Silica Nanoparticle Assembled, Microscale Porous

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Dopamine/Silica Nanoparticle Assembled, Micro-Scale Porous Structure for Versatile Superamphiphobic Coating Fang Li, Miao Du, and Qiang Zheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00036 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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Dopamine/Silica Nanoparticle Assembled, Micro-Scale Porous Structure for Versatile Superamphiphobic Coating Fang LI, Miao DU*, Qiang ZHENG MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China KEYWORDS: Superamphiphobic coating, Dopamine, Various substrates, Ice templation, Porous structure

ABSTRACT: Artificial superamphiphobic surfaces, which could repel both water and low-surface-tension organic liquids, have still been limited to particular kinds of materials or surfaces thus far. In this work, a kind of micro-scale porous coating was developed. Taking dopamine and hydrophilic fumed silica nanoparticles as initial building blocks, micro-scale porous coating was constructed via ice templation. Polydopamine bound silica nanoparticles together to form porous structure network and rendered the coating with potent for further post-functionalization. After two-step CVD,

the

micro-scale

porous coating

changes from

superhydrophilic to

superamphiphobic, exhibiting super-repellency to droplets with surface tension of 73 mN/m ‒ 23 mN/m. The influences of concentration of initial dopamine, hydrophilic fumed silica nanoparticles, and dry condition on the formation of the porous structure have been studied to optimize the condition. Coatings with different pore size and *: Address correspondence to [email protected]

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pore height have been fabricated to find out the relationship between the structure parameters and the repellency of the porous coatings. Only with optimal pore size and pore height can the porous coating display superamphiphobicity. Compared with nano-scale, the micro-scale structure favors the achievement of superamphiphobicity. Given the outstanding adhesive ability of polydopamine, the superamphiphobic coatings have been successfully applied to various materials including artificial materials and natural materials.

Super-repellent surfaces, inspired by natural water-repellent surfaces such as lotus leaves, red rose petals, and water striders, have drawn considerable interests both in fundamental studies and applications.1-6 Hierarchical structures accompanied with low-surface-energy materials can render surfaces with large static contact angles (SCAs > 150°) to water droplets.7-9 However, simply roughened surfaces are not sufficient to repel oils or organic liquids which have much lower surface tensions (γ) than water. Those that show high SCAs (> 150°) and low sliding angles (SAs < 10°) to oils (organic liquids) are named as superoleophobic surfaces.5,6 The surface that could repel both water and oils (SCAs > 150°, SAs < 10°) are termed as the superamphiphobic surface. As the abundant variety of oils (including other organic liquids), their γ can be dramatically different from each other.5 Herein, we only term the surfaces that can repel both water and oils (γ < 30 mN/m) as superamphiphobic


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surfaces. For surfaces with super-repellency to organic liquids, special topographies such as “re-entrant curvature”, “overhang structure”, or “doubly re-entrant structure” were developed because certain topographies could trap air pockets, inducing formation of negative Laplace pressure and preventing organic liquids from penetrating.10-18 In this case, organic droplets could not reach the bottom of the structure, and thus be suspended on certain surfaces. Recently, it was reported that springtails (Collembola), a kind of nature organisms, could repel oils.19 The mushroom or serif “T” structure which can be found in the skin of springtails again emphasizes the importance of special surface structure: pin three-phase contact line and prevent low-surface-tension liquids from contacting the substrate.20,21 According to the reported works, plasma treatments and chemical etching are the most widely used techniques to fabricate superamphiphobic surfaces.12,18,22,23 Besides, other methods such as photolithography,24 spraying,25,26 coating on textiles and membranes,27-30 etc., have also been utilized to prepare superamphiphobic surfaces. Note that although lots of efforts have been made, most reported approaches are considerably destructive to the substrates.29-35 Meanwhile, superamphiphobic surfaces have only been achieved on relatively few materials (mostly on silicon, silica, textiles, and meshes), which are either easy for constructing surface patterns by surface treatment methods or with pores (or patterns) intrinsically. Challenge still remains in fabricating superamphiphobic surfaces onto different substrates via a more green, gentle, and versatile approach. The “re-entrant” structure and other similar structures have been widely studied


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and used to fabricate superamphiphobic surfaces, and all the clues indicated that the air pockets trapped beneath the droplets were significant for oil repellency as they could induce a negative Laplace pressure.5,23,36 Porous structure, which is filled up with lots of air in its vacancies or pores, is thus potential for oil repellency.37,38 Another advantage of using porous structure is that it can be easily constructed via, e.g., ice templation method which has been regarded as a facile and green method without any pollution.39-44 Meanwhile, the porous structure is more potential than the “re-entrant” structure to be applied onto substrates made from different kinds of materials.37 Yet, the reported porous structures made from ice templation or generated via supercritical methods (aerogels are always fabricated via this way) also have serious limitations. They are mainly generated in bulks and hard to be further coated onto other substrate surfaces, such as metals, ceramics etc. Given the current status that superamphiphobic surfaces could only be fabricated on several particular substrates, a green and versatile method is demanded to make it possible to fabricate porous coatings onto various substrates.5,6 More importantly, the relationship between structure parameters and the repellency of the porous structure has not been figured out, hindering the development of super-repellent porous materials. Mussels (Figure 1A), one of the natural organisms, can secrete adhesive proteins which help attach to wet surfaces and adhesion-resistant materials such as polytetrafluoroethylene (PTFE).45,46 The amino acid composition which could be found in 3,4-dihydroxy-L-phenylalanine (DOPA, Figure 1B) and lysine were crucial for the excellent adhesion capability of the mussels.46, 47 DOPA was also reported to


