Light-Driven Wettability Tailoring of Azopolymer Surfaces with

80126 Naples, Italy. § Center for Nanoscale Systems, Harvard University, 9 Oxford Street, Cambridge, Massachusetts 02138, United States. ACS Appl...
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Light-Driven Wettability Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts Stefano Luigi Oscurato,† Fabio Borbone,‡ Pasqualino Maddalena,† and Antonio Ambrosio*,§ †

Dipartimento di Fisica E. Pancini, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, Via Cintia, 80126 Naples, Italy ‡ Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, Via Cintia, 80126 Naples, Italy § Center for Nanoscale Systems, Harvard University, 9 Oxford Street, Cambridge, Massachusetts 02138, United States S Supporting Information *

ABSTRACT: The directional light-induced mass migration phenomenon arising in the photoresponsive azobenzenecontaining materials has become an increasingly used approach for the fabrication of controlled tridimensional superficial textures. In the present work we demonstrate the tailoring of the superficial wettability of an azopolymer by means of the light-driven reconfiguration of an array of imprinted micropillars. Few simple illumination parameters are controlled to induce nontrivial wetting effects. Wetting anisotropy with controlled directionality, unidirectional spreading, and even polarization-intensity driven two-dimensional paths for wetting anisotropy are obtained starting from a single pristine pillar geometry. The obtained results prove that the versatility of the lightreconfiguration process, together with the possibility of reversible reshaping at reduced costs, represents a valid approach for both applications and fundamental studies in the field of geometry-based wettability of solid surfaces. KEYWORDS: azopolymer, mass migration, three-dimensional photopatterning, anisotropic wetting, unidirectional spreading

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INTRODUCTION Inspired by Nature, it has long been known that the topography of the superficial textures is a crucial parameter in determining the wetting properties of solid surfaces,1−4 and many studies have been oriented toward the proper description of the effects resulting from liquid interaction with rough surfaces. The Wenzel5 and Cassie−Baxter6 models are the most commonly used thermodynamic models able to predict the contact angle of a sessile liquid drop onto a rough surface on the basis of the geometrical parameters of the liquid/solid textured contact area.7−11 However, a great variety of complex wetting phenomena have also been recently identified, which are driven by peculiar superficial topographies and sometimes not accurately described by the standard wettability models. For example, experimental situations have been reported where the pinning effect of the liquid at the sharp edges of the superficial asperities dominates the wetting state,12−14 and peculiar superficial architectures having re-entrant curvatures are demonstrated to induce superficial omniphobicity.15−17 Furthermore, asymmetric superficial textures are able to induce directional wetting anisotropy18−22 or even to drive anisotropic23−26 or unidirectional liquid spreading.27−29 In this framework, the fabrication of complex superficial topographies has acquired a growing remarkable importance. Textured solid surfaces for wettability studies are typically © 2017 American Chemical Society

fabricated by standard techniques, including photolithography, electron beam and focused ion beam lithography, self-assembly, and soft lithography.8,28,30−32 Recently, a new promising approach for the design of controlled three-dimensional superficial textures was delineated by the directional lightinduced mass migration phenomenon arising in photoresponsive azo-benzene-containing materials.33−37 Under illumination with UV/visible light, the azobenzene molecules exhibit photoisomerization cycles between the trans and cis states, which are characterized by a difference in the threedimensional occupied molecular volume.38,39 This microscopic conformational change drives a reorganization of the host material, typically a polymer with embedded azochromophores, and results in a macroscopic mass displacement.38,39 The observed material motion is highly directional and occurs in the preferential direction of the illuminating light polarization. Besides the thoroughly reported40−49 surface relief gratings (SRG) inscribed onto the surface of azo-containing films by means of both uniform and patterned illuminations, lightinduced anisotropic mass migration has been recently used to design more complex superficial topographies onto this class of Received: June 5, 2017 Accepted: August 14, 2017 Published: August 14, 2017 30133

