Article pubs.acs.org/cm
A Strategy of Antifogging: Air-Trapped Hollow Microsphere Nanocomposites Mingqian Zhang, Lei Wang, Shile Feng, and Yongmei Zheng* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China. S Supporting Information *
ABSTRACT: Excellent durable antifogging film, which is composed of hollow microspheres and ZnO nanorods, is fabricated via combining airless spray and crystal growth methods. Because the thermal insulation effect of hollow microspheres strengthens the superhydrophobicity of the micro- and nanostructure composite film at lower temperatures, durable antifogging is realized successfully. When the film is placed in a low-temperature, highhumidity environment, supercooled tiny water droplets cannot spread on it and can easily coalesce and jump off from the surface, exhibiting the robust superhydrophobic and antifogging performance for a significantly long time. This investigation provides insight into designing structured thermal insulation surface materials to realize antifogging that can be applied into microdevices used in cold, high-humidity environments.
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INTRODUCTION Antifogging is very significant in our daily life and industry, e.g., prevention of metal surfaces from corrosion and contaminants, equipment insulation capacity, etc.,1−9 and could greatly reduce economic losses. Much effort has been spent investigating antifoging properties on surfaces of materials10−16 and elucidating the mechanism of fog formation. Various methods have been developed to fabricate the antifogging materials. For example, Watanabe et al.17,18 prepared a photocatalytic TiO2 nanoparticle coating that became superhydrophilic under UV irradiation, and Gao et al.19 created a dry-style superhydrophobic antifogging surface by soft lithography. We demonstrate that antifogging surfaces can be developed via two routes: in a superhydrophilic manner or in a superhydrophobic manner. However, either superhydrophilic or superhydrophobic manner would fail in the long term in reality, because the solid−gas interface would usually be substituted by a solid−liquid interface, accordingly, leading to a change in solid−liquid−gas three-phase lines in wet contact mode for droplets.20−22 In particular, at low temperatures, antifogging of a superhydrophilic surface is invalid and leads to collapse of droplets due to complete wetting on the surface. As for the superhydrophobic surface, a feasible way to prevent the surface from wetting to retard fog formation or drive fog droplet coalescence for shedding off is sought. In fact, it is crucial to improve the ability of antifogging at low temperatures to wet the surface. According to the classical nucleation theory,23−25 a supercooled tiny water droplet would condense and grow only when two conditions were satisfied. The first is the existence of the condensation nucleation points, and the other is that the phase transition energy is transferred from the supercooled water droplet to the substrates at lower temperatures. © 2017 American Chemical Society
Controlling the formation of antiwetting related to the design of a microstrucutre surface is still challenging. In particular, the durability, stability, and robustness of performance on the antifoging properties under low-temperature, high-humidity conditions were examined. Here, we present an “enough-air-trapped” strategy to design antifogging structures of a film, which is composed of soda lime borosilicate glass hollow (SGH) microspheres and ZnO nanorods (denoted as ZnO-on-SGH). We reveal its excellent superhydrophobic and antifogging properties, because the air trapped in the nanostructure and in the hollow microsheres greatly improved the air cushion of the material to keep the robust superhydrophobic and antifogging performance for a significantly long time even in a low-temperature, highhumidity environment. We demonstrated that a supercooled tiny water droplet would not spread yet can easily coalesce and jump from the surface, because much more air is trapped inside and outside micro- and nanostrucutres, which stabilizes the superhydrophobicity. The study offers an ingenious method of maintaining superhydrophobic performance in a low-temperature, humid environment by slowing the heat transfer process to improve the ability of antifogging and anti-icing materials, which would be significant for development for applications.
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EXPERIMENTAL SECTION
Fabrication of the Artificial Compound Eye Structure. The soda lime borosilicate glass hollow (SGH) microspheres and soda lime borosilicate glass (SG) microspheres were purchased from 3M Received: December 6, 2016 Revised: March 17, 2017 Published: March 17, 2017 2899
DOI: 10.1021/acs.chemmater.6b05139 Chem. Mater. 2017, 29, 2899−2905
Article
Chemistry of Materials
hydrophobic filament, and the results of the test were recorded using a CCD camera. Thermal Infrared Test of the Droplet on Films. The ZnO-onSGH film, the ZnO-on-SG film, and PVC tape were placed on the cooling sample stage of the contact angle system at a temperature of −18 °C. After 10 min, one droplet with a volume of 10 μL was placed on the surface using a microsyringe, and the results of the test were recorded using a thermal infrared imaging camera (FLIR A665sc).
