Mussel-Inspired Modification of Honeycomb Structured Films for

Article pubs.acs.org/JPCC

Mussel-Inspired Modification of Honeycomb Structured Films for Superhydrophobic Surfaces with Tunable Water Adhesion Xiang Yu, Qi-Zhi Zhong, Hao-Cheng Yang, Ling-Shu Wan,* and Zhi-Kang Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: In this paper, we report novel superhydrophobic surfaces with tunable water adhesion by combining musselinspired surface chemistry and bioinspired multiscale surface structures. Highly ordered honeycomb porous films were prepared from a diblock copolymer polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) by the breath figure method. Removing the top surface layer of the honeycomb films leads to pincushion-like surfaces. The films were coated with polydopamine after completely prewetting by ethanol, followed by the reaction with 1H,1H,2H,2H-perfluorodecanethiol for fluorination. As a result of the surface modification, both the honeycomb and the pincushion-like surfaces are superhydrophobic with a water contact angle higher than 150°. Interestingly, the former is highly adhesive, on which water droplets are pinned at any tilted angles, whereas the latter is relatively low adhesive. Calculations indicate that the honeycomb films are in the Cassie state, whereas the pincushion-like surfaces are in the metastable Cassie state. On the basis of these superhydrophobic surfaces with different adhesive properties, no-loss transportation of water droplets has been demonstrated.

1. INTRODUCTION The wettability of solid surfaces is a property that plays significant roles in various practical applications.1,2 It is wellknown that a superhydrophobic surface of a lotus leaf enables self-cleaning. Inspired by this, Onda et al. reported superwaterrepellent fractal surfaces with low surface energy materials on rough surfaces.3 Afterward, it was revealed that the self-cleaning property of a lotus leaf derives from the large water contact angle and the small sliding angle, which makes water droplets slide very easily. In addition to self-cleaning, such surfaces are also useful in anti-icing and antifogging. Differing from a lotus leaf that has a low adhesive superhydrophobic surface, some biological surfaces with high adhesive superhydrophobicity have also been found, which include gecko feet, red rose petals, peanut leaves, etc.4,5 By mimicking the surfaces of these natural species, a variety of superhydrophobic materials with high water adhesion have been fabricated. Water droplets with spherical shape can be pinned on a high adhesive superhydrophobic surface at any tilted angles even when it is turned upside down. As a result, high adhesive superhydrophobic surfaces show promising applications in microfluidic systems and no-loss transportation of microdroplets.1,2 Inspired by natural biological surfaces, a large number of materials have been created through the rational design of multiscale roughness and surface chemistry. Micro/nanostructured surfaces can be fabricated by various techniques. As an example, films with honeycomb structures can be facilely prepared via the breath figure method in which condensed water droplets act as dynamic, green templates.6−9 Yabu et al. prepared highly ordered honeycomb films from a fluorinated copolymer.10 After peeling off the top layer using an adhesive © XXXX American Chemical Society

tape, the exposed pincushion-like structure possessed a water contact angle as high as 170°. If the pore size further decreases to about 300 nm, the honeycomb film itself became superhydrophobic and transparent.11 Superhydrophobic surfaces were also obtained through the formation of pincushion-like structures from a specially designed waxy-dendron-grafted polymer via the breath figure method.12 For honeycomb films prepared from nonfluorinated polymers, superhydrophobic surfaces can be constructed through simple postmodification such as CF4 plasma treatment.13 Recently, Heng et al. investigated water adhesion, for the first time, on honeycomb films that have a water contact angle about 120°, and proposed that the negative pressure induced by the sealed air in the pores is a crucial factor for the high water adhesion.14 Most recently, Li et al. prepared honeycomb films from amphiphilic fluorinated pentablock copolymers by the breath figure method, which show water contact angles of about 110°; the corresponding pincushion-like surfaces have higher contact angles in the range of 130° to 140°. They found that the surface is of high water adhesion.15 It is not known yet whether a superhydrophobic honeycomb or the pincushion-like surface that has water contact angles higher than 150° is high or low water adhesive. Surface chemistry is also important to the wettability and adhesion properties of a film, besides surface roughness. For example, treatments by CF4 plasma or UV−O3 endow honeycomb films with totally different wettability; the former Received: December 30, 2014 Revised: January 30, 2015

