Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Light-Driven Transformation of Bio-Inspired Superhydrophobic Structure via Reconfigurable PAzoMA Microarrays: From Lotus Leaf to Rice Leaf Fei Gao,† Yuan Yao,† Wei Wang,† Xiaofan Wang,† Lei Li,*,‡ Qixin Zhuang,† and Shaoliang Lin*,† †
Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ College of Materials, Xiamen University, Xiamen 621005, China S Supporting Information *
ABSTRACT: Light-driven transformation from isotropic superhydrophobicity to anisotropic superhydrophobicity was accomplished through bio-inspired modification and reconfiguration on poly[6-(4-methoxy-4′-oxyazobenzene)hexyl methacrylate] (PAzoMA) microarrays. In this study, ordered PAzoMA microarray film was fabricated via the reverse breath figure (RBF) method. After gold nanoparticles sputtering and subsequent modification with self-assembly of 1H,1H,2H,2H-perfluorodecanethiol (FSH), the obtained lotus-leaf-inspired film showed isotropic superhydrophobicity with self-cleaning property due to the hierarchical structure and low surface free energy. Upon irradiation with linearly polarized light (LPL), the microspheres were elongated along the direction of polarization and exhibited anisotropic superhydrophobicity resembling rice leaf. With the increase of illumination time, the axis ratio became larger, and anisotropy sliding was more obvious. This research enriches responsive bioinspired superhydrophobicity and further provides a promising candidate for smart water harvesting.
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INTRODUCTION Superhydrophobicity1−5 has attracted great attention because of the increasing application prospects in oil−water separation,6,7 self-cleaning,8 water harvesting,9 etc. Over billions of years of evolution and selection, nature has created numerous functional superhydrophobic matters, such as lotus leaf and rice leaf. The hierarchical branch-like nanostructures on micropapillaes and wax layer with low free energy jointly endow lotus leaf superhydrophobicity with ultralow water adhesion and selfcleaning property.10 Anisotropic hierarchical structures in rice leaves promote unidirectional water droplet sliding.11 This anisotropic wettability is promising in directional droplets transport and microfluidic devices.12−14 Currently, two methods mimicking natural structures have been developed to fabricate bio-inspired superhydrophobic surfaces. One is to roughen the surface of low-surface-energy materials, and the other is to modify the rough surface with low-surface-energy materials.15−18 Smart superhydrophobic surface could be prepared by stimuli-responsive materials combining with surface roughness to alternate wetting behavior through the reconfiguration of functional molecules or rearrangement of microstructure under external stimuli.19−22 A considerable amount of research has focused on switchable wettability23−25 from superhydrophobicity to superhydrophilicity or tunable liquid adhesion26−28 between pinned state and roll-down state. However, there still remains a challenge to transform bio-inspired superhydropho© XXXX American Chemical Society
bic surface from isotropic to anisotropic. The critical procedure is the construction of smart microstructure that could convert from isotropic into anisotropic in one-dimensional order upon external stimuli. Photoresponsive azobenzene-containing polymer29−34 is a promising candidate for realizing the transformation form isotropic structure to anisotropic structure based on directional mass-migration effect and photoinduced orientation upon linear polarized light (LPL).35−40 This is ascribed to that the transition moments of azobenzene moieties being arranged almost perpendicularly to the polarization direction, named the Weigert effect.35,36 For example, the azopolymer colloidal spheres could be deformed to ellipsoids, spindles, and finally rods after LPL irradiation.35,36 In our previous research, azopolymer composed micro/nano structure including micelles, microparticles, microporous films, or hemispherical arrays could be photomanipulated or elongated along the polarization orientation due to the reconfiguration of azobenzene units and generation of photofluidization.41−45 Herein, we adopted light-reconfigurable azobenzene-containing microarrays to accomplish the transformation from bioinspired lotus leaf structure to rice leaf structure. A homopolymer with azobenzene moiety in the side chain, Received: January 10, 2018 Revised: March 16, 2018
A
DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Chemical structure of PAzoMA homopolymer via ATRP. (b, c) SEM images, (d) size distribution, and (e) AFM 3D topography of PAzoMA microspheres fabricated via the reverse breath figure method.