Company, and the substrates could be tape or other materials that can withstand a high-pressure, 90 °C environment (the substrate used in this paper is polypropylene film). First, the crystal seeds were prepared as follows. First, 2.195 g of Zn(Ac)2·2H2O (SCRC, A.R.), 20 mL of ethylene glycol monomethyl ether (SCRS, A.R.), and 0.6108 g of monoethanolamine (Beijing Chemical Plant, A.R.) were mixed and stirred for ∼30 min until the solids were completely dissolved. Then, an accurately weighed 10 g portion of SGH microspheres was added to a 0.3 g crystal seed solution, and the mixture was sufficiently stirred and subjected ultrasonication to obtain a homogeneous sol. Using an airless spray process, we prepared a surface with a SGH microsphere monolayer dispersion. The process for dealing with the binding force between crystal seeds and polymer substrates made it possible to avoid high temperatures for strengthening the binding force between the crystal seed and the polymer substrates. The mother liquid was prepared as follows. First, 100 mL of deionized water, 0.35 g of hexamethylene tetraamine, and 0.74 g of Zn(NO3)2•6H2O were mixed together and stirred with a magnetic stirrer for 10 min. The substrates with a crystal seed were moved into the reactors, and then the mother liquid was poured into the reactor. The reactor was placed in an oven that was kept 90 °C for 12 h. The substrates were removed, successively washed with deionized water and ethanol while the temperature of the reactor dropped to room temperature, and then dried in the oven. The surface was cleaned with a plasma cleaning solution (PDC-32 G, HARRICK PLSMA) at high power for 15 min to enhance the chemical activity. To decrease the surface energy of the ZnO nanorod arrays, the surfaces with nanorods were modified by FAS-17, which is a low-surface energy material. We then fabricated the ZnO-on-SG composite structure using soda lime borosilicate glass (SG) microspheres by the same method. Scanning Electron Microscopy. The morphology of the ZnOon-SGH surfaces was characterized by a field-emission scanning electron microscope (JEOL 6700F). Measurement of Thermal Conductivity. The thermal conductivities were measured using the three-layer method by a Hot Disk (TPS 2500 S), which is a new measuring device developed for lightweight insulators and superinsulators. Measurement of the Wettability of the Micro- and Nanostructure Film Surface. Water contact angles (CAs), advancing contact angles (ACAs), receding contact angles (RCAs), and sliding-off angles on the film were tested with an optical contact angle meter system (Dataphysics SCA40). The temperature of the cooling stage could be controlled from −10 to 30 °C, and the samples were fixed on the cooling stage using copper foil double-sided conductive tape. Droplets (5 μL in volume, deionized water) were gently placed on the film surface via syringe through mechanical vibration. Because the surface tension of the droplet is greater than its own gravity and the ultralow adhesion of the film, the droplet could not stick on the film surface without mechanical vibration. Antifogging Test. The biomimetic ZnO-on-SGH film with an area of 50 cm2 was placed on the sample stage of the contact angle system. The film temperature was tuned to 0, −2, −4, −6, −8, and −10 °C by the cooling controller of the contact angle system, and an ultrasonic humidifier was used to regulate the environmental relative humidity at 90% and mimic a fog environment composed of numerous tiny fog droplets. A precision electronic autobalance was used to monitor the change in the quality of the film with time at a given temperature, and the results of the test were recorded using a CCD camera. Striking and Bouncing Test. The biomimetic film was placed on the cooling sample stage of the contact angle system at a temperature of −10 °C. After 10 min, one droplet with a volume of 10 μL was placed on the surface using a microsyringe from a height of 2 cm, and the results of the test were recorded using a CCD camera. Coalescene and Bouncing Test. The biomimetic film was placed on the cooling sample stage of the contact angle system at a temperature of −10 °C. After 10 min, two droplets with identical volumes of 7 μL were placed on the surface using a microsyringe. Then one droplet was pushed toward the other using a super-
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RESULTS AND DISCUSSION An antifogging film (i.e., ZnO-on-SGH) can be fabricated by the process illustrated in Figure 1a. First, an airless spray
Figure 1. Scheme of the fabrication process of artificial mosquito compound eyes and structure and morphology of the micro- and nanostructure. (a) The fabrication process mainly includes two steps, which are microstructure construction of SGH microspheres on the polypropylene film by spin-coating and nanostructure construction of ZnO nanoarrays by crystal growth. (b) SEM image of artificial of mosquito compound eyes, which was covered by an amount of uniform SGH microspheres. It could be observed that the average diameter of SGH microspheres is ∼40 μm. (c) SEM image of a single SGH microsphere with nanoarrays. This special micro- and nanostructure benefits from the large amount of trapped air that enhances the superhydrophobicity and low slide angle. (d) XRD pattern of the film. (e) Water droplet on the surface. The CA can reach 160°. A water droplet could not enter the interspace of the film; nanoarrays supported the droplet, and the film became superhydrophobic and exhibited low adhesion.
