Microdrop-Assisted Microdomain Hydrophilicization of

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Microdrop-Assisted Microdomain Hydrophilicization of Superhydrophobic Surfaces for High-Efficiency Nucleation and Self-Removal of Condensate Microdrops Dandan Xing, Feifei Wu, Rui Wang, Jie Zhu, and Xuefeng Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19868 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Microdrop-Assisted Microdomain Hydrophilicization of Superhydrophobic Surfaces for High-Efficiency Nucleation and Self-Removal of Condensate Microdrops Dandan Xing1,2,†, Feifei Wu2,†, Rui Wang2, Jie Zhu2, and Xuefeng Gao1,2* 1

School of Nano Technology and Nano Bionics, University of Science and Technology of

China, Hefei 230026, P. R. China 2

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou

215123, P. R. China †These

authors contributed equally.

*Corresponding Author: [email protected]

Abstract Superhydrophobic-hydrophilic hybrid surfaces have attracted intensive interest due to their significant academic and commercial values. However, almost all reported microdomain hydrophilicization methods rely on costly micropatterning techniques that need special instrument. Here, we report a microdrop-assisted method for microdomain hydrophilicization of low-adhesive superhydrophobic surface and demonstrate its utility in high-efficiency nucleation and self-removal of condensate microdrops. Micrometer-sized fogdrops containing polyvinyl alcohol molecules can be selectively captured by breath figures of superhydrophobic surfaces with specific sizes and spatial distributions and convert into desired hydrophilic microdomains after thermal evaporation. After exploring the influence of hydrophilic microdomains’ distributions and sizes to surface wettability, adhesion and condensation dynamics, we achieved an optimal hybrid surface, which owns 240% average microdrop density, 387% microdrop self-removal rate and 75% average microdrop diameter as compared to the contrast superhydrophobic surface with uniform chemistry nature. This method is dispensed with special equipment, easy to implement, very cheap and eco-friendly,

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which would help develop other superhydrophobic-hydrophilic hybrid surfaces with different functions such as water harvesting, dehumidification and heat exchange.

Keywords superhydrophobic surface, breath figure, microdomain hydrophilicization, highefficiency nucleation, condensate microdrop self-removal

1 Introduction Controlling nucleation density and self-removal efficiency of condensate microdrops via bioinspired super-wettability surfaces have attracted great interest due to their extremely important academic and commercial values, e.g., used for improving the efficiency of power generation, desalination, air conditioning and thermal management.1-4 In principle, spatial distribution and density of condensed microdrops can be controlled by patterned surfaces with wettability contrast, e.g., introducing hydrophilic microdomain patterns on hydrophobic5,6 or superhydrophobic surfaces.7-13 Due to the peripheral superhydrophobicity induced contact line confinement, the wetting mode of condensate on the hydrophilic microdomains would transit from flat liquid films into spherical drops. Grown-up condensate microdrops can self-remove in a jumping way via mutual coalescence as long as coalescence-released excess surface energy exceeds energy dissipation caused by solid-liquid interface adhesion.3,14 Recently, He et al. have demonstrated the feasibility of controlling the preferential nucleation, confined growth and efficient self-removal of condensate microdrops via ethanol-assisted modification of hydrophilic polyvinyl alcohol (PVA) into patterned superhydrophobic microholes.12 Likewise, Hou et al. also demonstrated that the preferential nucleation, confined growth and efficient self-removal of condensate microdrops can be realized by inlaying patterned silicon micropillars with hydrophilic silicon oxide caps into a superhydrophobic nanoneedle structure.13 Clearly, these two methods essentially need photolithography technique to achieve patterned hydrophilic microdomains with desired density and spatial confinement at 2 ACS Paragon Plus Environment

