A Facile All-Solution-Processed Surface with High Water Contact

Research Article www.acsami.org

A Facile All-Solution-Processed Surface with High Water Contact Angle and High Water Adhesive Force Mei Chen,†,‡ Wei Hu,§ Xiao Liang,†,‡ Cheng Zou,†,‡ Fasheng Li,∥ Lanying Zhang,*,†,‡ Feiwu Chen,§ and Huai Yang*,†,‡ †

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China ‡ Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, People’s Republic of China § Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ∥ Department of Chemistry, Dalian Medical University, Dalian 116044, People’s Republic of China S Supporting Information *

ABSTRACT: A series of sticky superhydrophobicity surfaces with high water contact angle and high water adhesive force is facilely prepared via an all-solution-processed method based on polymerization-induced phase separation between liquid crystals (LCs) and epoxy resin, which produces layers of epoxy microspheres (EMSs) with nanofolds on the surface of a substrate. The morphologies and size distributions of EMSs are confirmed by scanning electron microscopy. Results reveal that the obtained EMS coated-surface exhibits high apparent contact angle of 152.0° and high water adhesive force up to 117.6 μN. By varying the composition of the sample or preparing conditions, the sizes of the produced EMSs can be artificially regulated and, thus, control the wetting properties and water adhesive behaviors. Also, the sticky superhydrophobic surface exhibits excellent chemical stability, as well as long-term durability. Water droplet transportation experiments further prove that the as-made surface can be effectively used as a mechanical hand for water transportation applications. Based on this, it is believed that the simple method proposed in this paper will pave a new way for producing a sticky superhydrophobic surface and obtain a wide range of use. KEYWORDS: polymerization-induced phase separation, epoxy microspheres, hierarchical structures, high apparent contact angle, high water adhesive force co-workers have revealed that the “petal effect” represents the Cassie impregnating wetting state, in which water penetration into the microroughness results in high adhesion with the solid surface, whereas the presence of nanoroughness provides a high apparent CA.5,14 In more detail, Bhushan et al. revealed that hierarchically structured plant surfaces can have both adhesive and nonadhesive properties at the same time with a high apparent CA, and large scale of the microstructure are more beneficial to water penetration, leading to high adhesion between water and the solid surface.15,16 Since the discovery of the petal effect, its great potential applications in dew collection as a water source for residence in arid regions, antidrip function for greenhouse films in agricultural countries, and liquid transportation without loss or contamination for microsample analysis have drawn extensive attention.17−19 Great efforts have been paid to

1. INTRODUCTION Superhydrophobic surfaces with high solid/liquid adhesion have attracted great interest in recent years, because of their fascinating properties resulting from the unique multiple hierarchy structures.1−3 On such surfaces, water droplets present an apparent contact angle (CA) of >150°, but will not roll off, even when the surface is turned upside down. This means that such surfaces possess a strong adhesive force to water. Such unique property is initially found in rose petals and therefore is termed as the “petal effect”, i.e., a superhydrophobic surface with a large contact angle hysteresis (CAH) and high adhesive force (AF) to water.4−8 Inspired by nature, great efforts have been developed to better understand these super water-repellent surfaces,9−13 especially the wettability and adhesion behaviors, because of their scientific value and economic applications. The discovery of novel hierarchical micropapillae and nanofolds on the petal surfaces of red roses leads to the proposal that these multiscale structures might be the reason for the unique properties observed. Based on the experimental observations and theoretical analysis, Jiang and © 2017 American Chemical Society

Received: May 25, 2017 Accepted: June 14, 2017 Published: June 14, 2017 23246

DOI: 10.1021/acsami.7b07429 ACS Appl. Mater. Interfaces 2017, 9, 23246−23254

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Chemical Structures of (a) Tetrathiol, (b) Diglycidylester, and (c) DMAP Used in This Study, Respectively; (d) Parameters of the LCs Used in the Study

Table 1. Materials Composition and Physical Parameters of the EMS Surface sample A1 A2 A3 A4

a

composition, tetrathiol/ diglycidylester/LCs/ DMAPb (wt %)

average size of EMSs (μm)

surface roughness, Rac (μm)

contact angle, CAd (deg)

advancing contact angle, θAd (deg)

receding contact angle, θRd (deg)

contact angle hysteresis, CAH (deg)

solid surface energy, σS (mJ/m2)

adhesive force, AFe (μN)