method26−28 was adopted to obtain a SGH sphere microstructure. A homogeneous sol composed of SGH microspheres and ZnO seed crystals together were sprayed by airless spray equipment onto the substrate surface on a large scale (Figure S1). Subsequently, ZnO nanostructure was fabricated on the microstructure SGH sphere surface by the crystal growth method.11,29,30 The surface structures were observed by scanning electron microscopy (SEM), as shown in panels b and c of Figure 1. It is noteworthy that SGH microspheres form uniform ∼40 μm layers spread out on the entire film (Figure 1b), and every SGH microsphere was densely covered with ZnO nanoarrays (Figure 1c). The ZnO nanoroads have a diameter that ranges from 50 to 100 nm and a height of ∼2 μm on average. The X-ray diffraction (XRD) pattern confirmed the formation of ZnO nanorods (JCPDS Card No. 36-1451) in the hybrids (Figure 1d). Remarkably, no apparent diffraction peak could be identified at the spectrum, indicating that ZnO nanorods were efficiently deposited on the SGH surface, perfectly suppressing the stacking of microsphere layers. The density of ZnO nanoarrays on the microspheres and the gaps among the neighbor microspheres are similar to those of mosquito compound eyes, which function uniquely. After being modified to achieve a low-surface energy material by using FAS17, the contact angle of water on the film reaches 163° (Figure 2900
DOI: 10.1021/acs.chemmater.6b05139 Chem. Mater. 2017, 29, 2899−2905
Article
Chemistry of Materials
superhydropobic property and low adhensive force of the surface, even though some tiny droplets with a certain sufficiently small diameter could enter the void space within the hierarchical structure. Then the tiny droplets merged into the larger droplet that was sprayed on the surface of the film under the effect of the adsorption force between the droplets and rolled away or jumped up from the surface. Therefore, the mass of the film changes little, and the film surface remains dry, indicating that the ZnO-on-SGH surface displays excellent hydrophobic antifog performance above −6 °C. The mass of the film on a large scale of 50 cm2 changed in the range of 0.34−0.39 g (e.g., 70 s (as shown in Figure 5b). The droplet on the ZnO-onSGH film even remained at a temperature above the ice point until ∼1020 s. At 1608 s, it began to freeze and had a freezing time longer than that of the droplet on the ZnO-on-SG film. The thermodynamic phase transition driving force from water to ice is F:37 ⎛p⎞ F = −ΔG = −kT ln⎜⎜ v ⎟⎟ ⎝ pvs ⎠
Figure 5. (a) Thermal infrared imags of the droplets on three surfaces during the heat transfer dynamic process. (b) Temperature variation curve of the droplets on three surfaces during the heat transfer dynamic process. The droplet on the PVC tape first froze at 538 s and was completely frozen instantly. Then, the droplet on ZnO-on-SG film remained in a supercooled state to 1033 s and had a freezing time of >70 s. The droplet on the ZnO-on-SGH film had the longest supercooled state time of 1608 s and a freezing time of 1747 s. The measurement error was reasonable and 155°. Also, the mass did not change significantly in the mimic fog environment for 10 h; droplets with different diameters could roll off the surface easily with vibration or tilt with an angle of 5°. These results indicate that the film retains its excellent antifogging property and could be used for application as a waterproof and antifogging material.
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CONCLUSIONS In summary, a film composed of SGH spheres and ZnO nanorods was fabricated successfully to achieve an antifogging effect. We demonstrated that the micro- and nanostructures induced tiny condensed droplets to merge with each other easily and then jump off or roll away from the surface, to prevent surface wetting. In particular, the design that employs the hollow microspheres to construct the microstructure greatly slows the transfer of heat from the droplets to the substrate, which leads the surfaces of the film that have stable antifogging performance for up to 80 min at a lower temperature of −10 °C. This investigation provides insight into designing structured thermal insulation surface materials to introduce an antifogging property that can be applied to microdevices used in cold, high-humidity environments, which can be used to develop a robust superhydrophobic antifogging coating for 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/acs.chemmater.6b05139. Figures S1−S3 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yongmei Zheng: 0000-0002-8379-8876 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is supported by the Key Program of National Natural Science Foundation of China (21234001), the National Research Fund for Fundamental Key Project (2013CB933001), and the National Natural Science Foundation of China (21473007).
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DOI: 10.1021/acs.chemmater.6b05139 Chem. Mater. 2017, 29, 2899−2905
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DOI: 10.1021/acs.chemmater.6b05139 Chem. Mater. 2017, 29, 2899−2905