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microscale. To date, almost all reported methods that can implement microdomain hydrophilicization of superhydrophobic surfaces, e.g., microcontact printing,6 mold-assisted photodegradation,9,15 mold-assisted photoinitiated graft polymerization,16-18 superhydrophobic micropattern-assisted PVA absorption8 and organic solvent-assisted micropatterning,7,12,19 rely on photolithography and other microfabrication techniques that have micropatterning ability. However, current micropatterning techniques are costly and require special instruments (e.g., photolithography and reactive ion etching setups) inaccessible to most labs. Therefore, it would be very significant to explore innovative methods that can implement microdomain hydrophilicization of superhydrophobic surfaces without the any assistance of traditional microfabrication techniques. Condensation is a common natural phenomenon and widely used in industry for thermal energy transport. Condensate microdrops with specific sizes and spatial distributions, called as “breath figures”, 20-23 have also been widely utilized in the past two decades as a versatile template for creating polymeric microporous films, which can find significant applications such as cell culture, filtration and ion transport. Inspired from this, we propose a microdropassisted method for modifying hydrophilic microdomains on the superhydrophobic surface, which basic principle is shown in Figure 1a. Micrometer-sized PVA fogdrops are captured by condensate microdrops of breath figures on the superhydrophobic surfaces and converted into desired hydrophilic PVA microdomains after thermal evaporation. In sharp contrast, fogdrops impacting other bared dry regions would instantly bounce off.24,25 Spatial distributions of breath figures can be converted into that of hydrophilic microdomains. Compared with traditional micropatterning methods, our method has apparent advantages: dispensed with special instruments, easy to implement, very cheap and eco-friendly. Such a modification is very significant, e.g., having remarkable merits in high-density nucleation, rapid coalescence and timely self-removal of condensate microdrops, as shown in Figure 1b. Compared with the contrast superhydrophobic surface with uniform chemistry property (Left), the introduction of 3 ACS Paragon Plus Environment


hydrophilic microdomains on superhydrophobic surface (Right) can not only decrease energy barrier of heterogenous nucleation but provide more nucleation sites. The increase of nucleation density can facilitate mutual coalescence of adjacent condensate microdrops at shorter separation distance, which means that merged microdrops can self-remove at smaller sizes. Meanwhile, the decrease of departure size of condensate microdrops would facilitate dynamically release more bare sites for new nucleation, growth and self-removal. That is, microdrop self-removal and self-renewal rate of hybrid surfaces per unit time per unit area may be higher as compared to the contrast superhydrophobic surface.

2 Results and Discussion It’s easily understood that the sizes and spatial distributions of condensate microdrops on the superhydrophobic surfaces are closely related to nanoarchitecture features and condensation conditions, e.g., condensation time, surface subcooling degree and ambient humidity. To verify the feasibility of our method, we chose aligned zinc oxide nanoneedles with condensate microdrop self-removal function as the contrast low-adhesive superhydrophobic surface14,26 (two-step growth process, morphology and XRD spectrum of nanoneedles see Figure S1) and a fixed pretreatment condition (e.g., Peltier temperature ~5 ºC, air temperature ~25 ºC, relative humidity ~70% and condensation time ~60 s) to make similar sizes and distributions of breath figures (chosen reason see Figure S2 and its caption). By means of high-speed and high-resolution microscopic imaging, our studies have demonstrated that PVA-contained fogdrops indeed can instantly bounce off once contacting bared “dry” regions but can be captured once contacting condensate microdrops (see Figure S3). In this case, we explore the influence of spraying time to the sizes and densities of as-prepared hydrophilic microdomains while fixing the same condensation pretreatment and heating conditions. Figure 2a shows a representative environment scanning electron microscope (ESEM) image of the as-prepared superhydrophobic-hydrophilic hybrid surfaces, corresponding to the spraying time of 10 s. It 4 ACS Paragon Plus Environment

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is evident that these characteristic black dots only emerging on the hybrid surfaces are just our modified hydrophilic PVA microdomains, which can be verified by energy dispersive X-ray spectroscopy tests (Figure S4). Figures 2b and 2c show magnified ESEM images of a single hydrophilic microdomain and its surrounding superhydrophobic region without PVA, respectively. The as-prepared ZnO nanoneedles have the characteristic interspace of 42.58 ± 16.2 nm, top diameter of 14.2 ± 2.4 nm, end diameter of 72.0 ± 8.0 nm, and height of 2.65 ± 0.16 µm. Different from the method of ethanol-assisted micropatterning that made hydrophilic PVA penetrate into superhydrophobic microholes,12 our method can realize the modification of PVA on the tips of nanoneedles. From the viewpoint of reducing solid-liquid adhesion of hydrophilic microdomains, our method should be more advantageous since lower adhesion facilitates the self-removal of condensate microdrops at smaller scale.3,14,26 Since the sizes and densities of hydrophilic microdomains can apparently influence both the nucleation density and self-removal efficiency of condensate microdrops, we explore the influence of spraying time to the sizes and densities of hydrophilic microdomains. It’s found in our experiments that the sizes and densities of hydrophilic microdomains can increase with the extension of spraying time, as shown in Figure 2d-f and Figure S5. As compared to ESEM imaging (Figure 2a), fluorescence imaging can more intuitively present the distribution of hydrophilic microdomains. Rhodamine B molecules are added into PVA-contained fogdrops to form fluorescence-labelled microdomains (Detailed fabrication process sees Experimental section).19 As shown in Figure 2d-f, Rhodamine B-labelled hydrophilic PVA microdomains present red while superhydrophobic regions present black. When spraying time increased from 10 s, 30 s to 180 s, the average diameters of hydrophilic microdomains would increase from ~4.96 μm, ~5.03 μm to ~5.48 μm, accompanying with the increase of density from ~2.5 × 104 cm-2, ~3.7 × 104 cm-2 to ~6.2 × 104 cm-2. Clearly, the size and density of hydrophilic microdomains can be tuned by spraying time.