25.00/25.00/50.00/5.00 20.00/20.00/60.00/5.00 15.00/15.00/70.00/5.00 10.00/10.00/80.00/5.00

4.30 5.03 7.03 9.27

4.783 4.554 3.975 3.764

152.0 150.5 146.7 141.3

156.0 154.7 150.1 148.7

92.6 88.3 78.6 75.6

63.4 66.4 71.5 73.1

0.567 0.644 0.925 1.170

103.4 105.5 113.2 117.6

a Samples A1−A4 were cured at 353.15 K. bThe amount of DMAP is the total weight of monomers. cAverage error = ±0.180 μm. dAverage error = ±2.1°. eAverage error = ±2.0 μN.

meet the requirement of treating a wide range of surfaces and simplify the tedious procedures in preparing these superhydrophobic surfaces with high solid/liquid adhesion. However, achieving such a target is still a great challenge, because these functional surfaces have elaborately designed multiscale structures, but the PIPS method usually results in a lack of control of the structure.42 In this paper, taking advantage of the PIPS method, we present a facile all-solution-processed (ASP) surface with high apparent water CA and high water AF (i.e., sticky superhydrophobicity) from a PDLC system containing epoxy/ mercaptan monomers and liquid crystals (LCs) for the first time. Different from previous methods,20−32 the current method is much more universal, since epoxy resin provides great adhesiveness to a variety of substrates including metallic, nonmetallic surfaces, and most of the polar plastic, and can be processed under robust conditions facilely, yielding micrometer- and nanometer-scale hierarchical structures of epoxy microspheres (EMSs) on the surfaces. This proposed approach can be used as a representative model and provide a new insight into surface treatment. We anticipate that such behaviors of sticky superhydrophobicity with controllable morphology will offer the surface with fascinating and practical applications.

develop different methods for the preparation of these superhydrophobic surfaces with high solid/liquid adhesion artificially, including electrodeposition,20 hydrothermal preparation,21 lithography,22 the solution immersion process,23,24 and many more.25−32 However, all these methods are subject to some certain limitations, such as severe conditions, tedious fabrication, expensive materials, or poor durability. Consequently, a more robust, facile, and universal surface treatment strategy that can be applied to a variety of surfaces is much more attractive and desirable. Polymerization-induced phase separation (PIPS), which involves polymerization in an initially homogeneous polymeric syrup containing reactive monomers and nonreactive molecules and forms a phase-separated structure simultaneously, is normally simple and accomplished within one step.33 It has been extensively applied in a broad range of polymer-based materials, such as polymer-dispersed liquid crystals (PDLC),34−37 thermoplastics-toughened epoxy resin,38,39 porous cyanurate,40 or dicyclopentadiene thermosets.41 Furthermore, because of the compatibility of the polymeric syrup with a variety of substrates ranging from rigid glass to flexible films, this method can be compromised with solution-based techniques such as spin-casting, inkjet printing, and spraying. Accordingly, the PIPS method may be a good candidate to 23247


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Scheme 2. Schematic Illustration for the Proposed ASP Surface Treatment: (a) A Homogenous Mixture of the Polymeric Syrup; (b) Spin-Casting the Syrup onto a Substrate; (c) Thermally Curing the Syrup To Form EMSs within the LCs; (d) Dipping the Substrate in Cyclohexane To Fully Extract the LCs; and (e) the Obtained Sticky Hydrophobic Surface Coated with a Layers of EMS

