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Article Cite This: ACS Omega 2019, 4, 6947−6954
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Development of Durable, Fluorine-free, and Transparent Superhydrophobic Surfaces for Oil/Water Separation Chaolang Chen, Ding Weng, Shuai Chen, Awais Mahmood, and Jiadao Wang* Sate Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China
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S Supporting Information *
ABSTRACT: Although artificial superhydrophobic materials have extensive and significant applications in antifouling, selfcleaning, anti-icing, fluid transport, oil/water separation, and so forth, the poor robustness of these surfaces has always been a bottleneck for their development in practical industrial applications. Here, we report a facile, economical, efficient, and versatile strategy to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surfaces by combining adhesive and hydrophobic paint, which is applicable for both hard and soft substrates. The coated substrates exhibit excellent superhydrophobic property and ultralow adhesion with water (contact angle ≈ 160° and sliding angle 150° and SA < 10°) even after 325 abrasion cycles, displaying great mechanical abrasion resistance of the coated glass slide. This ferroconcrete structure superhydrophobic coating exhibited outstanding performance in enhancing the mechanical robustness. The coated glass slide with six spraying cycles could withstand over 325 abrasion cycles, which is nearly 10 times longer than that reported in previous refs 44 and 51. Additionally, the characterization of the glass slides coated with six spraying cycles including their appearance, wettability, and surface morphology is shown in Figure 7. It can be seen that more and more black substance appears on the surface with the increase of abrasion cycle, which can be attributed to the residual of abrasion grains from sand paper. After 600 times of abrasion cycle, although the WCA of the as-prepared glass slide is slightly decreased, its surface was still completely protected by spray adhesive/SiO2 nanoparticle composite coating, showing excellent mechanical robustness. Oil/Water Separation. This universal superhydrophobic and superoleophilic coating fabrication method could be utilized to prepare oil/water separation materials. By using the spray “adhesive + hydrophobic paint” method, we coated the hydrophobic silica nanoparticles on porous substrate surfaces (e.g., stainless steel mesh and sponge). Then, the coated porous substrates were employed to selectively filtrate or adsorb oil from the oil/water mixture. After spray coating treatment, the mesh wires were wrapped by SiO2 nanoparticles and showed hierarchical rough structure (Figure S5). Moreover, the coated mesh exhibited excellent water-repellent property and had a low sliding angle of ∼2° (Figure S6). The oil/water separation experiments are conducted, as shown in Figure 8a,b. When the oil/water mixture was poured into the upper tube, it can be seen that only the oil (CCl4) could permeate the as-prepared mesh and flow inside the beaker placed below. All the water was hold above the surface because of the superhydrophobic property of the coated mesh (see Figure 8a and Movie S4). Hence, the coated mesh could realize the separation of oil and water by a simple filtering method. On the contrary, when the oil/water mixture was drained onto the pristine mesh, both oil and water could pass though the mesh, resulting in the failure of separation (Figure 8b
ΔPC = −
2γL cos θ rp
(2)
where γL is the surface tension of the liquid and θ is the intrinsic contact angle of the liquid on the flat surface. The rp represents the pore radius. As inferred from eq 2, the superhydrophobic surface can withstand a certain external pressure because of ΔPC > 0 (Figure S6a). In the meanwhile, the oil can spontaneously pass through this superoleophilic surface because of ΔPC < 0 (positive capillary effect), as shown in Figure S6b. Furthermore, the oil can be trapped into the rough structure and form oil film on the surface during the separation process, which will prevent the water droplet contacting with the solid surface, thus resulting in under-oil superhydrophobicity. Therefore, the water cannot permeate through the surface that immersed in oil (Figure S6c). If the superhydrophobic surface is drenched in water, the air will be trapped into the rough structure and form an air layer between surface microstructure and surrounding water.53 In this scenario, once the oil was dropped on this surface, it will enter into this thin air layer and quickly spread out under water pressure and capillary effect (Figure S6d). Therefore, the superhydrophobic and superoleophilic porous materials could allow the oil to pass through freely, whereas repel water completely. Intrusion pressure is a crucial property for the oil/ water separation material, which is defined as the maximal pressure that the material surface can support. In order to ensure the separation of oil and water, the external liquid pressure should be kept less than intrusion pressure. The intrusion pressure of as-prepared meshes with different pore sizes is shown in Figure 9b; it can be seen that the coated mesh could support a 6952
DOI: 10.1021/acsomega.9b00518 ACS Omega 2019, 4, 6947−6954
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Article
SKLT2018C06). We also acknowledge the support of this work from the State Key Laboratory of Tribology for Information Science and Technology, China.
high external pressure up to 3430 Pa, and their intrusion pressure decreased with the increase of pore size.
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CONCLUSIONS In conclusion, we have presented a facile and versatile strategy to prepare environmentally friendly, mechanically robust, and superhydrophobic surfaces with excellent transparency by the spraying method, which is applicable for both hard and soft substrates. The coated substrates exhibited excellent superhydrophobic property and ultralow adhesion with water. Moreover, they retained superhydrophobicity even after 325 sandpaper abrasion cycles, showing outstanding mechanical robustness. Furthermore, the coated surfaces can also be employed to separate oil/water mixtures because of their characteristics of being simultaneously superhydrophobic and superoleophilic. In short, it is believed that this superhydrophobic surface fabrication method is significant, promising, and feasible for plenty of 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/acsomega.9b00518. Water droplet rolling on the as-prepared glass slide surface with a tilt angle of ∼1.5° (AVI) Water droplet making contact and losing contact with the as-prepared glass slide surface (AVI) Dripping an oil droplet on the coated glass slide surface (AVI) Oil/water mixture separation test of pristine mesh (AVI) Oil/water mixture separation test of as-prepared mesh (AVI) SEM image and EDX analysis of Super 75 located on the silicon wafer and hydrophobic SiO2 nanoparticles; FTIR spectra of Super 75 and hydrophobic SiO2 nanoparticles and XPS spectrum of as-prepared coating; water contact angle and sliding angle of coated glass slides with different spraying cycles; transmittance curves and optical images of coated glass slides with different spraying cycles; SEM images of pristine mesh and coated mesh and EDX measurements of pristine and coated mesh; water droplet rolling on the as-prepared mesh surface and water droplet making contact and losing contact with the as-prepared mesh surface; and schematic of separation mechanism of as-prepared porous superhydrophobic and superoleophilic materials (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86-01062796458. ORCID
Chaolang Chen: 0000-0001-7538-7527 Jiadao Wang: 0000-0001-7801-9180 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (grant nos. 51775296, 51375253, and 51703116) and State Key Laboratory of Tribology (grant no. 6953
DOI: 10.1021/acsomega.9b00518 ACS Omega 2019, 4, 6947−6954
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DOI: 10.1021/acsomega.9b00518 ACS Omega 2019, 4, 6947−6954