Compared with transparent glass substrates for fluorescence imaging, copper substrates with high thermal conductivity are more suitable for exploring the condensation performance of nanostructure surfaces. Accordingly, superhydrophobic zinc oxide nanoneedles, used as the contrast sample (called as S1), were grown on copper substrates via the same nanosynthesis and hydrophobicization conditions (Figure S6), where only pretreated seed layer is different: electroplating zinc film for copper substrates to reduce lattice mismatch26 while sintering zinc oxide nanoparticle film for glass substrates.14 To explore the influence of sizes and distributions of hydrophilic microdomains to condensation performance of superhydrophobic surface, we carried out a series of regulation experiments and selected three hybrid surfaces corresponding to spraying time of 10 s (S2), 30 s (S3) and 180 s (S4) as exemplified samples (Figure S7) for presenting the structure-property variation trend. Detailed fabrication process refers to Experimental section. We first measured contact angles (θ) and adhesive forces (F) of these four samples, as shown in Figure 3 and Figure S8. The θ and F values of the contrast S1 sample are 160º and 3.12 μN, respectively. With the increase of spraying time, the θ values of hybrid surfaces slowly decreased to 157º for S2, 151º for S3 and 146º for S4, but their F values dramatically increased to 13.13 μN for S2, 24.50 μN for S3 and 64.38 μN for S4. Subsequently, we evaluate condensation dynamics of three hybrid surfaces (S2, S3 and S4) and the contrast S1 surface via optical imaging, as shown in movie S1. Interestingly, coalescence/jumping and coalescence/no-jumping events can be found on all surfaces. Nojumping events of S1 surface are rare, which may be ascribed to a small number of surface defects, while no-jumping events occurring on the hybrid surfaces are considerable, which is the results of introducing hydrophilic microdomains. Figure 4a shows coalescence/jumping and coalescence/no-jumping phenomena occurring on S2 surface as an example (more details see Figure S9-S12). Through data analysis, we can obtain the numbers of coalescence events (CE), coalescence/jumping events (CJE) and coalescence/no-jumping events (CNJE) of all surfaces, as shown in Figure 4b. CE number (i.e., the sum of CJE and CNJE numbers) can 6 ACS Paragon Plus Environment

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reflect the difference of nucleation density of nanosurfaces. Statistic CE and CJE numbers present the same trend of first increase and then decrease but CNJE numbers progressively increase. As for three hybrid surfaces, S2 surface owns the highest CE and CJE numbers, which are 2.23 and 2.50 times higher as compared to those of S1 surface, and its CNJE number is the lowest, all of which are highly desired. Note that all hybrid surfaces own higher CE number than S1 surface. However, the CNJE number of S3 and S4 surfaces are ~212% and ~274% of S2 surface while their CJE number are only ~45% and ~13% of S2 surface. Both the increase of CNJE number and the decrease of CJE number are undesired. Clearly, as compared to S3 and S4 hybrid surfaces, S2 surface is optimal, showing superior abilities in high-efficiency nucleation and self-removal of condensate microdrops Our studies have indicated that the solid-liquid interface adhesion of hybrid surfaces can dramatically increase with the increase of sizes and densities of hydrophilic microdomains, which is disadvantageous to high-efficiency self-removal of condensate microdrops. That is, the introduction of hydrophilic microdomains necessarily bring about more undesired CNJE, despite their inherent merits in increasing nucleation densities. Accordingly, an ideal superhydrophobic-hydrophilic hybrid surface should own the maximum CE and CJE as well as the minimum CNJE, as compared to the contrast superhydrophobic surface. Clearly, S2 surface is just such an ideal surface as compared to S3 and S4 surfaces. However, this is unusual since the number of hydrophilic microdomains (i.e., initial nucleation sites) of S2 surface essentially is not the most as compared to that of S3 and S4 surfaces. Remarkably, the solid-liquid adhesion of S2 surface is the lowest since the size and density of hydrophilic microdomains on the S2 surface are the smallest. It’s known that lower adhesion facilitates the self-removal of condensate microdrops at a smaller scale.3,14,26 That is to say, S2 surface can dynamically release more nucleation sites per unit area and per unit time. This should be the reason why S2 surface has the most CE and CJE and the least CNJE. In sharp contrast, the number of hydrophilic microdomains on S4 surface is the most while their average diameters 7 ACS Paragon Plus Environment