Scheme 1. (a) Fabrication Procedure of PAzoMA Microspheres via the Reverse Breath Figure (RBF) Method, (b) BioModification of PAzoMA Microspheres for Superhydrophobic and Self-Cleaning “Lotus Leaf” Structure, and (c) Linearly Polarized Light (LPL) Irradiation of (b) To Obtain Anisotropic Sliding “Rice Leaf” Structure
our knowledge, this is the first report about the photostimuli transformation from isotropic superhydrophobicity to anisotropic superhydrophobicity. As an application of lightreconfigurable azobenzene-containing microarrays, it has rarely been studied to manipulate bio-inspired polymer surface through such a convenient and effective strategy that shows promising in light-driven water harvesting.
poly[6-(4-methoxy-4′-oxyazobenzene)hexyl methacrylate] (PAzoMA), was employed to fabricate ordered microspheres via the simple reverse breath figure (RBF) technique. The microarray was further treated with gold sputtering and subsequently functionalized with self-assembled FSH monolayer to obtain superhydrophobicity and self-cleaning property similar to natural lotus leaf. Under irradiation of visible LPL, the microspheres elongated along the polarized direction, and anisotropic wetting including static contact angles (CA) anisotropy and dynamic sliding angles (SA) anisotropy was achieved like rice leaves. Additionally, the aspect ratio of elongated microspheres could be tuned by LPL illumination conditions, and the dependence of sliding anisotropy on the aspect ratio has been systematically investigated. To the best of
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EXPERIMENTAL SECTION
Materials. PAzoMA was synthesized via atom transfer radical polymerization (ATRP) method as reported previously.46 Figure 1a depicts the chemical structure of the homopolymer with side chain of azobenzene. The 1H NMR and GPC characterization results are shown in Figure S1. The molecule weight (Mn) is 1.3 × 104, and the polydispersity index (PDI) Mw/Mn is 1.4. 1H,1H,2H,2H-PerfluorB
DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) Scheme of bio-modification of PAzoMA microspheres. (b) SEM image of gold sputtering on microspheres. (c) EDS image of subsequent modification with self-assembly of FSH on (b). (d−g) Optical images of contact angles on the surfaces of PAzoMA microspheres, PAzoMA microspheres followed by gold coating, PAzoMA microspheres followed by gold coating and modification of FSH, and glass substrate followed by gold coating and modification of FSH, respectively. odecanethiol (FSH) and ethanol were purchased from Adamas Reagent, Ltd. Ultrapure water with a specific resistivity of 18.2 MΩ· cm was used in all of the experiments. Other reagents were analytically pure and used without further purification. Fabrication Procedures. The fabrication process of transformation of bio-inspired superhydrophobic structure is depicted in Scheme 1. It includes three steps: preparation of microspheres via RBF, bio-inspired modification of microspheres, and polarized deformation of modified microspheres. PAzoMA was dissolved in chloroform with concentrations from 2 to 20 mg mL−1. Microscope glass slides were cut into 0.8 cm2 pieces and ultrasonic cleaned in ethanol for 30 min. 20 mL of MeOH, EtOH, or H2O was added into a vial beforehand to obtain a saturated vapor in a cap-sealed glass vessel. Microspheres were fabricated by casting 30 μL of polymer solution onto a glass substrate placed in the vessel. Microsphere films were obtained after solvent evaporated completely. The glass substrates with microsphere array were sputtered with gold nanoparticle by a Hitachi E-1010 for 20 s at current intensity of 20 mA and then completely immersed in 5 mM ethanol solutions of 1H,1H,2H,2H-perfluorodecanethiol (FSH) for 24 h.47 After fluorinated modification, the samples were vertically washed three times with ethanol to ensure no residuals and then dried under vacuum at room temperature overnight. To investigate the light-induced deformation of RBF spheres, the samples were irradiated by a polarized light of area source from a LED lamp (COINU UVLED) with the wavelength of 450 nm and square polaroid. After irradiation with intensity of 613 mW cm−2 for various times, the light source was shut down, and the samples with different aspect ratios were obtained. All these experiments were carried out under ambient conditions. Characterization. Surface morphology of the films was observed by a field emission scanning electron microscope (FESEM, Hitachi S4800) with accelerating voltage of 15.0 kV. The distribution of element composition was mapped by an energy dispersive spectrometer (EDS). The 3D profiles and surface roughness were obtained by atomic force microscopy (AFM, PARK/XE-100) using the noncontact mode under ambient conditions. CAs of 5 μL of distilled water on the prepared films were measured by the pendent drop method using a JC200D3 contact angle system (Powereach). SAs were measured by slowly rotating sample stage until 7 μL of water droplets begin rolling
down. The CAs (SAs) perpendicular to and parallel to the elongated direction of microspheres were respectively defined as CA⊥ and CA∥ (SA⊥ and SA∥). Each angle value was averaged by at least nine records from three different samples. The self-cleaning experiment was performed by placing a water droplet to roll across the slightly tilted surface and take off coarse aluminum oxide (Al2O3) powder that randomly distributed on the sample surface as containment.