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have the ability to form strong covalent and non-covalent interactions with substrate surfaces.48 2-(3,4-dihydroxyphenyl) ethylamine (dopamine), possessing both catechol and amine groups, has been regarded as a powerful bio-inspired adhesive (Figure 1C).49-54 Dopamine can be polymerized in solutions which are buffered to a pH close to marine environment (10mM Tris, pH 8.5) or under the existence of oxidants, such as NaIO4.46,55-57 The chemical versatility of the polydopamine layer makes it possible to further react with a variety of molecules containing amino groups and mercapto groups.46,58,59 Recently, dopamine has been widely used to modify material surfaces and particles with different wettability and

render them

with chemical

versatility.47,57,60,61 Up to now, dopamine has just been regarded as a kind of simple surface modification chemicals, and has never been directly used as the main “building block” to fabricate porous structure. Moreover, to the best of our knowledge, dopamine has never been employed to fabricate superamphiphobic surface thus far. In this work, an organic-solvent-free approach to construct a coating with micro-scale porous structure relied on the outstanding binding capability and water solubility of dopamine is developed. Like constructing a building (Figure 1D), the hydrophilic fumed silica nanoparticles (HiFSNs, yellow clusters in Figure 1D) and dopamine (black circles in Figure 1D) were used as the building blocks to form the porous structure. Then, by cryogenic processing, designed porous structure network was formed in an icing-induced way. With further freeze-drying, the ice templates were removed and thereafter the coating with porous structure was acquired. After treatment with fluoroalkylsilane (Figure 1E), the coating displayed excellent


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repellency to organic liquids with low γ. Besides, we have also fabricated the coatings with different porous structures to study the influences of porous structure parameters on surface repellency. Based on theoretical analysis, the relationship between porous structure parameters and the repellency of the structure was figured out. Taking the advantage of the remarkable binding capability of dopamine, we also showed that this approach could be successfully employed to fabricate porous superamphiphobic coatings on various solid substrates.

RESULTS AND DISCUSSION Porous structure constructed from dopamine and HiFSNs. After adding NaIO4 solution into the HiFSNs and dopamine dispersion, dopamine was oxidized and polymerized into polydopamine. The characteristic peak in 280 nm shifting to higher wavelength indicated the formation of dopaminequinone (Figure 2A). Thereafter, the dopaminequinone was transformed into leukodopaminechrome via intramolecular cyclization.62 It was further oxidized into dopaminechrome, which could be verified by a weak and broad peak at 460 nm in the UV-vis spectrum.56,61,63 The dopaminechrome was further transformed into 5,6-dihydroxyindole via intramolecular rearrangement and eventually went on polymerization. Hydrogen bond was believed as the strong interaction between polydopamine and HiFSNs to draw them together (see supporting information).64,65 TEM (Figure 2C‒E) accompanied with energy dispersive X-ray detector (EDX, resolve N peaks in Figure 2B, Figure S3D and E), X-ray photoelectron spectroscopy (XPS, Figure 4A and C), dynamic


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light scattering (DLS, Figure S3C), TGA (Table S1), and FT-IR (Figure S4) results indicated that HiFSNs were indeed bound by polydopamine. The polydopamine bound HiFSNs and the corresponding dispersion displayed different colors from the original ones (Figure 2F and G, and Figure S1). During the cryogenic processing, ice crystals gradually formed, generating porous structure network (Figure S5). During freeze-drying process, the ice templates were removed, leaving the porous structure network on the substrate. Dark lines or areas in phase contrast microscope (PCM) images indicated the porous structure networks and light areas indicated the vacancies because light could not transmit through the networks but the vacancies. Only the structure that was exactly located at the focal plane could be clearly detected (red dashed line marked in Figure 3A and B). Otherwise, the image would be fuzzy (orange dashed line marked in Figure 3A and B). The porous structure was composed of polygons, mostly hexagons and pentagons with dimensions of tens to hundreds of microns. The superficial surface morphology of these micro-scale porous coatings was further observed by SEM. The superficial SEM image (Figure 3C and D) matched well with the corresponding PCM image (Figure 3A). The inside structure of the coating was also observed via SEM. The convex part in SEM image clearly indicated the porous structure networks (Figure 3E), while the concave parts corresponded to the vacancies which were occupied by the ice templates before freeze-drying. The porous structure network walls were composed of abundant HiFSNs (Figure 3F).


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Achievement of superamphiphobic coating. The polydopamine layer renders the porous structure with the possibility for further chemical modification. Just after freeze-drying, the porous structure could be completely wetted by water, i.e., superhydrophilic, due to numerous amino and hydroxyl groups presented on the surfaces of the polydopamine bound HiFSNs clusters. The two-step CVD treatment was applied to the porous coating (Figure 1E). Firstly, the coating was CVD treated with