surface profile measurement system (Bruker, Model Contour GT-I 3D). Apparent CA values were measured on a Data-Physics OCA 20 CA system at ambient temperature. The advancing and receding CA values were obtained by dispensing and retracting the volume of the water droplet on the surface, respectively. The acidic and basic water droplets were aqueous solutions containing hydrochloric acid and sodium hydroxide, respectively. The pH values of these droplets were measured by a pH meter (Mettler Toledo, Model FE28-3). The adhesive force was measured using a high-sensitivity microelectromechanical balance system (Data-Physics DCAT 11, Germany). Briefly, a 4.0 μL deionized water droplet was first suspended with a metal ring. The EMS-coated substrate then was placed on the balance stage. The stage was moved upward at a constant speed of 0.01 mm s−1 until the EMS layer contacted the water droplet. The surface then was further raised by a distance of 0.2 mm to ensure a complete contact. Afterward, the sample surface was gradually lowered at 0.01 mm s−1, and the change in the force was monitored using the balance. The force first increased, reached a maximum, and then decreased sharply when detachment occurred. During the process, the maximum force recorded was defined as the adhesive force. The measurement on each sample was repeated five times at different locations on one surface. The pencil hardness of the EMS surface was determined by the method described in ASTM Standard D 3363, which is a standard test method for film hardness using the pencil test.

2. EXPERIMENTAL METHODS 2.1. Materials. Diglycidyl 1,2-cyclohexanedicarboxylate (diglycidylester, 95%), pentaerythritol tetra(3-mercaptopropionate) (tetrathiol, >85%), and 4-dimethylaminopyridine (DMAP, 99%) were purchased from Tianjin Heowns Biochemical Technology, Tokyo Chemical Industry, and Aladdin, respectively. The nematic LC mixture used as solvent in this study was made in-house by mixing different liquidcrystalline monomers to reach a desirable temperature range and suitable viscosity. The chemical structures of the materials and the parameters of the nematic LC solvent are listed in Scheme 1. 2.2. Preparation of the EMSs Layers. To investigate the factors that influence the morphology and microsphere size of the surface, we tried to vary the materials compositions, as well as preparation conditions. As shown in Table 1, as well as Table S1 in the Supporting Information, different LC contents, curing temperatures, and accelerator contents were attempted for the Sample A, B, and C series, respectively. In a typical procedure, 0.2 g Diglycidylester and 0.2 g Tetrathiol were introduced into 0.6 g of LC solvent by vigorous stirring at 343.15 K for 60 s to form a homogeneous solution. After that, 0.02 g of DMAP was quickly added into the above mixtures and stirred for 10 s at room temperature. A total of 0.2 mL of the above mixtures was spin-cast onto a large (20 mm × 20 mm) glass substrate at 1500 rpm for 2 s. Subsequently, the substrate was put into an oven at 353.15 K for 2 h. Then, after it was cooled to room temperature, the substrate was dipped in cyclohexane (AR) for 2 weeks to fully extract the LC solvent. Finally, a sticky superhydrophobic surface was obtained after the substrate was dried under vacuum for 48 h. 2.3. Measurements. The morphologies, the size distributions, and the composition information on the EMSs were observed by scanning electron microscopy (SEM) (Hitachi, Model S-4800) equipped with an ImageJ as a software tool and an energy-dispersive X-ray spectrometry (EDS) system. Thin layers of gold were coated onto the surfaces to eliminate any electric charge problem. In each measurement of the EMS size distributions, more than 100 microspheres were taken into account. The morphology of the EMS after the curing step was observed by environmental scanning electron microscopy (ESEM) (Quanta, Model 200FEG). The optical textures of the surfaces were observed by polarizing optical microscopy (POM) (Carl Zeiss, Model Axio Vision SE64) equipped with a hot stage (Linkam, Model LK-600PM) calibrated to an accuracy of ±0.1 K (see Figure S3 in the Supporting Information). Fourier transform infrared (FT-IR) spectra were obtained on an FT-IR spectrophotometer (Bruker, Model Vector-22) using a KBr pellet over a range of 4000− 600 cm−1. The sample surface roughness was characterized with a

3. RESULTS AND DISCUSSION The detailed procedure of the proposed ASP surface treatment is illustrated in Scheme 2. Typically, one drop of homogeneous syrup containing epoxy monomers, thiols, liquid crystals (LCs), and a trace amount of accelerator was spin-cast onto a substrate (in this paper, we used glass as the substrate, unless specified otherwise) (see Schemes 2a and 2b). Then, phase separation was induced by thermal curing between epoxy monomers and thiols hardeners, forming EMSs with uniform size in LCs (Scheme 2c). Finally, a superhydrophobic surface with high water adhesion coated by densely packed EMSs was obtained after dipping the substrate in cyclohexane to fully extract the LC solvent (see Schemes 2d and 2e). Scanning electron microscopy (SEM) was used to systematically examine the morphology of the coated EMSs layers 23248