are almost the same with the other two hybrid surfaces (Figure S5). That is to say, S4 surface own the largest solid-liquid interface adhesion, which necessarily results in the lowest coalescence/jumping and self-renewal frequency. Accordingly, it can be easily understood that the total number of coalescence events (occurring at microscale) on S4 surface is the lowest although S4 surface own the highest initial nucleation density (occurring at nanoscale). It’s known that ESEM imaging owns far higher spatial resolution as compared to optical imaging. Accordingly, we used ESEM to further characterize the condensation mass transfer properties of the optimized superhydrophobic-hydrophilic hybrid surface (S2) and the contrast superhydrophobic surface (S1). Figure 5a shows distinct condensation dynamics of S1 and S2 surfaces (More details refer to movie S2). As compared to S1 surface, S2 surface owns much higher microdrop density and microdrop renewal frequency. Condensate microdrops on the S2 surface can circularly generate and self-remove in an apparently higher rate, which is impressive as watching movie S2. To more intuitively present the condensation performance difference of S1 and S2 surfaces, we conducted statistical analyses into the average densities, diameters and self-removal rates of condensate microdrops on these two surfaces, as shown in Figure 5b-d. The average drop densities of S2 and S1 surfaces are ~3.8 × 103 mm-2 and ~1.6 × 103 mm-2, their average drop diameters are ~16 μm and ~12 μm, and their drop self-removal rates are ~4.9 × 107 s-1·m-2 and ~1.2 × 107 s-1·m-2, respectively. As compared to S1 surface, the drop density and self-removal rate of S2 surface increased to 240% and 387% while average drop diameter reduced to 75%, all of which are desired. As compared to the contrast superhydrophobic surface, the optimized hybrid surface showed high-efficiency nucleation and self-removal abilities of condensate microdrops. The high-efficiency self-removal concept has two basic meanings: more condensate microdrops can self-remove and they can more timely self-remove with relatively smaller departure diameter.


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3 Conclusions In conclusion, we have demonstrated that high-efficiency nucleation and self-removal of small-scale condensate microdrops can be realized via a novel microdrop-assisted method for microdomain hydrophilicization of superhydrophobic surfaces. To our knowledge, this is the first report of implementing microdomain hydrophilicization of superhydrophobic surfaces without any assistance of traditional photolithography or micropatterning techniques. Compared with previously reported microdomain hydrophilicization methods,5-13,16-19 our method presents apparent advantages: dispensed with special equipment, easy to implement, very cheap and eco-friendly. Clearly, our research opens a door for large-area and low-cost fabrication of high-performance superhydrophobic-hydrophilic hybrid surfaces with diverse functions such as water harvesting, dehumidification and heat exchange if further integrating with photopolymerization techniques that can form cross-linked hydrophilic polymer covalently-bonded with the substrate in the near future.[27-30] 4 Experimental Section Fabrication of superhydrophobic surfaces. Glass slides and copper plates with the sizes of 2 cm × 4 cm were ultrasonically rinsed in acetone, ethanol and deionized water for 5 min, respectively, and dried by nitrogen gas flow. To reduce lattice mismatch, glass slides were coated with a layer of ZnO nanocrystal seeds by three-cycled treatment of dip-coating in an ethanol solution of 0.005 M zinc acetate and annealing at 350 ºC for 20 min. Likewise, copper plates were coated with a layer of Zn films via electrodeposition in an aqueous solution of 0.1M KCl and 0.2 mM ZnCl2 for 5 min under the potential of -1.8 V. Subsequently, aligned ZnO nanoneedles can be grown on the seeded surface of glass slides and copper plates in an aqueous solution of 0.25 M Zn(NO3)2·6H2O and 2 M NaOH at 60 ºC for 30 min. To achieve surface superhydrophobicity, the as-prepared nanosamples were further modified with 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane molecules (Adamas Reagent Co., Ltd., Switzerland). The nanosamples were placed together with a glass cup filled with 10 μL fluorosilane liquid 9 ACS Paragon Plus Environment