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RESULTS AND DISCUSSION Morphology of Microspheres via RBF. Breath figure is a versatile and effective technique to fabricate highly ordered porous structure with condensed water droplets as templates.48 If ethanol or methanol is employed as the atmosphere vapor, microspheres will be obtained instead of porous structure, namely reverse breath figure.49 PAzoMA was herein adopted to fabricate microspheres via RBF. Honeycomb-like breath figure array was also prepared under the same condition with conventional water atmosphere (Figure S2b). The influence of solvent, substrate, and polymer concentration on the arrangement and size of microspheres was investigated. As shown in Figures S3−S5, microspheres with diverse sizes and arrangement were obtained in different conditions according to the distinct evaporation rates and surface tensions. The optimized condition for fabricating ordered PAzoMA microspheres via RBF is to cast 5 mg mL−1 CHCl3 solution on glass slide under ethanol atmosphere. As shown in Figure 1b, PAzoMA microspheres with uniform size and regular arrangement were fabricated under ethanol vapor successfully. These microspheres were in hexagonal arrangement (Figure 1c). Figure 1d presents the frequency statistic (red columns) of the spheres size distribution and fitted curve (black line) with a Gaussamp function. The average diameter of the microspheres is ca. 2.24 μm with narrow distribution. The 3D topography (Figure 1e) obtained by AFM showed relatively uniform and regular arrangement of PAzoMA microspheres, which is in accordance with SEM observation (Figure 1b,c). C
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Figure 3. (a) Colored water droplets placed on the modified PAzoMA RBF film. (b−e) Snapshots of self-cleaning experiment with Al2O3 powder spreading on the surface of bio-modified PAzoMA film.
Figure 4. (a) SEM images of PAzoMA microsphere by LPL 30 min. (b) Shape of a colored water droplet placed on the sample. CA perpendicular to (c) and parallel to (d) the elongated direction.
Self-Cleaning Property of Lotus Leaf Inspired Microsphere Film. Figure 2a schematically illustrates lotus leaf inspired modification of PAzoMA microspheres through gold nanoparticle sputtering and self-assembly of FSH. Figure 2b shows the surface topography of single PAzoMA microsphere after gold coating. Gold nanoparticles of approximately 20 nm diameter were observed on the microspheres, and the surface roughness was consequently enhanced by the hierarchical nanostructure. Meanwhile, the existence of gold particles allowed the self-assembly of FSH via S−Au bonding to achieve low free energy modification. Figure 2c shows the EDS mapping of PAzoMA microsphere modified by gold nanoparticle and FSH. Gold and fluorine are both homogeneously distributed on its surface. CA of water on the samples are measured and compared (Figure 2d−g). The obtained PAzoMA microspheres film has a CA of 136° (Figure 2d). Gold nanoparticle sputtering on the PAzoMA microspheres exhibit an increased CA of 145° due to higher roughness (Figure 2e). After being immersed in ethanol solution of FSH and reacted for 24 h, the final PAzoMA microspheres with gold coating and FSH modification exhibit excellent superhydrophobicity with CA of 156° (Figure 2f), which confirms the successful lotus leaf inspired modification. CA on glass substrate followed by gold coating and modification of FSH is merely 118° (Figure 2g), demonstrating the significance of microstructure to obtain superhydrophobicity.