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Apart from the LC content, other parameters, such as the curing temperature and the content of accelerator, also impacted the size of the EMSs. During the process of PIPS, curing temperature plays an imperative role, which not only impacts solubility but also the curing rate of the system.46−48 As shown in Figure S2 in the Supporting Information, when the curing temperature is below the critical solubility temperature (343.15 K), a heterogeneous phase structure was observed. In contrast, homogeneous phase separation was obtained when the curing temperature was greater than the solubility temperature. Because of the improvement of the reaction rate at higher temperature, the sizes of the EMSs decreased as the curing temperature increased (see Table S1 and Figure S2 in the Supporting Information). Moreover, the content of accelerate, which can largely influence the curing rate in the process of phase separation, also affects the morphologies of the EMSs. As the accelerant content was increased from 2% to 10% in the syrup, which was attributed to the enhancement of the curing rate, the sizes of the EMSs decreased obviously, which was in accordance with the results obtained by increasing curing temperatures. However, attributed to the extremely fast gelation rate of the syrup, further increasing the accelerant content to 15% resulted in microspheres with nonuniform size (see Table S1 and Figure S3 in the Supporting Information). More interestingly, there existed nanofolds on the top of each surface of the EMSs (inset images in Figure 1). We speculated that, during the curing process, the EMSs were swelled by LCs, which led to the development of stress. When the LCs were extracted by dipping the substrate in cyclohexane and subsequently drying under vacuum, nanofolds on the EMS surface resulting from the relaxation of stress due to various types of underlying mechanical instabilities were consequently formed.49−51 To confirm the proposed formation mechanism of EMS nanofolds, environment scanning electron microscopy (ESEM) was used to characterize the morphology of EMSs after the curing step. From ESEM observation, we found that the surfaces of EMSs were smooth after the curing step as shown in Figure S4 in the Supporting Information, indicating that the nanofolds were generated during the dipping and drying processes. In addition, the chemical structures and compositions of the flat epoxy resin and the EMS layer after extracting LCs were characterized by energy-dispersive X-ray spectroscopy (EDS) and FT-IR (as shown in Figure S5 in the Supporting Information). The EDS results showed that the intensity of elemental C and O in the as-prepared EMSs was largely increased, compared with flat epoxy resin, which can be only contributed by the LC solvent swelled into the EMSs. Figure S5b shows the FT-IR spectra of the flat epoxy resin and the EMS layer. Specifically, the vibrational bands at 3490 cm−1, 2938 cm−1 ∼ 2854 cm−1, 2562 cm−1, 1730 cm−1, 1170 cm−1, and 1030 cm−1 were attributed to ν(O−H), ν(C−H), ν(S−H), ν(CO), ν(C−O−C), and ν(C−O−H), respectively. The appearance of vibrational band at 2215 cm−1 (assigned as the stretching vibration of −CN) indicated that a trace amount of LC residuals still remained inside the EMSs, which was in agreement with the EDS results and supported the hypothesis that the EMSs were swelled by LCs. Generally, flat epoxy resin is a hydrophilic material with an apparent CA of ∼79° (see Figure 2a).52 However, the asprepared EMS layer in this study exhibited superhydrophobic properties. Taking Sample A2 as an example. Figure 2b shows the shape of a water droplet on the as-prepared EMS layer,

with LCs content ranging from 50% to 80% in the Sample A series, as shown in Figure 1. Results showed that the formed

Figure 1. (a−d) SEM photographs and (e−h) size distribution of the EMSs of Sample A1 (panels (a) and (e)), Sample A2 (panels (b) and (f)), Sample A3 (panels (c) and (g)), and Sample A4 (panels (a) and (e)). The insets are magnified views of the corresponding SEM photographs of the EMSs.