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thaw cycles and heated to 70 °C under nitrogen. After 48 h the polymer was precipitated in methanol and purified by multiple precipitation from chloroform/hexane. For UV/visible absorption spectra acquisition (recorded with a Jasco V560 spectrophotometer, see Figure S1), transparent amorphous thin films were prepared by spin coating polymer solutions in 1,1,2,2-tetrachloroethane onto glass slides. Thermograms and molecular weight distribution are reported in the Supporting Information. PDMS stamp fabrication. The PDMS stamps were molded from a commercially available silicon master fabricated by electron beam lithography and chemical etching. The fabricated silicon wafer was exposed to vapors of trichloro(1H,1H,2H,2H-perfluorooctyl)silane for superficial antisticking treatment. The silanization process was realized by enclosing the silicon master and a beaker containing a few drops of the silanizing liquid in a hermetically closed box, kept at 125 °C for 90 min. Then, the box was opened and the temperature raised at 150 °C for a further 90 min in order to remove the vapor excess. The PDMS (Sylgard 184, Dow Corning) mixture was prepared by mixing the precursor and the curing agent in a 10:1 weight ratio. After degassing in a vacuum chamber for 2 h, the PDMS mixture was gently poured onto the silicon master and cured at 80 °C for 90 min before being carefully released from the silicon master (Figure S4). The flat transparent PDMS capping layer (1−2 mm thickness) was obtained by molding the PDMS mixture onto a flat polished silicon wafer and at 80 °C for 90 min. Replica of pillar texture onto the azo-polymer film. The pristine pillar array was fabricated by soft-imprinting from a solution of 10 wt % of the azo-polymer in N-methyl-2-pyrrolidone (NMP). A few drops of the solution were casted on a precleaned glass slide (10 min in acetone ultrasonic bath) and then covered by the PDMS stamp. The mold was kept at 45° overnight in order to let all solvent molecules evaporate through the pores of the PDMS stamp. Finally, the PDMS stamp was carefully released and a replica of the silicon master texture was realized onto the polymer film (Figure S4). By means of this technique the polymer cloud be obtained in a stable amorphous state for the photoinduced reconfiguration experiments. Optical setup for pillar reshaping. A schematic representation of the optical setup used to reshape the pristine azo-polymer pillar arrays is shown in the Supporting Information (Figure S5). The illumination source was a solid state diode laser (Coherent OBIS 488 LS) at wavelength of 488 nm. The beam was first expanded to a diameter of about 3 cm and then adjusted via an iris before the sample in order to minimize the effect of the intensity gradient of the Gaussian laser beam. The sample was placed on a xyz translational stage equipped with a rotational stage to control the light incidence angle. The linear polarization of the laser beam was controlled by a half-wave retarder. When needed, radial or azimuthal polarizations were obtained by inserting a liquid crystal polarization converter (θ-cell by ARCoptix) beyond the half-waveplate in the optical path. Pillar reshaping was obtained with intensities in the 20−75 mW/cm2 range. Structural characterization. The pillar structural characterization was performed by scanning electron microscopy using a FEI Nova NanoSEM 450 apparatus. The samples were previously sputtered with a nanometric layer of an Au/Pd alloy using a Benton Vacuum Desk V deposition system. Bright-field optical images of the reshaped pillars were also collected through an inverted microscope equipped with a 40X, NA = 0.5 objective. Contact angle measurements. Water contact angle (CA) measurements were performed with a homemade setup and analyzed by the open source imageJ plugin DropAnalysis.63 A 1 μL water droplet was carefully deposited on the sample, and the digital image of the drop profile was collected about 5 s after the drop contact. The temperature and the humidity of the room during the measurements were 23 °C and 50%, respectively. The contact angle value is the mean of at least three independent measurements in different regions of the samples.