Water droplets containing pigment were employed to directly show wetting behavior on the sample surface. As shown in the photograph of Figure 3a, the upright spherical shaped water droplets manifested excellent water repellency of the modified PAzoMA microspheres. White Al2O3 powder on the slightly tilted surface in Figure 3b mimicked the contaminants in the self-cleaning experiment. A 10 μL water droplet rolled across the surface and took away Al2O3 powder on its pathway (Figure 3c−e), which exhibits similar selfcleaning behavior as the lotus leaf. Additionally, a water droplet rolling along three different direction of the modified PAzoMA microspheres was measured and shown in Figure S6. The SAx, SAy, and SAz were all approximately 10°, indicating isotropic sliding. In conclusion, isotropic superhydrophobic and selfcleaning property were successfully fabricated by a bottom up bio-inspired modification on PAzoMA microspheres film. LPL Irradiation Induced Anisotropic Wettability to Resembling Rice Leaf. Upon irradiation of visible LPL, the PAzoMA microspheres of the film were elongated along the polarized direction (Figure 4a). The deformation degree was quantified by average axial ratios (l/d) of the aggregates (estimated statistically from 50 microspheres in Figure 4a). After being irradiated by LPL for 30 min, PAzoMA microspheres were elongated with the l/d of 1.8. The water droplet exhibited different shape along the long axis or short axis directions. Figure 4b shows a water droplet was stretched along the polarized direction after 20 s equilibrium. The anisotropy D
DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. SEM image and CA of lotus leaf inspired PAzoMA microspheres irradiated by LPL after 6 h. Schemes of a water droplet rolling off the elongated microspheres along the perpendicular (b) and parallel (c) direction. Insets: respective sliding angle.
Figure 6. Typical SEM images of lotus leaf inspired PAzoMA microspheres irradiated by LPL with different times of (a) 3 h and (b) 12 h. (c) Relationship between aspect ratio (l/d) and illumination time. (d) The dependence of the sliding angle along the parallel and perpendicular direction on the aspect ratio of modified PAzoMA microspheres.
and 18°, resembling rice leaf with a sliding anisotropy about 15°. For the microspheres with less uniform sizes and arrangements, the contact angle on its surface was 151° (Figure S8a,b). The sliding angles along orthometric directions were 18° and 19°, indicating a similar isotropic superhydrophobicity (Figure S8b,c). When the microspheres were elongated upon LPL irradiation for 6 h (Figure S8d), the sliding angle parallel and perpendicular to the elongated direction was 17° and 20° (Figure S8e,f), respectively. The difference value was only 3°, exhibiting less anisotropic surface on account of the irregular structure and less difference between orthometric directions. Besides, the average axial ratios could be tuned by light irradiation conditions such as illumination time and intensity. Figures 6a and 6b show typical SEM images of bio-inspired PAzoMA microspheres irradiated by LPL after 3 and 12 h, in
resulted in different CA∥ and CA⊥ as 118° and 130°, respectively (Figures 4c and 4d). As mentioned previously, the CA of previous microsphere is 136°. The decrease of CA is ascribed to the surface collapse and reduced roughness. For lotus leaf inspired PAzoMA microspheres, the photoinduced deformation rate is relatively slower since gold nanoparticles covered on them. The SEM image in Figure 5a exhibits the deformation topography by 12 h LPL irradiation. The inset in Figure 5a shows the static CA of the microspheres after polarized light irradiation is about 152°. The elongated microspheres still possess superhydrophobicity and keep CA isotropy even though the microspheres have been stretched to an l/d value of 2.3. Figures 5b and 5c schematically illustrate a water droplet rolling on the deformed samples along parallel or perpendicular direction, corresponding to the snapshots exhibited in Figure S7. The SA∥ and SA⊥ are respectively 3° E
DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. Schematic illustration of three-phase contact line (TCL) of a water droplet on the surface of (a) PAzoMA microspheres, (b) LPL irradiation of (a), (c) PAzoMA microspheres modified by gold nanoparticle and FSH, and (d) LPL irradiation of (c), from the top view (top part) and side view (bottom part). Red line represents TCL, and the blue part represents water droplet.