3-aminopropyltriethoxysilane,

whose

amino

groups

reacted

with

the

polydopamine layer. At the same time, the ethoxy groups were hydrolyzed under certain moisture and lead to the formation of lots of silanol groups on the outside surface of the porous structure network. Secondly, the coating was CVD treated with fluoroalkylsilane. With the presence of silanol groups, fluoroalkylsilane could be grafted onto the porous structure network via intermolecular condensation with silanol groups, which generated Si–O–Si bonds. Through the two-step CVD treatment, a low-surface-tension shielding layer with fluoroalkyl groups was formed on the outside of the porous structure networks. The deposited fluoroalkyl groups could be clearly resolved via the significantly strong F peak in XPS spectra (Figure 4A and B). F atom occupied approx. 49.3 atom% of the porous coating. N peak was also strengthened by 3-aminopropyltriethoxysilane after two-step CVD (Figure 4C and D). In the corresponding FT-IR spectra, two strong peaks of CF2 stretching vibration could be found at 1206 cm−1 and 1155 cm−1, respectively (Figure S7). Structure that could trap air pockets is crucial for the repellency of surfaces or coatings.4-6,23,66-68 The vacancies of porous structure are in favor of trapping


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considerably amount of air pockets. And the two-step CVD treatment rendered the porous structure with low-surface-tension shielding layer. Combining these two factors (air pockets and low-surface-tension shielding layer), the micro-scale porous coating (Figure 4E) was believed to display excellent super-repellency to both water and organic liquids. As expected, after CVD treatment, the superhydrophilic micro-scale porous coating changed to superamphiphobic. Liquids with γ ranging from 73 mN/m to 23 mN/m all exhibited SCAs larger than 150° on the CVD treated coating (Figure 4F). These droplets can easily roll off from the coating (SAs below 10°). The excellent anti-wettability could be more clearly reflected by the nearly spherical profile and shape of the droplets deposited on the coating (Figure 5). For comparison, another freeze-dried coating was directly CVD treated with fluoroalkylsilane, i.e., one-step CVD. A dramatic decrease in SCAs happened when olive oil droplet was deposited on the one-step CVD treated coating (Figure S8A). Its repellency was not as good as the coating treated with two-step CVD. Presumably, amino groups of 3-aminopropyltriethoxysilane could be grafted onto the structure via chemical bond while hydroxyl groups of 1H,1H,2H,2H-perfluorodecyltriethoxysilane could not. Only through two-step CVD can fluoroalkylsilane be firmly grafted onto the porous structure. Meanwhile, there are more CF3 and CF2 on top of the coating after two-step CVD than after one-step CVD (Figure S9), generating an outer layer with much lower surface energy. To demonstrate the robustness of the coating, the superamphiphobic coatings were impinged by water from the height of 10 cm. The as-prepared porous


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superamphiphobic coating can stand approx. 200 g of water impinging (Figure S8B). Increasing the initial concentration of HiFSNs could make the coating stand 1100 g of water impinging (more detailed in the supporting information).

Effect of porous structure parameters on super-repellency. Up to now, roles of the porous structure parameters have not been well understood yet probably because porous structure are not as regular as “re-entrant” or “T” structures which are uniformly constructed by etching or templation methods.15,20 In this paper, however, we found out that there’s rational relationship between the parameters of porous structure and the repellency of porous coatings. And it could be used to direct the fabrication of porous superamphiphobic surfaces. To simplify the parameters of porous structure, only two parameters, i.e., the pore diameter (D) and pore height (H) were considered. First of all, three porous coatings with different D and H were prepared. Droplets including water and oils were deposited onto the three different porous coatings and SCAs (SAs) were measured. The superamphiphobic coating denoted

the

above-discussed

micro-scale

porous

coating

which

exhibited

super-repellency to droplets with γ from 73 mN/m to 23 mN/m. The as-prepared micro-porous coating was fractured to gain the bottom layer of the micro-scale porous structure coating, in which H was smaller than the original one. The fractured surface was then treated with the two-step CVD. Interestingly, the coating (Figure S10) with only bottom layer of the micro-scale porous structure (same as Figure 3E) could show super-repellency to droplets with γ ranging from 73 mN/m to 32 mN/m. The droplets


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on the coating displayed SCAs larger than 150° and SAs below 10° (Figure 4F). Noting that the lowest γ of the droplets showing super-repellency on it was 32 mN/m. It failed to repel n-hexadecane (γ = 27.5 mN/m). It means that the coating with a bottom layer of the micro-scale porous structure only exhibited super-repellency to droplets with γ > 30 mN/m and the bottom layer was thus denoted as quasi-superamphiphobic coating here. The third coating was prepared by depositing the same volume of polydopamine bound HiFSNs clusters dispersion on glass slide and it was then dried under ambient condition. Instead of forming micro-scale pores, the coating possessed nano-scale porous structure (Figure 3G and H). Significantly, the nano-scale porous coating only displayed super-repellency to droplets with γ ranging from 73 mN/m to 45 mN/m (Figure 4F). Those droplets showed SCAs beyond 150° and SAs below 10° when they were deposited on the nano-scale porous coating. Thus, the third coating was denoted as superhydrophobic coating. The structure features of each porous coating are sketched in Figure 6 and summarized as follows: the superamphiphobic coating possesses micro-scale porous structure with large H; the quasi-superamphiphobic coating has a bottom layer (thin) of micro-scale porous structure, i.e., small H; the superhydrophobic coating possesses nano-scale porous structure. The pore diameter of the nano-scale porous superhydrophobic coating (D0) was around or slightly larger than the HiFSNs particle diameter (2R) (Figure 6A). The pore diameter of the quasi-superamphiphobic coating (D1) was significantly larger than the thickness of the porous structure network wall (T1). T1 was also much larger than the HiFSNs particle diameter, i.e., T1 > 2R. It was


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found out that the pore size and the thickness of the porous structure network wall did not change in different cross-sections after analyzing the pore size and the thickness of the porous structure network wall between the superficial layer (Figure 3C and D), inside layer (Figure 3B) and bottom layer (Figure 3E). Therefore, the pore size of the superamphiphobic coating (D2) is also significantly larger than the thickness of the porous structure network wall (T2). Meanwhile, D1 is basically equal to D2, and T1 is also equal to T2. The quasi-superamphiphobic coating only possesses the bottom layer of the micro-scale porous structure, indicating that the pore is incomplete, so the pore height of the quasi-superamphiphobic coating (H1) is smaller than that of the superamphiphobic

coating

(H2).