materials.39,50 Relying on both the approaches of film illumination with spatially structured intensity patterns43,44,51 and light-driven reconfiguration of superficial prepatterned microvolumes,33−37 a great variety of superficial topographies can be realized by tuning a few illumination parameters, such as the intensity distribution, the polarization state, and the incidence angle of light. This method allows a very peculiar reversibility of the light-reconfiguration process36,51 and also gives a high degree of versatility in the superficial geometry tailoring. Recently our group proposed phenomenological models able to deterministically predict the topography of the surface-reliefs (holographic52 and spontaneous47) arising onto the free surface of azopolymer thin films under different illumination configurations.52 Furthermore, the models describe also the asymmetric geometries induced by the light-driven reconfiguration of symmetric prepatterned three-dimensional micropillars,35 giving an unprecedented insight into the applicability of this class of materials for fabrication of engineered surfaces. Examples about the surface wetting tailoring based either on the properties of an azobenzene molecular photoswitch53−58 or on the anisotropic behavior induced by the sinusoidal SRG inscribed onto azopolymer films have already been reported.18 Moreover, the potentialities of the light-induced three-dimensional reconfiguration of superficial microposts on the surface of azomaterial films have been immediately recognized35 and a very recent work59 clearly demonstrated the ability of tuning the wetting anisotropy of the surface in a reversible way, even if the proposed description of the wetting state is not completely convincing. In the present work we use the light-driven mass migration in order to reconfigure a prepatterned array of azopolymer micropillars so as to induce a controlled directional wetting anisotropy. The wetting state of a water droplet onto the pristine cylindrical pillar array is recognized on the basis of the pinning effect of the triple-phase contact line (TCL) at the sharp pillar edges, and a recent model14 is applied to describe the observed static contact angle. The dependence of the anisotropic TCL deformation of the sessile droplet on the lightinduced pillar asymmetry is also presented and ascribed to the directional energy barriers the TCL encounters in its motion toward different directions onto the textured surface.18 Unidirectional anisotropic wetting is also demonstrated and the potentiality of using polarization and intensity patterns is exploited with the aim to design light-driven two-dimensional wetting pathways onto the surface of a single prepatterned azopolymer pillar array. Finally, the directional anisotropic wetting is transformed into symmetric directional and unidirectional liquid spreading by a superficial hydrophilization treatment. The proposed experimental situations clearly demonstrate the possibilities offered by the azomaterials in designing complex superficial topographies on a large scale for wettability studies and also for possible applications in microfluidics,35,60 photonics,39,61 and biology.62

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EXPERIMENTAL SECTION

Azo-polymer preparation and characterization. The reagents for the synthesis were purchased from Sigma-Aldrich and used without further purification. The azo-polymer was synthesized by radical polymerization of (E)-2-(4-((4-methoxyphenyl)diazenyl)phenoxy)ethyl acrylate, according to a previously reported procedure.52 Briefly, a solution of the monomer (1.00 g) and AIBN (0.0100 g) in 4 mL of anhydrous N,N-dimethylformamide was subjected to three f reeze and 30134

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RESULTS AND DISCUSSION Water contact angle of the pristine azobenzene pillar array. SEM images of the typical as-prepared textured polymer surface are presented in Figure 1a. Cylindrical posts, arranged

composite solid/air interface formed by the solid surface and the air trapped into the roughness asperities. In this composite regime, the observed contact angle θC is given by the relation cos θC = f cos θ0 − (1 − f), where f is the fraction of the solid− liquid interface at the drop−surface contact area. The Cassie− Baxter model is usually associated with high values of the observed static CA and low values of CA hysteresis.7,8 In the actual experimental situation, Figure 1c suggests that the liquid droplet completely wets the surface asperities of the texture since no air gaps can be distinguished between the liquid and solid surface. Hence, the Wenzel model instead of the Cassie−Baxter model should be used to describe the wetting state of the system, as also supported by the quantitative disagreement between the measured θ and the estimated θC for the considered pillar geometry and by the absence of droplet roll-off at any tilt angle, meaning high CA hysteresis. However, also the Wenzel model gave inadequate results due to the observed texture-induced hydrophilic-tohydrophobic transition from the flat (Figure 1b) to the structured (Figure 1c) surface. Instead, the experimental framework is properly explained by assuming the TCL pinning rather than the wetted superficial area as governing the wetting state. Recently, Suzuki and Ueno14 proposed a new equation to describe the CA of liquid droplets lying in the wetted regime onto patterned surfaces, establishing a relation between the contact angle θ0 on the ideally flat surface of the same material, the geometrical parameters of the surface texture, and the ability of the sharp edges of superficial features in pinning the TCL, quantified by a pinning angle θ1. A description of the TCL pinning model by Suzuki and its application to the present experimental situation is given in the Supporting Information and leads to the predicted contact angle θ given by the relation:

Figure 1. Water contact angle variation induced by the azopolymer pristine pillar texture. (a) SEM images of the as-prepared array of cylindrical pillars fabricated by soft imprinting from the PDMS stamp. (b) Image of 1 μL water droplet deposited onto the flat azopolymer surface, which results in a measured contact angle of θ0 = 87° ± 3°. (c) Image of the same volume of water droplet cast onto the pristine pillar array resulting in the measured contact angle θ = 115° ± 3°.