epoxy resins were all spherical in shape and had a narrow distribution in size. In addition, a close relationship between the size of these EMSs and the LCs content in the syrup was observed. The average diameter of the EMS was 4.30 μm with an LC content of 50% in Sample A1. Increasing the LC content led to an increase in diameters from 4.30 μm to 9.27 μm, as shown in Figures 1e−h, indicating that the size of the EMSs could be facilely tuned by varying the LC content. This LCcontent-dependent effect on the size of EMSs can be ascribed to curing kinetics.43−45 In the initial development of the curing process, the isotropic syrup was separated into epoxy-rich and LC-rich domains, respectively. Obviously, the curing rate among epoxy seeds decreased as the LC content increased. Thus, as the curing process developed, these epoxy-rich domains had enough space to grow and sufficient time to gather into larger ones, resulting in a larger size of EMSs. However, as shown in Figure S1 in the Supporting Information, when the LC content decreased to 40%, a continuous polymer phase was observed, which coalesce into a co-continuous polymer/LC. By further decreasing the LC content, phase inversion occurred and polymer phase became continuous, because of the difficulty to coalesce into a continuous phase during the curing process. 23249


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Figure 2. (a) Behavior of water droplet on a flat epoxy resin surface with an apparent CA of 79°. (b−d) Shapes of water droplets on the asprepared EMS layer with different tilt angles: 0° (panel (b)), 90° (panel (c)), and 180° (panel (d)).

indicating its superhydrophobicity, with an apparent CA as high as 150.5°. Moreover, the water droplet did not slide and maintained the spherical shape when the substrate was tilted vertically (Figure 2c) or even turned upside down (Figure 2d), showing strong adhesion between the EMS layer and the water, which indicates that the CAH of the as-formed EMSs layer should be very large. Therefore, we used CAH and a highly sensitive dynamic contact angle detector to characterize the adhesion behavior of the layer. As shown in Table 1 and Figure 3, the advancing and receding contact angles of the surface are 154.7° (θA) and 88.3° (θR), respectively, i.e., CAH = 66.4°. As the CAH is a measure of energy dissipation during droplet movement, the solid surface energy (σS) can be estimated from the following equation using CAH:53 σS =

σLV(1 + cos θA ) 2 + cos θA + cos θR

where σLV is the water surface tension (72.8 mN/m). Therefore, the surface energy of the EMS layer is 0.644 mJ/ m2. The relatively low surface energy indicates a hydrophobic surface, which is consistent with the apparent CA. The corresponding AF of the layer to the water drop is as large as 105.5 μN. Such strong adhesion plus the superhydrophobicity of the EMS layer exhibits a typical petal effect, demonstrating the successful preparation of sticky superhydrophobic surface via the ASP treatment.5,54,55 Similarly, the wetting and adhesive properties of other surfaces with different size of EMSs were also systematically investigated. As listed in Table 1 and Figure 3, it can be seen that all the obtained surfaces exhibit a similar quasi-superhydrophobicity with apparent CA values of >140°, and the apparent CA values increase as the sizes of the EMSs decrease. However, an inverse trend between the CAH, σS, and AF versus the size of the EMSs was observed, which indicates that the water adhesion increases as the sizes of the EMSs increases. Previous work has demonstrated that the wettability and adhesion behaviors on a liquid/solid interface had a close relationship with surface chemical compositions and geometrical microstructures of the materials.23 In this work, the sticky superhydrophobicity of the EMSs layers can be attributed to the impacts of the unique microstructures of the surface morphologies, since the chemical composition of the EMSs

Figure 3. Dependence of the (a) apparent CA, (b) CAH, and (c) AF on the LC content.

would not change after polymerization. Combining the behaviors of sticky superhydrophobicity measured above and the hierarchical structures of the EMS layer in Figure 1, we considered that the EMS surface was closely resembled that of a rose petal. According to the Cassie impregnating wetting state model proposed by Jiang’s group,5 water droplets on the EMS surface in this work were inclined to impregnate into the large gaps between the microspheres on the layers but not into the nanofolds on the surface of the spheres (the key reason for generating superhydrophobicity was that the water could not wet the nanofolds), producing a large contact area at the interface between liquid and solid (as shown in Figure 4). Since the surface adhesion (F) was proportional to the contact area (A),8,18,56,57 this large liquid−solid contact area endowed the 23250


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Figure 4. (a) Hierarchical structures of the EMS-coated surfaces with big gaps on the substrate and the water droplet resides in the Cassie impregnating wetting state. (b, c) Surface roughness profile photographs of Sample A1 (panel (b)) and Sample A4 (panel (c)).