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Scheme 1. Schematic Illustrations of the Mussel-Inspired Surface Modification of Honeycomb Films via Dopamine Deposition Followed by the Reaction with a Fluorinated Thiola

The same procedure was applied to pincushion-like films formed by removing the top surface layer of honeycomb films from the position indicated by the dashed line. a

and ∼80% RH). Owing to the condensation of water vapor onto the solution surface during the evaporation of carbon disulfide, the transparent solution turned turbid in about 1 min. With the evaporation of carbon disulfide as well as the condensed water, the solution solidified into a white film with an area of around 1.5 cm2, which displayed uniform bright iridescent colors when viewed with a reflected light. The film was dried at room temperature before further modification. To prepare honeycomb films with pincushion-like structures, the as-prepared honeycomb films on the PET substrate were pressed by an adhesive Scotch tape, and then the top surface was completely peeled off. As illustrated in Scheme 1, the top surface will be separated from the thinnest and weakest position which is marked by the dashed line. As a result, the residual films on both the substrate and the adhesive tape have a pincushion-like structure. The films remaining on the substrate were used in this work. 2.3. Mussel-Inspired Surface Modification by Dopamine Deposition and Fluorination. The dopamine deposition and the subsequent fluorination processes were conducted using a slightly modified procedure (Scheme 1).26,29 In a typical experiment, dopamine hydrochloride was dissolved in a Tris buffer solution (pH = 8.5, 50 mM) at a concentration of 2 mg/mL. Honeycomb films of ∼1.5 cm2 were prewetted by immersing in ethanol for 1 min and then immediately transferred into 10 mL of the freshly prepared dopamine solution. The samples were gently shaken for a designated time ranging from 4 to 24 h at room temperature. Then, a fluorinated thiol, PFDT, in ethanol was directly added to the reaction solution, making a final concentration of 1 mg/mL for dopamine and 0.5−2 mg/mL for PFDT. The reaction was allowed to proceed for another designated time ranging from 6 to 24 h at ambient temperature. The fluorinated samples were taken out from the solution, washed three times with ethanol followed by deionized water three times, and then dried in a vacuum oven at room temperature overnight. 2.4. Water Contact Angle Measurements. Static, dynamic, and sliding water contact angles were measured using deionized water on a DropMeter A-200 contact angle system with digital image analysis software (MAIST Vision Inspection & Measurement Ltd. Co.) at room temperature. A

is superhydrophobic, whereas the latter may be superhydrophilic.13,16 In recent years, mussel-inspired surface chemistry has received considerable attention because of its simplicity and versatility.17,18 Typical examples include deposition of dopamine,19−22 polyphenols,23 the codeposition with polymers such as poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA), poly(N-vinylpyrrolidone) (PVP), polyethylenimine (PEI), and polyamine,24,25 and postmodification with thiols.26−29 In this work, we combined mussel-inspired surface chemistry and bioinspired honeycomb structures to fabricate superhydrophobic surfaces with different adhesion properties and demonstrated the potential application of the surfaces in no-loss transportation of microdroplets. The present work provides a facile approach to hierarchically structured superhydrophobic surfaces with tunable water adhesion.

