Blake and co-workers.43 Briefly, 0.05 g of MBA was dissolved in 1 mL of distilled water containing 100 μL of TEMED. This was followed by the addition of 1 mL of 2-hydroxyethyl methacrylate (HEMA solution, 99%) and thorough mixing. A 0.25 mL aliquot of ammonium persulfate in water (0.5 mol % with respect to HEMA monomer) was subsequently added. After mixing all of the components, the solution was cast on the top of the required breath figure film and allowed to cure for at least 12 h at room temperature. The casting of the hydrogel solution was followed by placing a glass coverslip on top of the solution. This allowed the solution to spread uniformly across the porous breath figure film surface. For all experiments, the cross-linker MBA was included at a 1:25 molar ratio of cross-linker to monomer. The control flat samples were prepared by casting the hydrogel solution on a glass coverslip with no breath figure film. The breath figure−hydrogel composite was then dipped in methylene chloride to dissolve away the breath figure and obtain the patterned pHEMA hydrogel. Contact Angle Measurements. Contact angle measurements were taken on a Ramé-Hart goniometer with DROPimage Advanced software. For all tests, a 3 μL drop of dichloroethane (DCE) was dispensed from a needle submerged into a distilled water environment and lowered into contact with a test surface. The use of DCE as a surrogate oil8,35 is based on the fact that its density (1.28 g/cm3) is greater than that of water thus allowing a drop of DCE to rest on a submerged surface without the requirement for an inverted setup, thus

simplifying the experimental procedure. Prior to the contact angle experiments, the hydrogels were submerged in distilled water for at least an hour. On the flat pHEMA surface and structured pHEMA surface, the oil rests on the surface and the needle is removed before taking a static contact angle measurement. In the cases of the advancing and receding contact angle measurements, DCE was added (advancing) or removed (receding) in 0.25 μL increments with the needle continuing to stay within the droplet. The contact angle was recorded after each increment. The reported advancing or receding measurement was taken when the contact area between the drop and test surface changed. (The contact area increases in the case of the advancing contact angle and decreases in the case of the receding contact angle.) Characterization Methods. The surface morphology of the breath figure films and the patterned pHEMA hydrogels was characterized with a field emission scanning electron microscope (SEM) (Hitachi S-4800). Prior to the SEM imaging of the dry samples at room temperature, all the samples were coated with a thin gold layer through sputtering (Polaron SEM coating system) set at 20 mA and 2.4 kV for 90 s. To image the swollen samples in the cryogenic mode (cryo-SEM), the sample was plunged in liquid nitrogen to enable vitrification. The cut-section samples were fractured at −130 °C using a flat edge knife. The samples were sublimed at −95 °C for 5 min to remove surface vitrified water and expose surface morphology and then sputtered with a gold−palladium composite at 11 mA for 88 s. Imaging was done at a voltage of 3 kV and a working distance of ∼8 mm.

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RESULTS AND DISCUSSION Breath Figure and Inverse Template Preparation. Figure 2 is a schematic of breath figure formation and the generation of their inverse templates, illustrating the hexagonally patterned honeycomb pores and the replicas that lead to hexagonally patterned hemispherical protrusions.44 In preparing the inverse templates in this work, the hydrogel precursor solution was cast and allowed to infiltrate into the pores of the template. Once the hydrogel solution completely cured C

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Figure 3. SEM images of the fabricated breath figures. (a) Top view of the polystyrene (PS) breath figure film with a high resolution image focusing on one pore in the inset. (b) Cross section of the PS breath figure film with a high-resolution image focusing on one pore in the inset. The PS breath figure has a monolayer of pores with an average pore diameter of ∼7.5 μm.

