Surface morphology and wettability
SEM investigations allowed us to determine the microstructural morphology of substrates before and after surface modification. The morphology of the superhydrophobic surfaces of wood, glass, metal, and other cellulose-based materials is shown in Fig. 1. The original wood and bamboo surface was smooth and contained a few vessels. The unmodified filter paper and cotton was composed of smooth cross-linked fibers. The SEM image shows that the untreated glass and copper surfaces were smooth without any feature. On the contrary, after modification by the two-component modifier solution, almost all treated substrate surfaces possess a rough structure characterized by a random distribution of particles having various sizes and shapes.
Moreover, after coating, all treated substrate surfaces had a WCA greater than 150º (spherical water droplets were standing on the treated substrate surfaces) and SA less than 10º (as shown in Table 1). It was indicated that superhydrophobicity was successfully obtained. Based on the analysis mentioned above, it can be confirmed that the two-component modifier solution can create similar hierarchical structures with a high WCA on different types of substrates regardless of their morphologies or sizes.
Also, the treated substrate surfaces exhibited superhydrophobicity against water and against common liquids, including milk, coffee, juice, soy sauce, and beer. Figure 2 shows the relation between the liquids and liquid repellency of coatings on various substrates (The substrates included wood, bamboo, cotton, filter paper, sponge, glass, textile, and copper). The coatings' contact angles on different substrates were larger than 150º for milk, coffee, juice, soy sauce, and beer. All the liquid droplets had a spherical shape and were well supported by the substrate surface treated with the two-component modifier solution, indicating good repellency towards these common liquids. The repellency towards common liquids implies the excellent anti-fouling potential of these superhydrophobic materials.
Table 1
Wettability of superhydrophobic surfaces on different substrates
Substrate
|
WCA (deg)
|
SA (deg)
|
Wood
|
154.1 ± 2.1
|
5.1 ± 0.9
|
Bamboo
|
153.6 ± 1.8
|
5.8 ± 1.3
|
Cotton
|
151.8 ± 1.6
|
8.2 ± 0.5
|
Filter paper
|
154.7 ± 2.5
|
7.3 ± 1.5
|
Sponge
|
151.3 ± 0.9
|
8.7 ± 1.6
|
Glass
|
155.1 ± 2.3
|
4.0 ± 0.8
|
Copper
|
152.6 ± 1.6
|
6.7 ± 1.2
|
Textile
|
151.4 ± 0.7
|
8.4 ± 1.5
|
Durability of the coated superhydrophobic surfaces
To be used in real life, the superhydrophobic surface should withstand a certain degree of damage. In this study, we conducted a comprehensive and systematic study on the prepared superhydrophobic surfaces' mechanical stability and durability.
In the past, most of the evaluation of the durability of superhydrophobic surfaces was carried out by immersing superhydrophobic materials in water (Lin et al. 2020). Nevertheless, few papers have explored the durability of superhydrophobic surfaces against moisture. Liquid water is larger than the pores formed in the rough structure, whereas moisture is small enough to penetrate the pores and attack the rough structure (Wu et al. 2016). In other words, the superhydrophobic surface only shows high hydrophobicity to liquid water, but cannot prevent the penetration of moisture. To assess the superhydrophobic surfaces' durability to moisture, eight types of superhydrophobic materials were investigated under an indoor environment, and a relative humidity (RH) of 50% for 30 days. Figure 3 shows the variation of the WCA and SA in an indoor environment. The WCA and SA almost retained their original states, indicating long-term durability in an indoor environment. Therefore, these superhydrophobic surfaces on various substrates exhibit long-term durability in an indoor environment. Hence, these superhydrophobic materials can be well applied in indoor environments.
To evaluate the mechanical stability of the superhydrophobic surfaces, a sandpaper abrasion test was performed on the superhydrophobic surface according to the previously reported methods (Chang et al. 2015; Tu et al. 2018; Jia et al. 2018). As shown in Fig. 4, eight types of superhydrophobic materials were rubbed with sandpaper (1500 mesh) by loading 10 g weights on the sample. The WCA and SA value of the resultant surfaces were measured at every 20 cm of abrasion length. Figure 4 shows the water-repellent ability of various superhydrophobic materials after 140 cm of abrasion. The results are as follows. Superhydrophobic wood after severe abrasion, remains superhydrophobic with a WCA greater than 150º and an SA less than 10º, indicating excellent mechanical robustness against sandpaper abrasion. The good mechanical stability of the superhydrophobic wood surface can be attributed to the sufficient thickness of the superhydrophobic coating to withstand the abrasion and the hydroxyl group on the surface of the wood, which forms a covalent bond with the -Si-H on the PMHS chains. Similar to wood substrates, the superhydrophobic coating on the surface of bamboo also had good mechanical stability. However, superhydrophobic materials, including cotton, filter paper, sponge, glass, textile and copper, could not maintain their original superhydrophobicity with decreased CAs of ~ 115º and SAs of ~ 60º, which may result from their low specific surface area and inability to form a covalent bond.
Apart from the sandpaper abrasion test, water immersion test and anti-freezing experiments were also conducted on superhydrophobic materials. Taking wood as the test sample, a anti-freezing test and water immersion test were carried out to further assess superhydrophobic surfaces (as shown in Fig. 5a-b). Briefly, Place the superhydrophobic wood in a low-temperature (minus 20 ℃) environment or immersion the superhydrophobic wood in water for a certain period of time (0 ~ 24 hours) to characterize the change in hydrophobicity. WCA and SA were measured every 3 hours. Figure 5a-b shows the variation of the WCA and SA after water immersion and low-temperature treatment. After 24 hours of water immersion and freezing, the WCA and SA almost retained their original states, indicating long-term durability under water or low-temperature environments. In addition, because many superhydrophobic surfaces easily lose their superhydrophobic properties in hot water (Li et al. 2016), water droplets with different temperature were used to study the hot water superhydrophobicity of the as-prepared surface (Fig. 5c). It can be clearly seen that even if water drops with a temperature close to 100 ℃ are dropped on the surface of the modified wood, it still has good water-repellency, and the water drops are spherical. In short, these superhydrophobic materials can be well applied in extreme environments.