2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis of polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) (PS247-b-PDMAEMA14, Mn = 27900 g mol−1, Mw/Mn = 1.24) by atom transfer radical polymerization was described elsewhere.30−32 Dopamine hydrochloride (Sigma-Aldrich, AP), tris(hydroxymethyl) aminomethane, and acetone (Sinopharm Chemical Reagent Co., Ltd., AP) were used as received. 1H,1H,2H,2HPerfluorodecanethiol (PFDT) was purchased from Aladdin (China) and used without further purification. Poly(ethylene terephthalate) (PET) films were kindly provided by Hangzhou Tape Factory (China) and cleaned with acetone for 5 min and repeated three times before use. Water used in the experiments was deionized and ultrafiltrated to 18.2 MΩ with an ELGA LabWater system. All other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. (China) and used without further purification. 2.2. Formation of Honeycomb and Pincushion-Like Films via the Breath Figure Method. The breath figure method was operated according to our previously reported procedure.33,34 The polymer, PS247-b-PDMAEMA14, was dissolved in carbon disulfide to form a homogeneous solution at a concentration of 3 mg/mL at room temperature. An aliquot of 50 μL of the polymer solution was drop-cast onto a PET substrate placed under a 2 L/min humid airflow (∼25 °C B

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ordered with surface pore diameters of 1.46−1.85 μm (Figure 1). Before being subjected to deposition of dopamine, the films

sample was initially placed on the stage of the contact angle meter, and a stainless needle was positioned in the path of the parallel light beam. A sessile drop of water with a volume of 2 μL was vertically deposited on the sample using the equipped syringe pump, and the static water contact angle was recorded and calculated using software. To evaluate the time dependence of water droplets, a homemade chamber was used. A film sample was placed in the chamber, and a few droplets (typically 8 droplets) were deposited around the measuring droplet beforehand to saturate the environment to minimize the effects of evaporation on the dynamic contact angles. Sliding contact angles were measured by mounting the film samples on a tilted stage which can be tilted in the range of 0− 180°, and the volume of water droplets is 8 μL. All measurements were executed at more than five different spots for each sample. The average values or typical curves are reported. 2.5. Transportation of Water Droplets. Transportation of water droplets was conducted on a stage that can move up and down. Briefly, a fluorinated film with pincushion structures was mounted on the stage, on which a water droplet with a volume of 8 μL was deposited. The droplet was uplifted to contact with another downward-facing surface such as a fluorinated honeycomb film. Then, the fluorinated film with pincushion structures was moved down, and the droplet was transported to the other surface. Other samples with different adhesive properties were also evaluated for transportation of water droplets. 2.6. Surface Morphology and Chemistry Characterizations. A field emission scanning electron microscope (FESEM, S-4800, Hitachi) was used to image the surface morphology of films at an accelerating voltage of 3 kV. Prior to observation, the samples were sputtered with gold using an ion sputter JFC-1100. The distribution of the fluorine element on the film surfaces was mapped by energy-dispersive X-ray (EDX) analysis. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FT-IR) measurements were carried out on a Nicolet 6700 FTIR spectrometer equipped with an ATR cell (ZnSe, 45°). Thirty-two scans were taken for each spectrum at a nominal resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI-5000C ESCA system (PerkinElmer, USA) with Al Kα radiation (1486.6 eV). In general, the X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 45°. The pass energy was fixed at 93.9 eV to ensure sufficient sensitivity. The base pressure of the analyzer chamber was about 5 × 10−7 Pa. Survey spectra (0−1200 eV) and core spectra that have much higher resolution were both recorded. Binding energies were calibrated using the containment carbon (C1s = 284.6 eV). Data analysis was carried out with the PHI-MATLAB software provided by PHI Corporation.