Figure 4. SEM images of the inverse templates are presented. The cut-section cryo-SEM image of the breath figure−pHEMA composite is seen in (a). In (b), the high-resolution cut-section cryo-SEM image of the breath figure−pHEMA composite is seen. The process of cutting breaks the brittle PS polymer film. This image confirms that when the hydrogel precursor is poured on top of the breath figure film, it completely infiltrates the pores. Dissolving the breath figure template in methylene chloride exposes the pattern on the surface of the pHEMA hydrogel. In (c), the top view of the pHEMA inverse template obtained from the PS breath figure film is presented with a high-resolution image focusing on one protrusion in the inset. The cut section of the pHEMA inverse template with a high-resolution image focusing on a cut protrusion in the inset in (d) verifies a monolayer of protrusions on the surface. Note: in the cut-section images, the protrusions are not exactly cut at their center. Thus, the top view image is a better representative of the size of the protrusions.

breath figure fabrication.31 The water droplets are sustained on the surface of the polymer solution, and upon complete evaporation, the polymer surface has the surface morphology of a honeycomb porous structure with a single layer of pores. The inset to Figure 3b is a close-up of the cut section and indicates that the pores are approximately spherical caps cut by a plane above the center of the sphere, leading to pore volumes in excess of a hemisphere (>(2/3)πr3).

overnight, the beadlike patterned hydrogels were obtained by dissolving the breath figure films in methylene chloride. Figure 3 illustrates the morphologies of breath figures obtained from a solution of PS in carbon disulfide with uniform pores approximately 7.5 μm in diameter. The oblique view shown in Figure 3b illustrates a monolayer of pores overlaying a dense base. Such monolayers are generated when polymer solvents with a density greater than that of water are used in D

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Figure 5. Cryo-SEM images of the swollen (in water) patterned pHEMA hydrogels. (a, b) Images focusing on several surface protrusions. (c) Highresolution cryo-SEM image of the area between the protrusions. (d, e) Cross-sectional images of the cut protrusions. (f) High-resolution crosssection cryo-SEM image of a cut protrusion. The protrusions of the patterned hydrogel upon swelling conform to spherical caps with a hexagonal base.

The dried hydrogel/breath figure composite was then dipped in methylene chloride for 5 min to selectively dissolve the breath figure substrate, resulting in the generation of the micropatterned pHEMA as seen in Figures 4c and 4d. The inverse replica of the template is observed on the surface of the pHEMA structure in Figure 4c. The pHEMA hydrogel in Figure 4c, patterned from the PS breath figures, has beadlike protrusions similar to the breath figure pore dimensions. The average diameter of the beadlike protrusions is ∼10 μm, whereas the pore size in Figure 3a is 7.5 μm. This disparity is due to the fact that the pores are not perfect hemispheres but rather spherical caps. Thus, the features of the micropattern developed on the surface of the hydrogels are dependent on the pore morphology of the breath figure template. Figure 4d illustrates a cross-sectional SEM of the pHEMA inverse template obtained from the PS breath figure template. Both Figures 4b and 4d indicate a single layer of protrusions on a hydrogel film contributing to the surface pattern. While the hydrogel morphologies described in Figures 3 and 4 represent the material in the dry state, the experiments with

The PS breath figure film was then used as a template to fabricate the micropatterns on the pHEMA hydrogels. The HEMA precursor solution together with the initiator and the cross-linker was poured on top of these breath figure films (molds) and allowed to cure overnight. To verify that the pores of the breath figure films were filled by the hydrogel precursor solution, the cross section of the hydrogel/breath figure composite was imaged using cryo-SEM. From the images in Figures 4a and 4b it is observed that the hydrogel precursor solution completely infiltrates the pores of the breath figure film. The breath figure template mold and the hydrogel act as complementary pieces of a jigsaw puzzle, where the protrusions of the hydrogel (dark gray) conform to the pores of the breath figure film (light gray). In these images it should be noted that the process of sectioning for imaging creates small breakages that are reflected in the offset pieces of the pore template seen in Figures 4a and 4b. Nevertheless, the cryo-SEM results are illustrative of full pore penetration by the hydrogel precursor and provide information on the nature of replica formation. E

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Figure 6. Profile of the oil droplet partially wetting the patterned pHEMA surface is illustrated in (a) and in (b); the position of the oil drop on top of the protrusions is geometrically estimated from the static contact angle of the flat sample. The schematic in (c) depicts the nature of the swollen protrusions. The protrusion is assumed as a spherical cap with a hexagonal base. When the oil drop partially wets the protrusions, the contact area is another spherical cap.