into a glass container (Φ145 mm × 70 mm). The container was sealed with a cap, then placed in an oven and maintained at 120 °C for 2 h. Note that glass substrates were used only for fluorescence imaging. Microdomain hydrophilicization of superhydrophobic surfaces. A microdrop-assisted method is smartly proposed for creating hydrophilic microdomains on the superhydrophobic surfaces, which involves three steps. First, breath figures with specific sizes and distributions were formed on the superhydrophobic surfaces under a unified pre-treatment condition: the temperature of Peltier cooling stage ~5 ºC; condensation time ~60 s (as shown in Figure S2); ambient temperature ~25 ºC and relative humidity ~70%. Subsequently, the pre-treated superhydrophobic surfaces with breath figures were used for collecting PVA-contained fogdrops for different time (e.g. 10 s, 30 s and 180 s). PVA-contained fogdrops were generated via atomizing a 0.2 wt% PVA aqueous solution in a humidifier (Bear electric appliance Co., Ltd., China). Finally, the hydrophilic PVA micropatches were in-situ formed on the superhydrophobic surfaces after drying treatment at 60 ℃ for 10 min on a hot stage. Note that hydrophilic PVA microdomains could be fluorescence-labelled by adding 0.2 g Rhodamine B into 0.2 wt% PVA aqueous solution (200 ml). Characterization: Surface morphologies of nanosamples were tested under environment scanning electron microscope (ESEM, FEI Quanta250 FEG, USA) at 20 keV. An inverted fluorescence microscope (Eclipse Ti-U, Nikon, Japan) was used to compare the distributions and sizes of Rhodamine B labelled hydrophilic PVA microdomains. Water contact angles and adhesive force were measured by a goniometer (DataPhysics OCA20, Germany) and a microelectromechanical balance (DataPhysics DCAT21, Germany), respectively. A high-speed microscopic optical imaging system with magnification of ×800 (Keyence VW-9000, Japan) was used. Average values of CE, CJE and CNJE numbers of S1, S2, S3 and S4 surfaces were obtained by testing three samples for each specification and randomly analysing 200 s period of condensation process on five different locations for each sample, which were captured in 10 ACS Paragon Plus Environment

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the same condition: the temperature of Peltier cooling stage ~5 ºC; ambient temperature ~25 ºC and relative humidity ~70%. To achieve ESEM imaging, the temperature of Peltier cooling stage and water vapor pressure were set to be ~270 K and ~4.5 Torr, respectively. To ensure better imaging contrast as well as alleviate drop heating effect, the electron beam voltage was set at ~20 keV. ESEM images were taken under the magnification of ×1000 at the frame rate of 1 fps. Exemplified by a statistic period of 140 s and a constant imaging area of 278 μm × 139 μm, average drop densities and diameters of all surfaces were obtained by averaging their transient values corresponding to each ESEM image taken every 10 s. Drop self-removal rates were obtained by counting the numbers of self-removed condensate microdrops per unit time per unit area.

Supporting Information Movie S1 shows condensation dynamics difference of the contrast superhydrophobic surface and three hybrid surfaces taken by optical imaging. Movie S2 shows condensation dynamics difference of the contrast superhydrophobic surface and the optimized hybrid surface (S2) taken by ESEM imaging. In-situ growth of closely-packed aligned ZnO nanoneedles on transparent glass surface (Figure S1) and copper surface (Figure S6); Microdrops diameters and densities of breath figures formed on superhydrophobic surfaces varied with condensation time (Figure S2); Time-lapsed optical image of typical bouncing and sticking behaviors as a polymeric fogdrop impacts the superhydrophobic aligned nanoneedle surface without and with condensate microdrops (Figure S3); EDX spectra of superhydrophobic area and hydrophilic PVA area (Figure S4); Statistic hydrophilic microdomains’ sizes and distribution of as-prepared hybrid surfaces corresponding to spraying time of 10s, 30s and 180s (Figure S5); Supplemented SEM images of copper-based hybrid surfaces (Figure S7); Measured adhesive force curves of the contrast superhydrophobic surface and three hybrid surfaces (Figure S8); Time-lapse optical images showing typical coalescence/jumping and 11 ACS Paragon Plus Environment


coalescence/no-jumping events on the contrast superhydrophobic surface and three hybrid surfaces (Figure S9-12). This material is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXXXX.