Figure 1. Top-down SEM images of honeycomb films with dopamine deposition time of (a) 0, (b) 6, (c) 18, and (d) 24 h, followed by a 24 h reaction with fluorinated thiol. Insets show images of water droplets (2 μL) placed on the films. (e,f) Pincushion-like surfaces without modification (e) and modified by 24 h dopamine deposition and 24 h reaction of fluorinated thiol (f). Insets show water contact angles and the photos. Scale bar: 10 μm.

were fully prewetted by ethanol, which makes three-dimensional conformal deposition possible, that is, modification on both the external surface and the pore walls of the films (Scheme 1). Deposition of dopamine at 2 mg/mL for 24 h slightly increases the hydrophilicity of honeycomb films, decreasing the apparent water contact angle from ∼115° to ∼88° (Figure 2b). Post-treatments of the polydopamine-coated films with 1H,1H,2H,2H-perfluorodecanethiol (PFDT) may produce fluorinated, hydrophobic surfaces (Scheme 1 and Figure 2). Some different treatment procedures have been reported. For example, it can be performed by a two-step procedure, that is, deposition of dopamine and then the reaction with thiols, and the polydopamine coatings may be treated with NaOH26 or added with triethylamine.17 A twostage protocol was also reported by directly adding thiols or H2O2 into the solution after a short period of precoating with dopamine.35−37 We here used a convenient two-stage process in which the rinsing step and changing the solution are avoided. To obtain optimal experimental conditions for the construction of hydrophobic surfaces, some factors, including reaction time and concentration of the thiol, were studied on honeycomb film surfaces. Figure 2a and insets in Figure 1a−d show that the water contact angles increase and then level off with the deposition time of dopamine. The reaction time of the fluorinated thiol also shows a similar effect (Figure 2b). As

3. RESULTS AND DISCUSSION 3.1. Mussel-Inspired Surface Modification. Catechol and the derivatives, especially dopamine, have been considered as a versatile platform in surface chemistry because they are able to form robust polymer films on a wide range of surfaces.18 In this work, honeycomb patterned porous films were prepared via the breath figure method according to our previously reported procedure.33,34 The breath figure process is very fast, and a piece of film takes only ∼1 min. The resultant films are highly C

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Figure 2. Apparent water contact angles on honeycomb films modified at different conditions. (a) Effects of deposition time of dopamine with a fixed 24 h reaction with fluorinated thiol. (b) Effects of reaction time of fluorinated thiol with a fixed 24 h dopamine deposition. (c) Effects of concentration of fluorinated thiol. All the films were fully prewetted using ethanol before modification.

range of 1100−1300 cm−1, arising from vibrations of −CF2− and −CF3 (Figure 3c). XPS results support the surface modification (Figure 4). A new peak at around 689.1 eV,

shown in Figure 2c, water contact angles increase first with the concentration of the thiol and then decrease gradually. As the mixture of dopamine and thiol was used in the second stage, too high concentration of thiol may hinder the self-polymerization and assembly of dopamine.38 A similar phenomenon was also found in the codeposition of dopamine with PEI.39 3.2. Surface Morphology and Chemistry. Figure 1 shows surface morphologies of films prepared under different conditions. A very uniform thin layer of polydopamine was formed on the film surface. Longer deposition time may cause some particles on the surfaces. The particles are aggregates formed by the self-polymerization of dopamine in the solution, and they adsorb onto film surfaces by gravity or interaction. Correspondingly, one can observe that the dopamine solution turns into black and opaque with time. Overall, it can be seen that the honeycomb porous structure remains well after the coating of polydopamine and fluorination. The pincushion-like structure also remained after the modification (Figure 1e and f). The surface chemistry of the films was analyzed by ATRFTIR, XPS, and EDX. Figure 3 shows ATR-FTIR spectra of the films. PS-b-PDMAEMA film is characterized with a strong peak at 1712 cm−1, which is attributed to the stretching vibration of carbonyl groups (Figure 3a). The polydopamine-coated film has a wider and stronger peak at 1601 cm−1 that can be assigned to CC resonance vibrations in the aromatic rings. The fluorinated film shows some sharp and strong peaks in the

Figure 4. XPS spectra of honeycomb films (a) without modification, (b) with 24 h dopamine deposition, and (c) with 24 h dopamine deposition followed by 24 h fluorination.