protrusions. The height, h, of this spherical cap can be found from the criterion of the flat surface contact angle through the geometrical relationship sin(θ − π/2) = (R − h)/R, with R being the sphere radius. We extrapolate this geometry to the actual geometry of the swollen hydrogel (Figure 6c). Here the base of each protrusion is hexagonal with area A = 3((√3/2) r12). The roughness factor, r*, and the fractional area of the drop in contact with the solid, f, are then obtained as follows, assuming that the base of each swollen protrusion is a hexagon. With the specific geometry shown in Figure 6, the roughness factor (r*) and the fractional area in contact with the oil drop ( f) can be determined by

oil contact are in the submerged state where the hydrogel is allowed to imbibe water and swell to the extent defined by the level of cross-linking. The cryo-SEM images of the submerged patterned pHEMA are shown in Figure 5. The top view images at increasing resolution (Figure 5a,b) indicate that swelling leads to a tighter packing of the hexagonally ordered protrusions with each protrusion having a hexagonal border. The corresponding cut section images in Figures 5d and 5e indicate that the protrusions can be approximated by the geometry of a spherical cap. Figure 5c is the high-resolution cryo-SEM of a gap between protrusions, and Figure 5f is the cut-section cryo-SEM of a single protrusion showing the merging with the adjacent protrusions. We note that the top view images of such hydrogel protrusions show similarity to the compound eyes of many insect species,45 and we note in particular the similarity to the compound eye of the Atlantic krill,46 a crustacean that is in the submerged state in its natural habitat of the ocean. Analysis of Wetting Behavior. Figure 6 is a schematic of an oil drop on the protrusions of the hydrogel with the appropriate representations of the contact with a rough surface. A hydration layer is assumed to exist between the submerged hydrogel and the oil drop. For the purpose of simplicity, the hydrogel with a thin hydration layer is treated as having an effective surface energy calculated from the oil contact angle, θ, of the submerged flat hydrogel. The patterned hydrogel has a hydration layer when submerged but will still have the same effective surface energy as it is independent of morphology. This effective surface energy is then used to estimate the contact angle of the rough surface in the Cassie−Baxter state. As Figure 6a illustrates, the intrinsic contact angle θ based on material properties without accounting for surface roughness is the angle made by the tangent to the protrusion and the horizontal plane, at the point of contact of the oil drop with the protrusion. As shown in Figure 6b, an interface in the form of a spherical cap is formed when the oil drop partially wets the

r∗ =

area of contact by oil 2πRh = projected area πr2 2

f=

πr 2 2πr2 2 flat area of contact = 2 = area of the hexagonal base A 3 3 r12

where r1 and R were estimated by size analysis of Figure 5f where the contours of the spherical cap are traced and the chord lengths from points of contact between the protrusions measured. Details of the morphology fitting analysis are shown in the Supporting Information (S2), with values of R, h, r1, and r2 determined to be 3.04 μm, 0.899 μm, 2.68 μm, and 2.16 μm, respectively. The roughness factor, r*, is then 1.173, and the areal fraction of contact, f, is 0.786. The contact angle is thus estimated from eq 1 as 149.9° and is in reasonable agreement with the experimentally determined value of 152.0 ± 1.4° (Figure 7). Figures 7a and 7b are images of the drop shape indicating the static contact angle of dichloroethane (DCE) on flat and rough surfaces of the pHEMA hydrogels. The contact angle of oil (DCE) on a flat pHEMA surface submerged in water is 134.8 ± 2.5° (Figure 7a). The introduction of surface roughness through inverse replica patterning on the hydrogels leads to F