Author information: †These authors contributed equally. *Corresponding Author: [email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests.

Acknowledgements This work was supported by National Key R&D Program of China (2017YFB0406100), National Natural Science Foundation of China (21573276), Youth Innovation Promotion Association CAS (2011233), and Natural Science Foundation of Jiangsu Province (BK20170007 and BK20170425). The authors thank Prof. Lei Jiang for his discussion. References: (1) Park, K. C.; Kim, P.; Grinthal, A.; He, N.; Fox, D.; Weaver, J. C.; Aizenberg, J., Condensation on Slippery Asymmetric Bumps. Nature 2016, 531, 78-82. (2) Cho, H. J.; Preston, D. J.; Zhu, Y.; Wang, E. N., Nanoengineered Materials for LiquidVapour Phase-Change Heat Transfer. Nat. Rev. Mater. 2016, 2, 16092. (3) Gong, X.; Gao, X.; Jiang, L., Recent Progress in Bionic Condensate Microdrop SelfPropelling Surfaces. Adv. Mater. 2017, 29, 1703002. (4) Chen, X. M.; Wu, J.; Ma, R. Y.; Hua, M.; Koratkar, N.; Yao, S. H.; Wang, Z. K., Nanograssed Micropyramidal Architectures for Continuous Dropwise Condensation. Adv. Funct. Mater. 2011, 21, 4617-4623. 12 ACS Paragon Plus Environment

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Figure 1 (a) Schematics showing a novel microdrop-assisted method for microdomain hydrophilicization of superhydrophobic surfaces. Top diagram shows that micrometer-sized fogdrops with PVA molecules can be selectively captured by condensate microdrops on the surface of superhydrophobic nanoneedles while bouncing off other bared dry regions. Bottom diagram shows the formation of hydrophilic PVA microdomains after evaporation of water. The spatial distributions of breath figures can be converted into those of hydrophilic microdomains. (b) Schematics showing that the hybrid surface (Right) owns more nucleation density and smaller departure diameter as compared to the contrast superhydrophobic surface (Left). Dotted circles denote the imminent coalescence and departure of adjacent condensate microdrops.


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Figure 2. Representative ESEM images of the as-prepared hybrid surface (a) and its local magnification images with (b) and without (c) hydrophilic microdomain. (d-f) Fluorescence images of hybrid surfaces, corresponding to spraying time of 180 s, 30 s and 10 s. Red dots denote hydrophilic PVA microdomains dyed with Rhodamine B while black background shows superhydrophobic surface without PVA. Clearly, the sizes and distributions of hydrophilic microdomains can be tuned by spraying time.



Figure 3. Histogram showing adhesive forces of the contrast superhydrophobic surface (S1) without spraying treatment and the hybrid surfaces corresponding to spraying time of 10 s (S2), 30 s (S3) and 180 s (S4). Insets showing their contact angles.


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Figure 4. (a) Lapse-time optical images showing representative coalescence/jumping event (CJE, top) and coalescence/no-jumping event (CNJE, bottom) of S2 surface. (b) Average values of CE, CJE and CNJE numbers of S1, S2, S3 and S4 surfaces. CJE and CNJE are denoted by red and green, respectively.



Figure 5. (a) Lapse-time ESEM images showing distinct condensation dynamics on S1 and S2 surfaces. Red and blue circles show the status of microdrops before and after coalescence. (b-d) Statistical average drop diameters (b), drop densities (c) and drop removal rates (d) of S1 and S2 surfaces. Clearly, the optimized superhydrophobic-hydrophilic hybrid surface exhibits remarkable high-efficiency nucleation and microdrop self-removal performaces as compared to the contrast superhydrophobic surface.


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Microdrop-Assisted Microdomain Hydrophilicization of

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