from fluorine 1s, can be observed. In addition to the peak at 284.7 eV (C 1s), an isolated peak appears at 291.8 eV, which is attributed to C−F (the left inset in Figure 4). Moreover, the sulfur 2p signal at 163.8 eV can also be found (the right inset in Figure 4). The surface chemistry and the distribution of fluorine element were also characterized by EDX spectroscopy and mapping (Figure 5). The results also demonstrated a fluorinated surface. It can be seen from the mapping image (Figure 5d) that the film underwent conformal modification in this work, instead of site-specific assembly which can be modulated by prewetting or preblocking.32 3.3. Surface Wettability. A flat polystyrene film has a water contact angle of about 89°.40 As a result of high surface porosity, the honeycomb film has a static water contact angle of about 115° (Figure 1a). It is well-known that a highly porous film may entrap air and stay in the Cassie state or the metastable Cassie state (a transition state from the Cassie state to Wenzel state). The apparent contact angles (θM) can be theoretically calculated using Cassie and Baxter’s law: cos θM = (1 − fpore ) × cos θpolymer + fpore × cos θpore

Figure 3. ATR-FTIR spectra of honeycomb films (a) without modification, (b) with 24 h dopamine deposition, and (c) with 24 h dopamine deposition followed by 24 h reaction with fluorinated thiol.

where f pore is the area fraction of pores at the surface; θpolymer is the water contact angle of a polymer in the form of a flat and D

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(Figure 7). It is interesting that water droplets pinned on both the original and fluorinated honeycomb film surfaces at any

Figure 5. EDX spectra of honeycomb films (a) without modification, (b) with 24 h dopamine deposition, and (c) with 24 h dopamine deposition followed by 24 h reaction with fluorinated thiol. (d) Distribution of the fluorine element on sample (c) surface by EDX mapping.

Figure 7. (a−d) Digital photographs of water drops on honeycomb films (a) without modification, (b) with 24 h dopamine deposition, (c) with 24 h dopamine deposition followed by 24 h reaction with fluorinated thiol, and (d) pincushion-like films with 24 h dopamine deposition followed by 24 h reaction with fluorinated thiol. (e,f) Photographs showing water drops on tilted surfaces of samples (c) and (d), respectively.

smooth film (∼89° for a PS film); and θpore is the contact angle of the pore, which is 180° in the Cassie state. The f pore can be estimated from the SEM images shown in Figure 1 (f pore = 0.39−0.47). The calculated water contact angles (112.3° ∼ 117.5°) are close to the experimental result, 115°, indicating that the film is in the Cassie state. The water contact angle on a fluorinated honeycomb film can be as high as 150°. If the honeycomb film was converted into a pincushion-like surface by removing the top surface layer, which has a pore area fraction more than 80%, the water contact angles on the original and fluorinated film surfaces increase to 133° (Figure 1e) and 160° (Figure 1f), respectively, and the water droplets are rather stable with time, as shown in Figure 6. Although both the fluorinated honeycomb and pincushionlike films have water contact angles larger than 150°, the adhesive property varies. Surfaces of sliding superhydrophobicity (low adhesion) and sticky superhydrophobicity (high adhesion) are different, typically, in the contact angle hysteresis. We measured sliding angles of water droplets on tilted surfaces