Obviously,

the

transparency

of

the

thin

quasi-superamphiphobic coating (Figure S10) is better than that of the thick superamphiphobic coating (Figure 4E). It is worth noting that only the thick micro-scale porous coating with larger pore size (D2) and pore height (H2) displays superamphiphobicity.

Relationship between porous structure parameters and super-repellency. When a droplet is deposited on the structured surface, two theoretical states, i.e., Cassie state and Wenzel state, are used to describe the contact state between droplet and surface structure.69,70 When a droplet is overhung on the topside of the structure, it is at the Cassie state. The Cassie’s SCA (θC) for a drop deposited on the coating can be written as,69

cosθ C = (1 − φair )cosθ Y − φair


(1)

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where θY is the Young’s SCA, and ϕair is the air fraction of the corresponding porous coating. Wenzel state describes the situation when the droplet is not suspended on the structure. In this case, liquid penetrates into the surface structure and completely contacts with the structured surface in the projected contact area. The Wenzel’s SCA (θW) can be written as,70 cos θ W = r ⋅ cos θ Y

(2)

where r represents the surface roughness factor. In Wenzel state, introducing of surface roughness (r) helps improve the wettability or the repellency of the structured surface compared to its pristine wettability. It means that a hydrophilic (or oleophilic) surface would be more hydrophilic (or oleophilic), while a hydrophobic (or oleophobic) surface would be more hydrophobic (or oleophobic) when they are both structured. θY of 90° is a commonly accepted critical SCA to judge whether the pristine surface is wettable or not (θY < 90°, hydrophilic or oleophilic; θY > 90°, hydrophobic or oleophobic). Equation (1) is only held when the droplet is exactly overhung on the topside of the structure. Otherwise, the droplet is not at the Cassie state. Cassie state is reported as a metastable state,14,17 and may become unstable under certain conditions. As has also been presented by some previous work, there are critical break-in points in surface tension for structured surfaces.18,21 SCAs for the droplet dramatically decrease when γ of the deposited droplet is below a critical value. Above the break-in point, the droplet is overhung at the Cassie’s state. Instead of varying with θY or ϕair, SCAs for different droplets are all beyond 150° with a relatively small deviation. Below the


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break-in point, SCAs would dramatically decrease and the deposited liquid gradually wets the surface. It seems that 150° is an apparent borderline in SCAs to judge whether the droplet is overhanging or wetting the structured surface. There are critical break-in points for the three coatings as well (Figure 4F). It is obvious that when the coatings display super-repellency, SCAs are all ranged from 150° to 165°. Once the coatings are not able to hold droplets to exhibit SCAs beyond 150°, obvious penetration happens, resulting in dramatic decrease in SCAs (Figure 4F). The transition is called Cassie-to-Wenzel transition.17 In the present work, SCAs for droplets deposited on the three coatings decrease significantly to values below 150° when olive oil (γ = 33.1 mN/m), n-hexadecane (γ = 27.5 mN/m), and n-hexane (γ = 18.4

mN/m)

were

deposited

on

the

superhydrophobic

coating,

quasi-superamphiphobic coating and superamphiphobic coating, respectively (Figure 4F). θY for olive oil, n-hexadecane, and n-hexane are all below 90° (Table 1). This indicates that the pristine surface is oleophilic. According to Wenzel state, simply roughened surfaces might be wetted by these liquids. Therefore, the only way to render the pristine oleophilic surface with superamphiphobicity is to pin the three-phase contact line at the topside of the surface structure, and to increase the air fraction so that the three-phase contact line could be kept away from touching the substrate. As the air pockets trapped beneath the droplets induce a negative Laplace pressure, the droplets would be kept on top of the surface instead of penetrating into the structure.5,23,36 Above the break-in point, the droplets are at the Cassie state.


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Accordingly, the theoretical SCAs can be calculated by Equation (1). As discussed above, although the measured SCAs do not vary with θY or ϕair, 150° is still a critical SCA that could be used to judge whether the droplet is overhanging or penetrating. Only when the droplet is at Cassie state can it exhibit SCAs larger than 150°. Theoretically, the calculated θC by Equation (1), i.e., θC,cal should also be larger than 150°. Specifically, if θC,cal is larger than 150°, the droplet is believed to be suspended on top of the structure (Cassie state). In contrast, if θC,cal is below 150°, the droplet is believed to impale into the structure (Wenzel state). According to the above discussion, the repellency of the coatings can be evaluated by theoretically calculated θC,cal.