cos θ = cos θ0 +

d + 2h cos θ1 p

(1)

where d, h, and p are respectively the diameter, the height, and the pitch of the fabricated azopolymer pillar array, while the pinning angle θ1 describes the equilibrium of the interfacial tensions at sharp edges (see ref 14 and the Supporting Information) and can be interpreted as the CA of a liquid film pinned at the edge of an ideally infinite long groove of the considered material. For the actual azopolymer pillar geometry, the measured values θ0 = 87° ± 3° and θ1 = 122.5° ± 0.1° give the estimated value θest = 144° ± 3° from eq 1. The predicted θest quantitatively agrees with the CA measured onto the as-fabricated azopolymer array and unambiguously recognizes the TCL as governing the wetting behavior of the present experimental situation. Equation 1 can also be used to interpret the observed CA of the symmetric structures in the experiments described in ref 59 (see Supporting Information). Wetting anisotropy on light-reconfigured asymmetric azopolymer micropillars. Directional asymmetry in the roughness of the solid surface causes anisotropic TCL distortion at the liquid−solid interface of a sessile droplet, resulting in a CA anisotropy observed along different directions of the TCL. In the case of the azopolymer prepatterned textures, significant anisotropy can be inscribed in the threedimensional superficial roughness by the directional lightinduced mass migration of the polymer under UV/visible light illumination.

in a square array of pitch p = 10.0 μm, diameter d = 4.6 μm, and height h = 2.0 μm, were homogeneously fabricated on a 1 cm × 1 cm area. Figure 1b and Figure 1c show respectively the images of 1 μL water droplets deposited onto the flat untextured polymer surface and onto the cylindrical pillar array, with the relative measured contact angles of θ0 = 87° ± 3° and θ = 115° ± 3°. The wetting state of the azopolymer textured surface was first assessed by comparing the measured water contact angle θ with the values predicted by the standard Wenzel and Cassie−Baxter models for the actual pillar geometry. In the Wenzel regime, the droplet completely fills the surface asperities (wetted regime) and the apparent contact angle θW onto the rough substrate is related to the contact angle θ0 of the liquid droplet deposited on the flat surface of the same solid by the relation cos θW = r cos θ0. The roughness parameter r is defined as the ratio of the actual area of the patterned surface and the geometrically projected one,8 and is always greater than 1. Therefore, the surface roughness in the Wenzel model always enhances the intrinsic hydrophilic/ hydrophobic behavior of the flat surface, while a textureinduced transition from the hydrophilic (CA < 90°) to the hydrophobic (CA > 90°) regime is not contemplated. Such a transition is possible in principle for droplets in the Cassi− Baxter state, in which the liquid drop lies suspended on the 30135

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Figure 2. Light-induced superficial asymmetry and directional wetting anisotropy. (a and b) SEM images and illumination configuration schemes of the azopolymer microposts reconfigured by a laser beam linearly polarized in two orthogonal directions (specified by the red arrows). Scale bar 5 μm. (c) Top and side views of a typical reconfigured microstructure, together with the definition of the posts’ asymmetry A after the light-driven reconfiguration process. (d) Measured asymmetry as a function of the exposure time at fixed illumination intensity of 75 mW/cm2. The uncertainty bars indicate the standard deviation of the measured asymmetry distribution of each sample. (e) Anisotropic water droplet deposited onto the asymmetric pillars presented in (f). The red and green arrows represent the directions of the long and short axes of the reshaped structures, respectively. (g and h) Side views of the droplet in the direction parallel (red) and orthogonal (green) to the asymmetric pillars. (i) Parallel (CA∥) and orthogonal (CA⊥) water contact angles for different values of the parameter A.

As previously reported,33−36,59 a linear light polarization turns simple cylindrical or cubic pillars into asymmetric threedimensional geometries, which become mainly elongated in the light polarization direction. The polymer motion starts at the top surface of the structures and, without any external constraint, gradually involves the whole volume of the microposts as the reconfiguration process goes on. However, as described in ref 35, the azopolymer flow can be spatially constrained at the top surface of the structures by illuminating the pristine pillar array through a transparent flat PDMS capping layer placed in tight contact with the top surface of the posts. In this configuration, the adhesion forces arising between the polymer and the capping layer prevent the polymer flow from heading toward the bottom surface of the film, and the light-driven material motion occurs mainly in contact with the flat PDMS layer, producing top-flat three-dimensional reconfigured structures.