tilted angles even when it is turned upside down with a tilted angle of 180° or 90° (Figure 7e). The corresponding pincushion-like films show different sliding angles. For an unmodified pincushion-like film, the static water contact is about 133°, and water droplets do not roll off at any tilted angles, which is very similar to the honeycomb films. However, the pincushion-like surface fluorinated with PFDT possesses lower water adhesion, and a water droplet of 8 μL is stable at a tilted angle of 43° but rolls off when it reaches 44° (Figure 7f). Compared with the honeycomb film that is high-adhesive, the pincushion-like surface shows much lower water adhesion. It has been demonstrated that the negative pressure induced by the volume change of sealed air pockets (i.e., the vacuum effect) as well as van der Waal’s attraction contributes to the large adhesive force of honeycomb film surfaces.14 This is similar to kinds of typical high adhesive superhydrophobic examples, that is, surfaces with aligned nanotubes or nanopillars. On the other hand, the adhesive behavior of a water droplet on a superhydrophobic surface is usually governed by the state of the three-phase-contact line (such as shape, length, continuity of contact, and amount of contact), van der Waals forces, and capillary forces.41 The unique surface structure of honeycomb films is another reason for the high adhesive property. The three-phase-contact line is longer on a honeycomb surface than a square pillar surface, which is usually fabricated by lithography. As a result, honeycomb films are high adhesive superhydrophobic. For the pincushion-like surfaces, the calculated contact angles based on Cassie and Baxter’s law (θM > 142.8° for f pore > 0.80) are higher than the experimental results (133°), revealing that they are probably in the metastable Cassie state. The pincushion-like surface is similar to a rose petal on which water droplets can penetrate into the large pores, leading to high capillary force and high adhesion force.42 Consequently,

Figure 6. Typical results of the evolution of water contact angles with time on pincushion-like surfaces before (a) and after (b) dopamine deposition and fluorinated thiol reaction. E

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films, show a water contact angle higher than 130° as well as high water adhesion. However, the mussel-inspired modified pincushion-like film with a water contact angle of 160° is of much lower water adhesion, on which no-loss transportation of water droplets was demonstrated. As the breath figure method is very simple, efficient, and versatile to various materials44−47 and substrates including flat or 3-dimensional surfaces,7−9 the present method is promising in applications such as microfluidic systems.

the pincushion-like surface is also highly adhesive, although the contribution of the vacuum effect is obviously smaller compared with honeycomb films. On the other hand, it is generally accepted that surface chemistry is important to the adhesive property. For example, introducing hydrophobic components can change a surface from water-adhesive to water-repellent.43 Therefore, the pincushion-like musselinspired modified film that has a fluorinated surface shows much lower water adhesion. 3.4. Microdroplet Transportation. The above-mentioned results reveal that the mussel-inspired surface modification endows honeycomb films and the corresponding pincushionlike films with different water adhesive properties. Such surfaces are useful in no-loss transportation of microdroplets. Figure 8a

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87953763. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21374100).

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REFERENCES

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Figure 8. Transportation of a water droplet (a) from a pincushion-like fluorinated film (bottom) to a fluorinated honeycomb film (upper), (b) from a pincushion-like fluorinated film to the original pincushionlike film, and (c) from a fluorinated honeycomb film to a pincushionlike fluorinated film.

demonstrates a typical transportation process. A pincushionlike fluorinated film that is low adhesive superhydrophobic was mounted on the stage of the contact angle meter. The water droplet on the surface was uplifted and contacted with a downward-facing fluorinated honeycomb film that has a high adhesive superhydrophobic surface. With lowering the low adhesive surface, the water droplet can be completely transported to the upper surface without any loss. The upper surface can be replaced by other high adhesive surfaces, such as unmodified honeycomb films and pincushion-like films (Figure 8b). However, the upper needs to be high adhesive surfaces. No-loss transportation is infeasible when transporting water droplets from a high adhesive surface to a low adhesive one, for example, from a fluorinated honeycomb film to a pincushionlike fluorinated film (Figure 8c).

4. CONCLUSIONS In summary, we designed novel superhydrophobic surfaces with tunable adhesive property by combining mussel-inspired surface chemistry and multiscale surface structures. Honeycomb patterned films with highly ordered surface pores turn to superhydrophobic after conformal deposition of dopamine followed by fluorination through the reaction with a thiol. The honeycomb films, even with a water contact angle higher than 150° after fluorination, are high-adhesive. Pincushion-like surfaces, prepared by removing the top layer of the honeycomb F

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DOI: 10.1021/jp513001k J. Phys. Chem. C XXXX, XXX, XXX−XXX