Table 1. Surface tensions and static contact angles for breakthrough-point liquids deposited on three coatings. Surface

Static contact angle a) (°)

tension

Glass Superamphiphobic Quasi-superamphiphobic Superhydrophobic

(mN/m)

slide b)

coating

coating

coating

Ethylene glycol

47.3

86.5

158.3

157.1

156.2

Olive oil

33.1

38.5

160.1

151.2

0 c)

n-Hexadecane

27.5

8.8

155.1

0 c)

0 c)

18.4

c)

c)

0 c)

Liquid

n-Hexane a)

0

0

c)

0

: For calculation, the average SCAs were listed in the table. The deviations of SCAs

were not listed in the table. b)

: The static contact angle measured on treated planar glass slide is Young’s static

contact angle, θY. c)

: The surface could be gradually wetted by the corresponding droplet.

In the case of porous structure, if ignoring the influence of coating thickness, ϕair


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can be approximately calculated by equations as follows (Figure S11 and S12),

φ air =

D0 2 R + D0

(3a)

φair =

D1 T1 + D1

(3b)

φ air =

D2 . T2 + D2

(3c)

or,

or,

ϕair of superhydrophobic coating (nano-scale porous) can be roughly estimated by Equation (3a), while ϕair of quasi-superamphiphobic coating and superamphiphobic coating can be estimated by Equation (3b) and (3c), respectively. As D0 is equal or slightly larger than 2R, ϕair of superhydrophobic coating is definitely larger than 0.5. Because the structure of the nano-scale porous coating is not regular enough, it is hard to assume ϕair. ϕair of the nano-scale porous coating has been studied in our previous work.71 ϕair of the evaporation induced nano-scale porous structure changed slightly with the concentration of fumed silica. Therefore, we took ϕair of 0.87 for the nano-scale porous coating. In the case of ethylene glycol droplet, substituting θY (86.5°) and ϕair (0.87) into Equation (1) we got θC,cal (149.5°), which was almost equal to 150°. The nano-scale porous coating indeed exhibits super-repellency to ethylene glycol as shown in Figure 4F. If deposit olive oil (θY = 38.5°) onto the nano-scale porous coating, we got θC,cal of 140.2°, being much smaller than 150° and implying that the Cassie-to-Wenzel transition probably happened. Actually, penetration did happen and the coating was wetted by olive oil, demonstrating that the theoretically


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calculated results agreed well with the measured results (Figure 4F). Similar to the nano-scale porous coating, the repellency to the droplets of the micro-scale porous coatings was also evaluated. From Figure 3A‒E we found out that pore size ranges from approx. 70 µm to approx. 200 µm. The intermediate value of micro-scale pore size was taken (both D1 and D2) as 150 µm. And T (both T1 and T2) was approx. 10 µm. According to Equation (3b) and (3c), we got ϕair of approx. 0.94. Thereafter, we acquired θC,cal for olive oil (153.3°) and n-hexadecane (151.7°). As a result, the micro-scale porous coatings (both thin and thick) were expected to be able to repel both olive oil and n-hexadecane. However, only the thick micro-scale porous coating with larger pore height (H2) could hold n-hexadecane at Cassie state. For the droplet of n-hexadecane on the thin micro-scale porous coating with much smaller pore height (H1), the Cassie-to-Wenzel transition happens. In order to explain these results, we carefully investigated the causes that induce droplet penetration. When a droplet is deposited on a structured surface, the three-phase contact line is planar microscopically if there is no force exerting on the droplet (dashed-dotted line in Figure 7A). When taking the Laplace pressure (∆PLap) into consideration, the three-phase contact line would move downwards and start to sag (solid line in Figure 7A). Here, the gravity of the droplet is neglectable because the pressure generated by gravity is much smaller than ∆PLap. There are two possible ways that could lead to Cassie-to-Wenzel transition. One is that the three-phase contact line is still pinned on the topside of the structure, but the bottom of the curved three-phase contact line touches the substrate, namely sag impalement (dashed line in


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Figure 7A).17,72,73 The other is that the three-phase contact line depins from the top side of the structure and moves downwards, namely depinning impalement (dotted line in Figure 7A).74-76 For simplification, we assume these polygon pores are circular. The driving force, ∆PLap, which leads to the break-in of the droplet can be calculated as,

∆PLap = 2κγ

(4)

where κ is the curvature at the sagging three-phase contact line (Figure 7C). κ is equal to 1/a, where a is the radius of curved surface. Nano-scale porous structure possesses much smaller pore diameter than micro-scale porous structure. Obviously, the possible a of the curved surface on the nano-scale pore could be much smaller than that on the micro-scale pore. Hence, the κ on the nano-scale pore is much larger than that on the micro-scale pore. As a result, ∆PLap of nano-scale porous coating is significantly larger than ∆PLap of the micro-scale porous coating. Consequently, oils are much easier to penetrate into the nano-scale porous coating. For the two micro-scale porous coatings, the three-phase contact lines have to remain pinned. Otherwise these coatings would fail to repel n-hexadecane, demonstrating that depinning impalement has not happened. Hence, depinning impalement is not the reason for the penetration of n-hexadecane on the thin micro-scale porous coating. The only possible reason is that the sag impalement has happened. The pore diameters for the two coatings are equal (D1 = D2), so the pore height (H) is responsible for the difference in repellency. The critical pressure for sag impalement is related to the maximum pressure which is allowed by the geometry of