Figure 2a−b show the SEM images of the pristine cylindrical pillars (of the type described in Figure 1a) reconfigured by a linearly polarized laser beam (λ = 488 nm) illuminating the azopolymer microposts through the transparent PDMS capping layer. The bottom panels of Figure 2a−b depict the schemes of the relative illumination configurations used in the lightreconfiguration processes. In each of the two experiments, the laser beam impinges on the pillar array at normal incidence, while the polarization direction (specified by the red arrows in the panels and in the sketches) is rotated by 90° for the reconfiguration of structures in Figure 2a with respect to those in Figure 2b. As expected, the linear polarization produces a symmetric displacement of the material at the top surface of the posts in the preferential direction of the light polarization, and the reconfigured architectures can be described as having a flat top “pseudoelliptic” section, with the major axis oriented in the light polarization direction (Figure 2c). The degree of induced 30136

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producing significant structural modifications and any decrease of the energetic barriers for the TCL motion in that direction. The measured parallel contact angle (CA∥, Figure 2i) shows instead a strong dependence on the pillar asymmetry A. As expected from energy barrier considerations, higher values of A induce a decrease of the observed contact angle in the parallel direction. A variation of about 20° was found in parallel CA∥ by increasing the posts’ asymmetry from 1.3 to 2.1, following an approximately linear trend (Figure 2i). For clarity, the image sequence of the water droplet profiles in the direction of the long and short axes of the reshaped pillars is reported in the Supporting Information (Figure S9 and Figure S10), together with the SEM images of the single pillar geometry for each corresponding asymmetry measured value. Unidirectional and bidimensional wetting anisotropy. The potentialities offered by the light-reconfigurable azobenzene microposts in controlling the superficial wettability in a nontrivial way are here demonstrated by the realization of unidirectional and spatially structured wettability paths. Unidirectional wetting anisotropy can be simply induced in any arbitrary direction over the reshaped pillar array by exploiting another straightforward degree of freedom of the illumination configuration: the beam incidence angle.35 Figure 3b−c shows the SEM images of an azopolymer pillar array reconfigured with the illumination configuration schematized in Figure 3a. The linearly polarized laser beam (the polarization direction is indicated by the red arrow) illuminates the sample through the PDMS flat capping layer at the incidence angle φ = 45°. In particular, the sample is rotated with respect to the laser beam around an axis contained in the sample plane, passing through the center of the patterned pillar texture and orthogonal to the linear polarization direction of the illuminating beam. Due to the nonuniform illumination of the pillars and shadow effects,35 the light-driven polymer mass migration becomes asymmetrical in the light polarization direction and gives rise to tilted structural reconfigurations. More specifically, the reshaped pillars are more elongated in the polarization direction toward the side of the incoming illumination beam (Figure 3a). A liquid droplet deposited on the obtained structures experiences an asymmetric TCL along the deformation direction. The image of a 2 μL water droplet (with 0.1 wt % Triton X-100 surfactant) deposited on the tilted structures is shown in Figure 3b. The droplet became elongated toward the reconfigured pillar slant direction with a measured contact angle difference of about 25° in the considered experiment. The dependence of the induced asymmetry direction in the light-reconfigured microposts on the polarization direction of the illuminating beam can be used to design a 2D asymmetry pattern on the same pristine pillar array. This can be achieved, in principle, by illuminating the sample with a pure 2D spatially varying polarization pattern or with sequential spatially structured illumination intensity distributions, coupled with a controlled polarization state. Such a bidimensional asymmetry translates into the bidimensional modification of the wetting behavior of the reconfigured azopolymer substrate, giving the possibility of designing different liquid drop shapes and orientations in the illuminated areas of the same pristine array. An example of a pure polarization bidimensional pattern is given by a configuration based on radial (R) or azimuthal (A) polarization states, in which the optical electric field of the light beam oscillates in a direction that is radial or azimuthal respect to the center of the spot.69,70 R and A polarizations are

