Nature-inspired superlyophobic surfaces

Abstract

For many years, people have been attracted by the self-cleaning property of the lotus leaf, and dream to develop man-made superhydrophobic surfaces. Many methods have been developed to produce a surface with a water contact angle higher than 150° and a low roll-off angle. In general, a superhydrophobic surface is prepared by combining high surface roughness and low surface energy. The challenge here is to develop a robust superhydrophobic surface with a simple process, so that it can be manufactured in an industrial scale and can be applied in our daily life. The aim of this PhD research is to develop a simple method to prepare a robust superhydrophobic surface. On the other hand, besides superhydrophobicity, oil repellency is also an attractive property which is essential to maintain self-cleaning property of a surface and has many potential applications. So another goal of this study is to investigate lipophobicity for superhydrophobic surfaces. In Chapter 2, we developed a polymeric film with very low surface energy through a self-stratification process. In this Chapter, we stressed that a low contact angle hysteresis is the most important factor for a superhydrophobic surface, and the low contact angle hysteresis only exists when the water droplet is in the Cassie state. The surface reorganization may be one of the reasons for the high water contact angle hysteresis on the polyurethane films with surface rich in perfluoroalkyl side chains. By mimicking the lotus leaf structure, we developed a raspberry-approach to synthesize superhydrophobic coatings with a dual-size surface structure in Chapter 3. The film was prepared by depositing well-defined silica-based raspberry-like particles on an epoxy-based polymer matrix, followed by the surface modification with PDMS. On this superhydrophobic film, advancing water contact angle is about 165°, and the roll-off angle of a 10-µL water droplet is about 2°. In addition, the size ratio between large and small particles of raspberry particles does not have a significant effect on the film wettability when it is larger than 10. The effect of the dual-size roughness on the surface wettability has also been studied by a free-energy modeling. In Chapter 4, a layer-by-layer (LbL) approach was developed to prepare a superhydrophobic surface, with the aim of improving the mechanical robustness of the film. Epoxy-based films were partially cured to a proper extent, followed by deposition of silica microparticles. After that, the films were completely cured. Subsequently, silica nanoparticles were grafted on the microparticle surface through amine-epoxy reactions. Finally, the surfaces were modified with a layer of PDMS to obtain their superhydrophobicity. To further increase the mechanical robustness of the films, the films were treated with SiCl4, which cross-linked the nano-and microparticles, and among nanoparticles. The films were then modified with a perfluoroalkylsilane to obtain superhydrophobicity. The partial embedment of the large silica particles in the polymer matrix and the cross-linking between large and small particles by SiCl4 have proven to be sufficient to obtain mechanically robust superhydrophobic surfaces. Lipophobicity on superhydrophobic surfaces was studied in Chapter 5. In this Chapter, we examine the lipophobicity on the superhydrophobic films with dual-size surface roughness. A surface with a dual-size roughness structure was modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane. Probe liquids with surface tensions ranging from 22.5 to 73 mN/m were prepared by mixing water and ethanol, and the contact angles of these probe liquids on the superhyphobic surface were examined. The contact angle results on such a surface are in a good agreement with the free-energy modeling study. On this surface, the contact angles of hexadecane, sunflower and oleic acid were 125°, 132° and 135°, respectively, indicating that the superhydrophobic surface is also lipophobic. However, this oil-repellent surface is still far from ideal due to the high oil roll-off angle. As an example of practical applications, a process of developing superhydrophobic textiles is described in Chapter 6. By in-situ growing silica microparticles to hydrophilic cotton textiles and using a hydrophobization (PDMS) step, the cotton textiles have been turned completely water non-wettable. Moreover, a superlipophobic textile was prepared by modifying the microparticle-covered cotton textiles with a perfluoroalkylsilane; the textile was then effectively turned superlipophobic.