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the porous structure.17 Here, pore height of the porous structure is the dominant parameter. We assumed the pores as circular for simplicity and the diameter of the pore is D in the case D >> T. If the pore height is large enough (H > D/2), the largest possible curvature (κmax) of the sagged three-phase contact line is attained for a semicircle centered on the topside of the structure, namely κmax = 2/D (Figure 8A). The maximum critical pressure difference which could lead to Cassie-to-Wenzel transition, ∆Pbreak-in, can be acquired relating D and γ by Laplace equation and presented as follows, ∆Pbreak −in =

4γ . D

(5a)

In the case of porous structure with smaller pore height (H < D/2), the bottom of the curved three-phase contact line touches the substrate before it reaches the maximum curvature which enables the maximum pressure (Figure 8B). In this case, the maximum critical pressure difference of the thin structure is considerably reduced to, ∆Pbreak −in =

16 Hγ . D 2 + 4H 2

(5b)

As the pores of the micro-scale porous structure are connecting pores. The pore height (H2) of the thick micro-scale porous coating (superamphiphobic) can be as large as the thickness of the coating. The measured coating thickness is approx. 200 µm, i.e., H2 = 200 µm, being much larger than D2/2 (75 µm). Hence, ∆Pbreak-in of thick micro-scale porous (superamphiphobic) coating (∆Pb,sc) is calculated by Equation (5a). The measured H1 (thin coating) is approx. 20 µm, being smaller than D1/2 (75 µm). Therefore, ∆Pbreak-in of thin micro-scale porous (quasi-superamphiphobic) coating (∆Pb,q-sc) is calculated by Equation (5b). Substituting D (both D1 and D2) of 150 µm


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and H1 of 20 µm into Equation (5a) and (5b) respectively, we got ∆Pb,sc of approx. 2.7×104γ and ∆Pb,q-sc of approx. 1.3×104γ. Therefore, for the same liquid (such as n-hexadecane) and same ϕair, the thick coating (superamphiphobic) with larger H could hold the droplets while the thin coating (quasi-superamphiphobic) with small H couldn’t. At last, the repellency of the thick micro-porous coating (superamphiphobic) was discussed. The ϕair of the thick micro-porous coating is approx. 0.94. According to Equation (1), even θY for a liquid, e.g., n-hexane (γ = 18.4 mN/m), is close to 0°, θC,cal could still be larger than 150°. It means that the thick micro-scale porous coating is expected to repel n-hexane as the ϕair is large enough to overhang the droplet at Cassie state. In fact, it fails. According to Equation (4), the maximum ∆PLap can be calculated as 4γ/D when κ reaches to its possible minimum value, 2/D. The maximum ∆PLap is not large enough to surpass the ∆Pb,sc (4γ/D), which means that sag impalement is not likely to happen. Therefore, the most probable reason is that the three-phase contact line depins from the porous structure network and moves downwards until it touches the substrate (dotted line in Figure 7). According to the above-discussed, only the micro-scale porous coating with optimized pore diameter and pore height can display superamphiphobicity.

Superamphiphobic coatings on different materials. Different materials possess different surface properties. It remains an issue of developing a versatile approach that can fabricate superamphiphobic coatings onto various material surfaces


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thus far. In this work, the powerful binding capability of the polydopamine renders the porous structure with the possibility to attach onto different substrates. We picked up four typical materials, i.e., ceramic, aluminum, poly(methyl methacrylate) (PMMA), and wood, representing inorganic nonmetallic materials, metallic materials, organic materials, and natural materials. These materials all can be successfully coated with the superamphiphobic coatings (Figure 9 and S13). Water droplets, olive oil droplets, and n-hexadecane droplets could roll off easily (SA < 10°).

CONCLUSIONS In summary, we have developed a novel, green, and effective approach to construct micro-scale porous structure. Here, dopamine was used as one of the main building blocks rather than a simple surface modification material. The other building block, HiFSNs can be bound by the polydopamine, and the formed porous structure was strengthened by the polydopamine as well. After further surface modification with

fluoroalkylsilane,

the

porous

structure

could

display

excellent

superamphiphobicity. The parameters of the porous structure (pore size and pore height) significantly influence the repellency of the coatings. The porous coating could only display superamphiphobicity when the pore size and pore height are optimized. Compared with nano-scale, micro-scale structure seems to favor the achievement of superamphiphobicity. Since the porous structure is made of HiFSNs and dopamine, the robustness of the structure or the superamphiphobic coating is significantly influenced by the binding strength of the polydopamine. Because of the


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outstanding binding ability of the polydopamine, the superamphiphobic coatings can be successfully applied onto different substrates. In this work, we proposed a novel way to construct superamphiphobic surface and foresee that this micro-scale porous superamphiphobic coating is quite potent in shielding materials from low surface energy liquids with broad application if the robustness and the stability of the porous structure can be further improved.

METHODS Materials. Dopamine hydrochloride (98%), sodium periodate, NaIO4 (AR) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Hydrophilic fumed silica nanoparticles (HiFSNs) were supplied by Degussa AG (Hanau, Germany), under the trade name of Aerosil® 380. Ultra-purified water with pH = 7 (resistivity 18.25×106 Ω·cm) was made in the laboratory. In this work, ultra-purified water was denoted as water for short. 3-aminopropyltriethoxysilane (98%) was purchased from Alfa Aesar Co., Ltd., Shanghai, China. 1H,1H,2H,2H-perfluorodecyltriethoxysilane (96%) was purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Preparation of micro-scale porous structure. HiFSNs (1:100, wt/wt) are first dispersed in water through 5 min of sonication (300 W). Obvious Tyndall effect is observed (Figure S1B‒D), indicating that the HiFSNs could be successfully dispersed in water with the help of abundant hydroxyl groups on the surface of the HiFSNs.77,78 Dopamine (1.0 mg/mL) is dissolved in the HiFSNs dispersion to prepare the Dispersion A. NaIO4 (100 mM) is dissolved in water to prepare the Solution B.