asymmetry in the reconfigured posts depends on the laser fluence during the illumination. In a simple experimental situation, textured surfaces with different directional asymmetries on the microscale can be obtained by tuning only the film exposure time at fixed illumination intensity. As far as the lightdriven pillar reconfiguration is realized through the PDMS capping layer, the geometrical description of the threedimensional architectures can be properly approximated by only considering the contour of the posts’ top surface, while the height can be assumed unchanged with respect to the pristine pillars (the shorter the exposure time, the more valid is this approximation). Therefore, the degree of induced asymmetry of the azopolymer pillars after the light-driven reconfiguration can be described by the asymmetry parameter A, defined as the ratio of the long axis l (indicated by the red arrow in Figure 2c) to the short axis s (green arrow) of the flat-top reshaped pillar surface. The value of the measured asymmetry resulting from the illumination of different pristine arrays with increasing exposure time at fixed laser intensity is plotted in Figure 2d. The parameter A for each sample is determined from SEM images as the mean of at least 100 asymmetry measurements of individual reconfigured pillars. The asymmetry A shows a nonlinear increasing trend as the exposure time increases, as also reported for different azobenzene-containing materials.36,59 However, the asymmetry increase is mainly related to the elongation of the structures in the long axis direction (65% maximum elongation) rather than to the contraction in the short axis (15% maximum contraction) (Figure S9). The asymmetric three-dimensional architecture of the reconfigured posts is able to induce a directional wetting anisotropy of the liquid droplets deposited on the surface, with a consequent directional dependence of the observed CA along the TCL. Such contact angle anisotropy can be interpreted as depending on the energy barriers that TCL encounters on moving from the center of the liquid drop toward the outer regions of the substrate.21,30,64−68 In this framework, the directions of the sample parallel to the long axis of the asymmetric microstructures are characterized by smaller energy barriers to the liquid propagation, which results in these directions being more energetically favored rather than the orthogonal ones. As a consequence, the TCL is mainly elongated in the direction of the long axis of the reconfigured structures, as reported in Figure 2e, which shows the photograph of a sessile water droplet (with blue dye) deposited onto the reconfigured asymmetric pillar array reported in Figure 2f. The red and green arrows indicate, respectively, the directions parallel and perpendicular to the illuminating light polarization that correspond to the droplet TCL elongation (red arrow) and pinning (green arrow) directions. The level of induced droplet TCL anisotropy depends on the asymmetry of the light-reshaped superficial micropillars, and it could be characterized by the differences of the water CA measured in the long (parallel CA, Figure 2g) and the short (orthogonal CA, Figure 2h) axis directions of the reshaped pillar as a function of the measured mean asymmetry A of the superficial microstructures. However, the orthogonal contact angle of a 1 μL water droplet (CA⊥, Figure 2i) does not show a significant dependence on the structure asymmetry, since the measured values at different degrees of structural asymmetry remain unchanged (within the experimental measurement errors) with respect to the pristine pillar array. This behavior can be explained with the small contraction of the pillar in the short axis direction after the posts’ reconfiguration, not 30137

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Figure 4. Radial and azimuthal wettability distributions onto the reconfigured micropillars. (a) Optical micrographs of different regions of the array of pillars reshaped with the radially polarized beam (scale bare 10 μm). The red dotted lines schematize the light polarization direction during the one-step reconfiguration process. (b) Photograph of the liquid droplet placed in different regions of the radially reconfigured array (Scale bar 1 mm). The droplets are elongated toward the center of the illuminated region in radial directions. (c) Micrographs and (d) photograph of the same experimental situation for azimuthal polarization of the illuminating beam.

Figure 3. Left−right contact angle asymmetry induced by the reconfigured tilted pillars. (a) Schematic presentation of the illumination configuration for slanted pillars reshaping. (b) SEM top-view image (scale bar 5 μm) and (c) side-view image of the tilted pillars. (d) Image of 2 μL water droplet with surfactant Triton X-100 0.1 wt % deposited onto the slanted pillar array. The measured contact angles are CALef t = 40° and CARight = 65°.

obtained in the present work by a liquid crystal polarization converter (θ-cell, ARCoptix) able to transform the linear polarization of the light beam in both R or A polarization states, depending on the direction of the original linear polarization with respect to the θ-cell axis.69,70 Radial and azimuthal reconfigurations of azopolymer pillars are realized by illumination of the prepatterned area (1 cm × 1 cm wide) with the properly modulated laser beam emerging from the θcell. The optical bright-field microscope images of the reconfigured pillars in different regions of the illuminated sample through radial and azimuthal polarization are shown in Figure 4(a) and Figure 4(c), respectively. The overlaid red lines schematically represent the directions of the electric field polarization states of the two illumination configurations. As far as the oscillation direction of the electric field can be locally approximated as linear for both R and A polarizations, neighboring pillars are deformed approximately in the same direction (this is more true as the distance from the beam center increases) as shown in each subpanel of Figure 4(a) and Figure 4(c). However, the deformation direction significantly changes in the millimeter-scale, and the long axis of the reconfigured pillars properly reproduces the radial or the azimuthal pattern of the illuminating beam polarizations. The induced two-dimensional