which the l/d value is 1.79 and 2.89, respectively. The relationship between l/d value and illumination time is shown in Figure 6c. The l/d value increases with the illumination time. It is noteworthy that longer illumination time than 18 h would induce the decrease of surface roughness and sharply destroy the ordered structure (Figure S9). Moreover, the dependence of sliding anisotropy on the l/d value was also investigated (Figure 6d). The growth of l/d value results in larger sliding angle along the perpendicular direction and smaller sliding angle along the parallel direction. That is more obvious sliding anisotropy could be obtained with the augment of l/d value. In terms of the initial PAzoMA microspheres, the l/d value is 1, and isotropic SA of water droplets along every direction is 10°. For the elongated microspheres with the l/d of 2.89, water droplets could roll off along the parallel direction at an extremely low SA of ca. 1°. In contrast, the SA perpendicular to the polarized direction is as high as 20°. The strategy provides a quite effective and convenient technique to regulate anisotropic sliding through LPL irradiation. In our further work, reversible transformation of bio-inspired superhydrophobic structure will be investigated through introducing cross-linkable segments in azopolymer. Additionally, azopolymers with differnet Tg values can also be adopted to fabricate the bio-inspired microspheres to accelerate the deformation rate. Transition Mechanism of Three-Phase Contact Line. The generation of static anisotropic wetting and dynamic anisotropic sliding could be explained as the change of wetting state and three-phase (solid−liquid−gas) contact line (TCL).50−52 In terms of the untreated film composed of isotropic arranged PAzoMA microspheres, the water droplet on its surface was in Cassie wetting state and would spread along random direction due to discontinue and unidirectional TCL (Figure 7a). Upon LPL irradiation, the microspheres were elongated in one dimension and exhibited anisotropic wetting (Figure 7b). According to the gradual disappearance of air trap and decrease of contact angle in Figure S10,30 it was deduced that LPL induced the water droplets on surface in Wenzel state and generated continuous TCL. Additionally, the distance of adjacent aggregates along the perpendicular direction turned much longer than that of parallel direction. The pinning effect hindered TCL motion across perpendicular direction with periodic stick−slip behavior. Much lower barriers along parallel direction caused the preferential rolling of water droplets.50−52
For the samples of lotus leaf inspired PAzoMA microspheres, high roughness and low free energy endowed the surface superhydrophobicity with a Cassie wetting state and air trapped beneath the water (Figure 7c). Water droplet could isotropically roll off due to the randomly distributed discrete TCL. After elongation by polarized light, the superhydrophobicity remained while dynamic SA anisotropy instead of static CA anisotropy appeared. The TCL for water droplet movement parallel to the elongated direction was more continuous than that of perpendicular direction on account of lower wetting/ dewetting energy barrier (Figure 7d).12,52 Consequently, the SA parallel to elongated direction is lower than the values perpendicular to the direction of elongation. With the increase of average l/d value, the TCL of parallel direction was more continuous than that of perpendicular direction, resulting in distinct SA anisotropy.
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CONCLUSIONS In summary, we successfully fabricated ordered PAzoMA microspheres via the RBF method. The microspheres were endowed superhydrophobicity and self-cleaning property similar to lotus leaf by bio-inspired modification with gold sputtering and self-assembly of FSH. Anisotropic wettability could be effectively achieved upon LPL illumination. The untreated microsphere film exhibited static anisotropic wetting with 12° difference between parallel and perpendicular to the polarized direction. Dynamic anisotropic sliding instead of static anisotropic wetting appeared for the lotus leaf inspired microspheres. The angle of water droplets rolling off along the parallel direction was 20° higher than that along the perpendicular direction resembling rice leaf. Besides, the anisotropic wettability became more obvious with the increase of axis ratio of the elongated microspheres. The study provides an effective and convenient strategy to manipulate bio-inspired surface and shows promising in light-driven water harvesting.
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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.macromol.8b00059. 1 H NMR results and GPC results of PAzoMA polymer; the influence of atmosphere, solvent, concentration, and substrate on the morphologies of PAzoMA film; scheme and snapshots of a water droplet rolling off the lotus leaf F
DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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inspired PAzoMA arrays along different direction; snapshots of a water droplet rolling off the rice leaf inspired elongated microspheres along the parallel and perpendicular direction; SEM images and sliding angles of irregular PAzoMA microspheres with and without LPL irradiation; SEM images of lotus leaf inspired PAzoMA microspheres irradiated by LPL with 18 h; snapshots and schematic illustration of a water droplet contacting the elongated PAzoMA microspheres (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S.L.). *E-mail:
[email protected] (L.L.). ORCID
Lei Li: 0000-0003-2732-9116 Shaoliang Lin: 0000-0003-3374-9934 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51622301, 51573046, and 51573088). Support from Projects of Shanghai Municipality (14SG29 and 17JC400700), Natural Science Foundation of Fujian Province (2014J07002), and Fundamental Research Funds for the Central Universities (222201717001 and WD1616010) is also appreciated.
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DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.8b00059 Macromolecules XXXX, XXX, XXX−XXX