superhydrophobic EMS layers with high adhesive property. As shown in Table 1, Figure 4, and Figure S6 in the Supporting Information, the surface roughness (Ra) slightly increases from 3.764 μm for Sample A4 to 4.783 μm for Sample A1. In accordance with the above-mentioned Cassie impregnating wetting state model and other reports,23 the wettability of the EMS layer changed from quasi-hydrophobicity to superhydrophobicity when the diameter of the EMS decreased, which can be ascribed to that the decreasing scales of the hierarchical structures resulted in an increase of the roughness of the surface. However, the relationship between the AF and the size of EMS exhibited a reverse tendency, which can be attributed to the shrinkage in the contact area between the liquid and the solid.23,56 Because of the harsh environmental conditions involved in practical use, the chemical stability and durability of the obtained surfaces was investigated in detail. Taking Sample A2 as an example, Figures 5a and 5b show the water apparent CA and AF values under different pH values. When the water pH increased from 1 to 14, there were no obvious fluctuations for these values and the average apparent CA and AF values were 150° and 105 μN, respectively. These results indicated that the pH value of the water droplet has little effect on the sticky superhydrophobicity of the surfaces, confirming that the surfaces have a robust chemical stability. In addition, such stability remained when the surface was set aside without special protection for at least three months (see Figure 5c), indicating that the surface possesses good durability.

Furthermore, by employing a pencil hardness test, the EMS layer possessed a pencil hardness of 2H, indicating a reasonable hardness in abrasion resistance.58,59 It has been demonstrated that a sticky superhydrophobic surface has great advantages in many industrial applications, such as microsample analysis,17,18,60 and droplet-based microreactors.23,61−63 The key techniques of these applications are based on the effectiveness of transporting a droplet from one surface to another. As shown in Figure 6, a water droplet was first deposited on a surface of Si substrate with an apparent CA of 115.0° and a sliding angle (SA) of 18.0°. The EMS-coated surface obtained with an LC content of 60 wt % then was used to touch and drag the droplet. Because of the high adhesion of the EMS surface, the droplet was completely and effectively transferred to the upper surface. Finally, the droplet was easily released and left on a hydrophilic surface (glass), indicating the practical applications as microreactors for the EMS surfaces.

4. CONCLUSIONS In conclusion, based on the method of PIPS, a facile ASP treatment was developed to prepare a sticky superhydrophobicity surface layer in a PDLC system for the first time. The wetting behaviors of the obtained layer deriving from the Cassie impregnating wetting state can be easily controlled by tuning the size of epoxy microspheres (EMSs) with hierarchical scales on the surface. Also, the as-made surfaces show good chemical stability and excellent long-term durability. Results from water droplet transfer experiment further demonstrated that such 23251


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layers can be effectively used as a mechanical hand in transportation without loss. Unlike the previous techniques, this present ASP method can be applied to functionalize many types of surfaces and processed in a robust condition facilely, attributed to the compatibility of epoxy resin with a variety of substrates, which enlightens a new way of sticky superhydrophobic treatment, and can be potentially used in important industrial applications.

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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.7b07429. EDS and FT-IR characterization of as-prepared epoxy microspheres (EMSs), the effects of the curing temperature and accelerator content on the final morphology of EMSs, and the ESEM photographs of the EMSs after the curing step (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L. Zhang). *E-mail: [email protected] (H. Yang). ORCID

Huai Yang: 0000-0002-3773-6666 Author Contributions

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 This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51333001, 51573003, 51372029, 51561135014), the Major Project of Beijing Science and Technology Program (Grant No. Z151100003315023).

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REFERENCES

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Figure 5. (a) Apparent contact angles and (b) adhesive forces for a water droplet with different pH values on Sample A2, indicating that the obtained surfaces are acid/base-resistant. (c) Long-term storage of the obtained surfaces without special protection.

Figure 6. Process of water transfer: (a) a 4.0 μL water droplet on a Si substrate, (b) the EMS-coated surface touches and adheres to the droplet, (c) the droplet is lifted by the surface, and (d) the droplet is transferred on a glass slide without loss. 23252


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A Facile All-Solution-Processed Surface with High Water Contact

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