wetting anisotropy of the reconfigured arrays is clearly observed from the photographs in Figure 4(b) and Figure 4(d), where the droplets of an ionic liquid (1-butyl-3-methylimidazolium iodide) deposited onto the reconfigured samples at fixed distance from the center are shown. The liquid droplets become anisotropically elongated in the direction of the local long axis of the reshaped posts, which varies angularly across the sample and globally reconstructs the two-dimensional radial and azimuthal patterns. Since the light-driven reconfiguration is induced only in the illuminated regions of the pristine cylindrical azopolymer pillars, a spatially modulated intensity of the illuminating beam can be used to induce surface asymmetries with spatial selectivity. As a consequence, a two-dimensional wetting anisotropy path can be in principle arbitrarily designed onto the pillar array using a proper combination of incidence angle, polarization state, and spatial distribution of the light intensity. The proof of concept of the droplet TCL deformation path design is demonstrated in the simple experimental situation illustrated in Figure 5. Here, a single pristine pillar array is illuminated in three different regions by a laser beam which is 30138

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and the slanted pillar elongation direction is schematized by the one-directional arrow pointing toward the left in the light polarization direction. Each of the illuminated regions has a defined directional asymmetry (Figure 5b) due to the different reconfiguration conditions which directly translates in a position dependent liquid droplet anisotropy over the pillar array. More complex intensity/polarization combinations can be easily conceived and experimentally realized, giving the possibility of exploiting the light-driven mass migration phenomenon in azobenzene-containing materials to effectively draw complex anisotropic bidimensional wetting distributions on single prepatterned macroscopic regions. The anisotropic wetting effects induced by the asymmetric geometries of the light-reconfigured posts can be even enhanced by a chemical modification of the azopolymer surface. For example, a superficial hydrophilization treatment of the reshaped posts is able to turn the directional wetting anisotropy in a directional spreading of water droplets. Oxygenplasma treatment is a commonly used technique for modification of organic polymer surfaces to induce or enhance hydrophilicity.71−73 By means of this treatment, aliphatic and/ or aromatic carbon on the surface can be readily oxidized to functional groups with C−O and/or CO bonds such as alcohol, ketone, and carboxylic acid derivatives.72 These groups are responsible for the induced polarity of the exposed polymer surface and increased hydrophilicity. As a proof, Figure 6 shows the photographs of the spreading of water droplets resulting from the O2-plasma hydrophilization treatment of the pillar array after the light-reconfiguration process. In particular, the posts in Figure 6a are reshaped with linear polarization at normal incidence angle, which results in the symmetrically elongated structures shown in the inset of Figure 6a. The liquid volume, initially dispensed in the position indicated by the red spot, flows symmetrically in the direction of the red arrows, which corresponds to the direction of the long axis of the reconfigured microposts. Despite the fact that the surface is

Figure 5. Two-dimensional wettability path induced by polarization/ intensity light patterns. (a) Optical micrographs of the different reconfigured pillar areas of the pristine array. (b) Photograph of the water droplets deposited in the relative numbered regions. The red double arrows of regions 1 and 2 represent the direction of the linear light polarization at normal incidence, while the single arrow in region 3 indicates a nonzero incidence angle producing pillars elongated in the direction of the arrow.

spatially cut by a rectangular slit in a stripe of light about 2 mm wide and constitutes the prototype of a simple structured intensity pattern. The polarization and incidence angle of the illuminating beam are tuned to achieve different reconfigured pillar geometries in each illuminated rectangular sample area (marked by the blue rectangle and labeled with progressive numbers). Figure 5a shows the optical micrographs of the reshaped pillars in each area, and the laser polarization direction is indicated by the red arrows in the relative numbered box. In particular, areas 1 and 2 are illuminated at normal incidence angle, while area 3 is illuminated at an incidence angle of 45°