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Dispersion A and Solution B (10:2, v/v) are mixed together and stirred for 18 h. During the reaction, dopamine is oxidized into dopaminequinone by NaIO4 as shown in Figure S2.46,50 The dispersion is diluted by water (1:100, v/v) for UV-vis analysis. The

dopaminechrome is further transformed into 5,6-dihydroxyindole

via

intramolecular rearrangement and eventually goes on polymerization. Then, polydopamine serves as the binding material to bind HiFSNs together, producing large secondary HiFSNs clusters. After 18 h, the as-prepared dispersion is dropped onto substrates, such as glass slides, ceramic, aluminum, poly(methyl methacrylate) (PMMA), and so on. Each 5 cm2 of the substrate is coated with 250 µL of the as-fabricated dispersion by automatic pipetting device. The coated substrate is frozen at −30 °C and stored at the same temperature overnight. Afterwards, the coated substrate is freeze-dried. Effect of initial dopamine and HiFSNs concentration. The initial concentration of dopamine and HiFSNs plays important roles in the formation of porous structures and their stabilities. Lower concentration of initial dopamine (0.1, 0.2 mg/mL) cannot generate enough polydopamine to bind HiFSNs together. After freeze-drying process, the remaining could not form a complete coating (Figure S6A). Complete coatings can be obtained by using dispersions with initial dopamine concentration beyond 0.5 mg/mL (Figure S6B). The coatings prepared by the dispersions with initial dopamine concentration of ≥ 1.0 mg/mL are stable enough to undergo the SCAs measurement. Higher concentration of initial dopamine (2.0, 5.0, 10.0 mg/mL) do not significantly improve the stability of the porous structure. Therefore, the initial concentration of


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dopamine is chose to be 1.0 mg/mL. Appropriate initial HiFSNs concentration is also very crucial for the formation of the porous structure. The polydopamine generated during the polymerization of dopamine bound HiFSNs together and make the size of HiFSNs clusters increase dramatically even the initial HiFSNs concentration is extremely small (1:100, wt/wt) (Figure S3C). Higher initial HiFSNs concentration (1:10, 1:20, 1:50, wt/wt) lead to lots of sediments because more HiFSNs are bound together by polydopamine to generate much larger HiFSNs clusters. Therefore, the coating with porous structure is prepared by the dispersion with the optimal concentrations of initial dopamine (1.0 mg/mL) and HiFSNs (1:100, wt/wt) in this work. Preparation of the superamphiphobic coating. The as-fabricated porous structure on the substrate surfaces are first put into a desiccator with 200 µL of 3-aminopropyltriethoxysilane. The desiccator is sealed and placed in an oven at 50 °C for chemical vapor deposition (CVD) treatment. After 12 h, the coated substrate is taken

out

and

evacuated

for

3

times

to

remove

the

unreacted

3-aminopropyltriethoxysilane in it. Then the coated substrate is put into the desiccator again with 200 µL of 1H,1H,2H,2H-perfluorodecyltriethoxysilane. The desiccator is then sealed and placed into the oven at 50 °C. After 4 h, the desiccator is taken out from the oven and evacuated for 3 times to remove the unreacted fluoroalkylsilane. We have picked 5 typical materials, i.e., aluminum, glass slide, ceramic, PMMA and wood as substrates to represent metallic materials, inorganic nonmetallic materials, organic materials, and natural materials respectively.


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Preparation of the nano-scale porous superhydrophobic coating. 250 µL of the as-prepared dispersion is deposited onto the glass slide (5 cm2) by automatic pipetting device. The sample is dried under ambient condition. After complete evaporation of water, the sample is treated with the same two-step CVD that is described in the “Preparation of the superamphiphobic coating” section. Preparation of the planar surface for measuring Young’s static contact angle (θY). The planar glass slide is first dipped into dopamine (1 mg/mL) aqueous solution. NaIO4 aqueous solution is then poured into the dopamine solution. Reaction parameters are kept the same. After the treated glass slide is washed and dried under N2, it is then CVD treated with the two-step CVD. Characterization. Ultraviolet visible spectrophotometry (UV-vis, CARY 100 Bio, Varian Inc., US) is used to analyze the reaction process of the dopamine. The amount of the polydopamine which binds HiFSNs is characterized via Fourier Transform Infrared Spectrometer (FT-IR spectrometer, VECTOR 22, Bruker Optics Co., Ltd., Germany). After the polymerization of dopamine, the polydopamine bound HiFSNs clusters are separated by sonication and washed by water for three times. The dried clusters are again dispersed in water (1:1000, wt/wt). The size of the polydopamine bound HiFSNs clusters is then measured by DLS (Zetasizer Nano ZS, Malvern Instruments Ltd., UK). The weight loss of different polydopamine bound HiFSNs clusters is measured by thermal gravimetric analyzer (TGA, Q50, TA Instruments, US). The structure of the porous structure is characterized by phase contrast microscope (PCM, BX51, Olympus Ltd., Japan), scanning electron microscope (SEM,


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S4800, Hitachi Ltd., Japan), and transmission electron microscope (TEM, HT7700, Hitachi Ltd., Japan, and JEM-2100F, JEOL Ltd., Japan). XPS (EscaLab 250Xi, Thermo Fisher Scientific, Inc., UK) is used to characterize the chemical composition for porous coating surfaces. The static contact angle (SCA) of the superamphiphobic coating is measured via a contact angle measurement device (Harke-SPCA, Peking Harke Experimental Instrument Factory, Beijing, China). The SCAs are measured by depositing 6 µL of droplets on the coatings.

ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Key Program, Grant 51333004).

Supporting Information Available: The supporting information is available free of charge via the Internet at http://pubs.acs.org.

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Graphic Table of Contents


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Figure 1. Schematic illustration of the nature origin, mussel, and fabrication process. (A) Photograph of the mussel and its byssus. This photograph is obtained from Wikipedia under the item of “mussel”. Chemical structures of (B) DOPA explored in the adhesive protein of mussel’s byssus, and (C) dopamine. Schematic depicting of (D) the formation of the porous structure, and (E) further surface fluoroalkylsilanization to the porous structure. Yellow clusters denote the branched hydrophilic fumed silica nanoparticles (HiFSNs). Separated black circles denote dopamine molecules. Connected black circles denote polydopamine molecules. 159x115mm (300 x 300 DPI)


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Figure 2. Characterization of the reaction and polydopamine bound HiFSNs clusters. (A) UV-vis spectra of the mixed dispersion during the different stages of the reaction. (B) EDX spectrum acquired for polydopamine. N peak indicates the existence of polydopamine. TEM images for (C) HiFSNs, (D) and (E) polydopamine bound HiFSNs clusters. Polydopamine is indicated by white arrows. Accompanied EDX spectra for polydopamine (Zone I, marked in red) is displayed in (B). EDX spectra for polydopamine bound HiFSNs clusters (Zone II, marked in cyan) is displayed in the supporting information. Photographs for (F) HiFSNs (white) and (G) polydopamine bound HiFSNs clusters (black). 159x159mm (300 x 300 DPI)


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Figure 3. Morphology and topography of the as-fabricated porous coating. PCM images of (A) superficial layer, and (B) inside layer of the porous structure. Red dashed lines indicated the micro-scale porous structure network which located at the focus point surface of the lens of the PCM. Orange dashed lines indicated the micro-scale porous structure network which did not located at the focus surface of the lens. (C) SEM image of the superficial surface morphology for the micro-scale porous structure. (D) Enlarged SEM image of the superficial surface morphology. (E) SEM image of the fractured micro-scale porous structure (corresponding to the bottom layer of the micro-scale porous structure). The inside network can be observed from the image. (F) Enlarged SEM image of the micro-scale porous structure network wall. SEM images captured in different areas of the nano-scale porous structure with (G) larger magnification and (H) smaller magnification. 126x64mm (300 x 300 DPI)


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Figure 4. XPS data for coatings before and after CVD and the measured SCAs for droplets deposited on different porous coatings. XPS spectra for micro-scale porous coatings (A) before and (B) after two-step CVD. N characteristic XPS spectra for micro-scale porous coatings (C) before and (D) after two-step CVD. (E) Photograph of an n-hexadecane droplet deposited on the superamphiphobic coating. (F) SCAs for drops with different surface tensions deposited on the superamphiphobic coating, quasi-superamphiphobic coating, and superhydrophobic coating. 144x131mm (300 x 300 DPI)


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Figure 5. Profiles, photographs, surface tensions, and SCAs for various droplets deposited on the microscale porous superamphiphobic coating. 159x48mm (300 x 300 DPI)


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Figure 6. Schematic illustration for structure features of (A) nano-scale porous superhydrophobic coating, (B) thin micro-scale porous quasi-superamphiphobic coating, and (C) thick micro-scale porous superamphiphobic coating. In the three images, the nanoparticle diameters are all 2R. In order to manifest the differences between structure dimension and particle diameter, the nanoparticle drawn in (B) and (C) was smaller than that in (A). 139x38mm (300 x 300 DPI)


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Figure 7. Simplified schematic illustration of the micro-scale porous structure from (A) side and (B) top view. The actual pores are polygons. For simplification, we assume the pores are circular. When no pressure applied on the droplet, it is overhung at Cassie state. With pressure the three phase contact line moves downwards and the interface sags (solid line). (C) Forces exert on the three-phase interface. γ is the surface tension. ∆PLap is the pressure exerted on the contact line. Sagging of the contact line is caused by ∆PLap. D is the diameter of the pore. H is the thickness of the structure. T is the thickness of the porous structure network. In the case of nano-scale porous structure, T is approx. 2R. 33x7mm (300 x 300 DPI)


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Figure 8. Schematic illustration for the curved three-phase contact lines when they were at their lowest position. (A) Droplet on the thick micro-scale porous coating (H > D/2). (B) Droplet on the thin micro-scale porous coating (H < D/2). 39x12mm (300 x 300 DPI)


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Figure 9. Photographs of water droplets (blue ink dyed), olive oil (yellow) and n-hexadecane droplets (colorless) deposited on micro-scale porous superamphiphobic coatings fabricated on (A) ceramic, (B) aluminum, (C) PMMA, and (D) wood. 66x40mm (600 x 600 DPI)


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Silica Nanoparticle Assembled, Microscale Porous

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