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CONCLUSIONS We have shown that the implementation of a surface pattern in the form of beadlike protrusions on the surface of pHEMA hydrogels makes the surface superoleophobic when submerged. The pattern was obtained in the form of inverse templates of breath figures. The pore morphology of the breath figure dictates the morphology of the features on the hydrogel surface, and the morphology of the submerged pattern is in the form of close-packed hexagonally arranged spherical caps. The oil contact angle of the inherently oleophobic flat pHEMA increases from 134.8° to 152° upon introduction of surface roughness, thereby imparting superoleophobicity to the material. Thus, the procedure represents a simple way to enhance oleophobicity on hydrogel surfaces. Because of its biocompatibility, pHEMA hydrogels are used extensively in biomedical applications as ocular implants,47,48 contact lens materials,49 coatings for wound and burn healing materials,50 and drug delivery materials.48 It is possible that such easily fabricated superoleophobic surfaces of pHEMA may serve to minimize protein adsorption and prevent biofouling.51−53 The applications to antifouling hydrogel based membranes and membranes for oil−water separation become a possibility. In principle, the method of replica molding can be extended to a variety of other polymers including natural biopolymers. In the Supporting Information section (S4) we have shown the creation of protrusions in a variety of polymers including chitosan, gelatin, carboxymethyl cellulose, and poly(vinyl alcohol). The Supporting Information section (S5) also shows the feasibility of creating double-sided patterns on hydrogel films using the inverse replica technique. The introduction of these protrusions will lead to enhanced oleophobicity of all these materials when submerged and is the objective of our continuing work that seeks to extend these concepts to the fabrication of hydrogel patterned surfaces.

Figure 7. Contact angles of a drop of dichloroethane (DCE) on (a) the flat unpatterened pHEMA and (b) pHEMA inverse template of the PS BF. Advancing contact angles were measured of (c) the flat pHEMA and (d) the patterned pHEMA. Receding contact angles were measured of (e) the flat pHEMA and (f) the patterned pHEMA. The patterned hydrogel has higher contact angles than the flat hydrogels and exhibits superoleophobicity (static contact angle >150°).

an increased oil contact angle of 152 ± 1.4° (Figure 7b) and transfers the material into the superoleophobic regime. Figures 7c−f illustrate the advancing and receding contact angles of both the flat and patterned pHEMA hydrogels. For the flat surface, the advancing and receding contact angles are 142.3 ± 1.5° and 101.0 ± 5.3°, respectively. For the patterned pHEMA, the advancing and receding contact angles are 151.0 ± 4.4° and 105.3 ± 8.6°, respectively, indicating an increase over the flat surface. The higher advancing and receding contact angles indicate the enhanced resistance of the patterned surface to the attachment of oil. The technique of creating a pattern on the surface of the hydrogel was extended by employing a second breath figure template made with a polymer blend of poly(ethylene glycol) and poly(lactic-co-glycolic acid) (PEG−PLGA). The template was prepared by dissolving the PEG−PLGA in methylene chloride. For conciseness, the procedure and observations are detailed in the Supporting Information (S3). There are distinct differences in the templates and in the spacings of the protrusions for the inverse replicas of the two templates, although each template (and its inverse replica) is fully reproducible. Pores of PS breath figure films are ∼7.5 μm, while those of PEG−PLGA are ∼2 μm while the interpore spacings change from 1.45 μm for PS films to 1.2 μm for PEG− PLGA films. The protrusions of the pHEMA inverse replicas from PEG−PLGA are less closely packed than those from PS. While the distinctions in template and inverse replicas are not readily understood, the observations bring up the possibility of using various templates to modulate the protrusion morphology of a single inverse replica material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03870. Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (V.T.J.). *E-mail [email protected] (N.S.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Science Foundation (Grant 1236089) is gratefully acknowledged. REFERENCES

(1) Chen, L.; Liu, M. J.; Lin, L.; Zhang, T.; Ma, J.; Song, Y. L.; Jiang, L. Thermal-responsive hydrogel surface: tunable wettability and adhesion to oil at the water/solid interface. Soft Matter 2010, 6 (12), 2708−2712. (2) Truman, P.; Uhlmann, P.; Frenzel, R.; Stamm, M. A Stack of Functional Nanolayers for Simultaneous Emulsion Separation and Sensing. Adv. Mater. 2009, 21 (35), 3601−3604.

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DOI: 10.1021/acs.langmuir.5b03870 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b03870 Langmuir XXXX, XXX, XXX−XXX