Figure 6. Directional liquid spreading on O2-plasma treated surfaces (15 W, 5 sccm, 1 min). (a) Symmetrically spreading droplet onto the symmetric reshaped pillars (optical micrographs in the inset). (b) Unidirectional droplet spreading in the direction of the slanted reconfigured pillars presented in the inset. The red spot represent the initial position of the droplets which then propagate mainly in the directions marked by the arrows. 30139

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made homogeneously superhydrophilic by the plasma treatment, the stronger energy barrier arising in the direction of the short axis is able to pin the spreading TCL along an approximately straight line parallel to the long pillar axis. As a consequence, a symmetric directional spreading is observed in the pillar long axis direction and hence in the illuminating light polarization direction. Unidirectional liquid spreading (Figure 6b) is instead achieved onto the tilted pillar structures, reconfigured at the laser incidence angle φ = 45° (inset Figure 6b). Also in this case the water droplet spreads in the direction of the pillar deformation axis (which corresponds to the light polarization direction), but the liquid flows mainly toward the tilted side of the reconfigured pillars and gives rise to a unidirectional liquid spreading. This asymmetric spreading behavior is even more highlighted by the thin liquid film propagating ahead to the droplet volume, which spreads on the left side of the droplet until it reaches the edges of the textured array, while it soon results in being pinned on the right side. Moreover, these results, coupled with the capability of the wetting path design demonstrated above, suggest that the lightreconfigured azopolymer microtextures can be used also to delineate bidimensional spreading pathways (Figure S11).

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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/acsami.7b08025. Azopolymer chemical scheme and the visible absorption spectrum; DSC diagram and molecular weight distribution of the azopolymer; fabrication steps for the pristine azopolymer pillars; schematic representation of the optical setup for the light-driven reconfiguration experiments; model describing the wetting state of the pristine pillar array; sequence of images relative to the relation of the wetting anisotropy with the light-induced superficial asymmetry; image of two-dimensional liquid spreading pathway inscribed on the azopolymer pillar array by the intensity-polarization structured illumination patterns. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stefano Luigi Oscurato: 0000-0002-1814-8033 Fabio Borbone: 0000-0001-7433-9267 Pasqualino Maddalena: 0000-0001-5823-3331 Antonio Ambrosio: 0000-0002-8519-3862

CONCLUSIONS

In summary, we have demonstrated that the intensitypolarization dependence of the light-driven mass migration phenomenon occurring in materials with embedded azochromophores can be used for tuning the three-dimensional geometry of soft-lithographic imprinted micropillars, hence producing controlled superficial wetting properties. In particular, the pinning of the TCL was recognized as governing the wetting state of the pristine cylindrical pillars, while the structures obtained by tuning simple illumination parameters such as the light polarization state and the beam incidence angle had been demonstrated to induce a directional controlled wettability anisotropy. Despite the relatively simple experimental fabrication framework, phenomena such as unidirectional anisotropy and spreading were achieved onto the reconfigured microstructures. Furthermore, spatially structured intensity and polarization patterns allowed the design of tailored bidimensional anisotropic wetting paths. These nontrivial wetting applications usually require expensive and multistep fabrication processes for tailoring the superficial geometry, and typically the obtained structures cannot be further tuned once fabricated. The soft imprinting technique used for azopolymer pristine texturing is based on the replicamolding from a single silicon master and permits the fabrication of hundreds of light-customizable samples, with a dramatic reduction of fabrication time and costs with respect to the standard lithographic methods. Besides the potential applications in many fields of research such as photonics, electronics, and biology, such reversible light-configurable surfaces can be used as a powerful tool for both experimental and theoretical studies relying on the topography-based superficial wetting design. Furthermore, the light-reconfigured azopolymer surfaces can be used as molding templates for soft-lithographic transfer of the texture on other materials, which can have improved mechanical and chemical properties for specific applications.

Author Contributions

A.A. and S.L.O. conceived the idea. S.L.O. conducted the lightreconfiguration and the wettability experiments under the supervision of A.A. and P.M. F.B. synthesized the polymer and performed the SEM characterization of the structures. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Dr. Fulvia Villani and Dr. Fausta Loffredo from ENEA (Portici, Italy) for the O2-plasma treatment and for the contributions on the preliminary material characterization. We thank Dr. Giovanni Dal Poggetto of Institute for Polymers, Composites and Biomaterials − National Research Council (CNR) of Italy, for the molecular weight determination.

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