The investigation of formation mechanism
Superhydrophobic surfaces were successfully synthesized via a novel method proposed in this paper. The formation mechanism of the superhydrophobic surface is described herein (illustration shown in Fig. 6). Firstly, a two-component modifier solution (A, B) was prepared and mixed to obtain the modifier solution. Then, the eight types of materials were immersed in the mixed solution for a certain period of time. Because the hydrophobic modifier PMHS (the PMHS chain contains a large quantity of reactive Si-H bonds and hydrophobic -CH3 groups) used in this experiment is easy to dehydrogenate with hydroxyl-containing substances in the presence of a catalyst, Kastredt catalyst was used. (Kastredt catalyst is the complex compound of platinum and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, which has very high catalytic activity for dehydrogenation between -Si-H bands of PMHS and hydroxyl groups of substances at room temperature) (Lin et al. 2018; Lin et al. 2020). Wood, bamboo, cotton, filter paper, SiO2, etc. all contain a large number of hydroxyl groups, so It can combine with PMHS to improve their hydrophobic properties. However, because the dispersant (absolute ethanol) used in the experiment also contains hydroxyl groups, it will also react with the hydrophobic modifier PMHS, which affects the reaction between PMHS and SiO2 and the base material, which will lead to the modifier solution failure. Therefore, an inhibitor was introduced in the experiment, which can well avoid the modifier solution's failure. As shown in Fig. 6, when the substrates are dried, the reaction between PMHS and SiO2, substrates, and the dispersant absolute ethanol is delayed due to the inhibitor's presence (as a ligand, the inhibitor will coordinate with the platinum atom in the catalyst and hinder the contact between the platinum atom and the reactant). During the drying process, the absolute ethanol gradually evaporates. After the absolute ethanol was completely evaporated and the inhibitor no longer had an inhibitory effect, the PMHS on the substrate surface reacted with SiO2 or the substrate to form a rough hierarchical structure (low surface energy materials combined with rough nanomaterials). Therefore, a superhydrophobic coating can be constructed on the surface of various substrates.
The application of superhydrophobic surfaces
Self-cleaning. Superhydrophobic surfaces with low adhesion to water will have good self-cleaning properties (Zhou et al. 2016). Water has a very low adhesion force on the superhydrophobic surface. The water droplets rolling on the superhydrophobic surface can take away the contaminants accumulated on the surface, mimicking the lotus leaf effect. Therefore, it can be considered that the contaminants accumulated on the superhydrophobic surface can be easily removed. Figure 7 shows the self-cleaning effect of filter paper and wood surface without and with superhydrophobic coating. Both filter paper and wood surfaces were contaminated by methyl blue powder. For modified filter paper and wood surface, as the water was dropping to the surface, the water droplets readily rolled off, removing the methyl blue powder and resulting in a dry and clean surface (Fig. 7(a5-a8 and b5-b8)). On the contrary, water droplets merged with the methyl blue powder and stuck to the original filter paper and wood surface, leading to a wet and polluted surface (Fig. 7(a1-a4 and b1-b4)). As shown in Movie S1 and Movie S2, methyl blue powder on the superhydrophobic filter paper and the wood surface can be rapidly and completely removed by spraying water onto it. In contrast, the methyl blue powder cannot be completely removed from the surface of original filter paper and wood.
Oil-water separation. The efficient separation of oil-water mixtures is a long-term problem and challenging environmental pollution such as crude oil leakage in the ocean from industrial production (Wu et al. 2016). Materials with special wettability, including superhydrophobic and superoleophilic properties, have been widely used in oil-water separations (Xue et al. 2011). Herein, the modified filter paper demonstrated superhydrophobic-super-oleophilic properties. So it was used to demonstrate potential applications for oil-water separation. As shown in Fig. 8, the oil-water separation experiment was carried out using filter equipment and superhydrophobic filter paper serving as filter elements. When the mixture of diesel and water is poured onto the superhydrophobic filter paper, the diesel rapidly diffuses and penetrates through the filter paper and falls into the beaker below. On the contrary, the water remains on the filter paper. In this manner, the effective separation of oil-water mixture was achieved. The separation efficiency was higher than 97%.
Finally, through a 20 cycles of oil-water separation test, the modified filter paper still has good oil-water separation performance, which also shows good durability. It also means that the as-prepared filtration material can be repeatedly used to separate oil-water mixtures.
Elimination of underwater oil. Herein, the superhydrophobic cotton was assessed for the elimination of underwater oil droplet. Figure 9 shows an oil red-dyed CCl4 droplet resting at the bottom of the water. When the superhydrophobic cotton was brought into contact with the water, its whole surface turned to silver-like. Furthermore, once the modified cotton contacted the oil droplet, part of the CCl4 was sucked into the cotton due to the oleophilic effect. The longer the contact, the more oil is loaded. Finally, when the cotton was taken out of the beaker, the water was cleaned up with no trace of dyed oil. Consequently, it can be concluded that superhydrophobic materials can be well applied in the field of oil-water separation, which can effectively solve the environmental pollution caused by oil-water mixing and the problem of oil-contaminated water in industrial production.