Controllable Broadband Optical Transparency and ... - ACS Publications

Sep 23, 2016 - ABSTRACT: Inspired by biointerfaces, such as the surfaces of lotus leaves and pitcher plants, researchers have developed innovative str...
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Controllable Broadband Optical Transparency and Wettability Switching of TemperatureActivated Solid/Liquid-Infused Nanofibrous Membranes Kengo Manabe,† Takeshi Matsubayashi,† Mizuki Tenjimbayashi,† Takeo Moriya,† Yosuke Tsuge,† Kyu-Hong Kyung,‡ and Seimei Shiratori*,† †

Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ‡ SNT Co., Ltd., 7-1 Shinkawasaki, Saiwai-ku, Kawasaki, Kanagawa 212-0032, Japan S Supporting Information *

ABSTRACT: Inspired by biointerfaces, such as the surfaces of lotus leaves and pitcher plants, researchers have developed innovative strategies for controlling surface wettability and transparency. In particular, great success has been achieved in obtaining low adhesion and high transmittance via the introduction of a liquid layer to form liquid-infused surfaces. Furthermore, smart surfaces that can change their surface properties according to external stimuli have recently attracted substantial interest. As some of the best-performing smart surface materials, slippery liquid-infused porous surfaces (SLIPSs), which are super-repellent, demonstrate the successful achievement of switchable adhesion and tunable transparency that can be controlled by a graded mechanical stimulus. However, despite considerable efforts, producing temperature-responsive, super-repellent surfaces at ambient temperature and pressure remains difficult because of the use of nonreactive lubricant oil as a building block in previously investigated repellent surfaces. Therefore, the present study focused on developing multifunctional materials that dynamically adapt to temperature changes. Here, we demonstrate temperature-activated solidifiable/liquid paraffin-infused porous surfaces (TA-SLIPSs) whose transparency and control of water droplet movement at room temperature can be simultaneously controlled. The solidification of the paraffin changes the surface morphology and the size of the light-transmission inhibitor in the lubricant layer; as a result, the control over the droplet movement and the light transmittance at different temperatures is dependent on the solidifiable/liquid paraffin mixing ratio. Further study of such temperature-responsive, multifunctional systems would be valuable for antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications, intelligent windows, and other devices. KEYWORDS: biomimetics, SLIPS, transparency, wettability, temperature-responsive materials

I

characteristics. A rough surface can be transformed into an ordered structure, a random pattern or a hierarchical structure by tailoring its topography from nano- to microscale

nspired by biointerfaces, such as the surfaces of lotus leaves,1 pitcher plants,2−4 mosquito eyes,5,6 honeycomb7,8 and spider silk,9,10 and motivated by both pure scientific interest and industrial applications, many researchers have explored innovative strategies for controlling the wettability and optical properties of surfaces. Surface topography has emerged as one of the most important variables influencing such © 2016 American Chemical Society

Received: June 30, 2016 Accepted: September 23, 2016 Published: September 23, 2016 9387

DOI: 10.1021/acsnano.6b04333 ACS Nano 2016, 10, 9387−9396

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ACS Nano Scheme 1. Schematic Diagram of a TA-SLIPS

a

The porous underlayer consists of CHINFs and PAA, formed via LbL self-assembly. The hydrophobic underlayer was produced by functionalizing the hydrophilic nanofibrous underlayer. The temperature-responsive, super-slippery surface was achieved by infusing solidifiable/liquid paraffin into the hydrophobic nanofibrous membrane.

perfluorinated lubricant oil. In addition, according to previous studies, SLIPSs generated using various combinations of hydrophobic underlayers and lubricant oils can exhibit different functions.26,27 Wong et al. designed a new form of cross-species, bioinspired SLIPS with thermal-healing properties.28 Jiang et al. fabricated organogel SLIPSs that exhibit temperature-driven water adhesion switching.29 Other examples of changing surface functions have been reported by our group, including a selfstanding gel SLIPS fabricated by a facile nanoscale-phase separation method,30 an antireflective SLIPS with a transmittance of 97%,31 and a biocompatible, antithrombogenic SLIPS produced using the layer-by-layer (LbL) self-assembly method.32 Therefore, to develop functional SLIPSs, the underlayer and lubricant oil should be carefully selected on the basis of the intended use. Aizenberg’s group developed the first smart, superslippery surfaces based on adaptive fluid-infused porous films with transparency and wettability that can be tuned via the application of a graded mechanical stimulus.33 Because increasing the films’ strain leads to the formation of a larger area of unfilled, open pores, more light is scattered at the liquid−air interface, resulting in lower light transmission in visible wavelengths. Simultaneously, the stretched film allows the surface to assume an undulating morphology, thereby immobilizing water droplets on the rough surface. The above studies suggest that SLIPSs can be used to fabricate smart surfaces with optical transparency and surface wettability that can be simultaneously controlled by the application of external stimuli if the structure of the outermost surface and the scattering factor of the lubricant layer are simultaneously controlled by varying the lubricant oil morphology or underlayer topography. A slippery fluid-infused film with tunable transparency, antifouling ability, and temperature responsiveness is expected to have applications in solar panels, medical devices, and intelligent windows.34,35 The concept underlying the development of these films was based on replacing the fluoride oils trapped in the surface gaps with paraffin that changes from a solid to a liquid when heated

architectures using top-down (e.g., lithography) or bottom-up (e.g., molecular self-assembly) manufacturing techniques.11,12 Precise control over nano-/microstructured material characteristics, such as size, shape, morphology, and composition, is beneficial for tuning the surface wettability,13 reflection,14 scattering,15 structural color,16 adhesion,17 and mechanical properties.18 The other main variable is a material’s chemical features; responsiveness to stimuli can typically be achieved via chemical modifications. Functional materials that exhibit surface property alterations according to external stimuli are called smart surfaces and have recently gained substantial interest.19,20 Conventionally, research on smart surfaces has focused on the switching of individual properties between two states, and a limited number of strategies that may lead to materials with dynamically adjustable characteristics have been reported, including chirality-triggered switching and biofunctionalization.21,22 For example, the wettability and transmittance of poly(N-isopropylacrylamide) (PNIPAAm) coatings readily change with variations in the environmental temperature because of the reversible contraction and stretching of PNIPAAm chains, which is triggered by the tunable hydrogen-bonding interactions in their surroundings.23 Therefore, PNIPAAm coatings are known as excellent multifunctional smart surfaces and have a history of being used as smart surfaces. Nevertheless, achieving sufficient optical precision and effective repulsion of low-surface tension liquids in solid surfaces remains challenging because of the difficulties associated with fine-tuning the surface structure and surface chemistry to meet the requirements of a smart surface. In contrast, liquid-infused surfaces (LISs) could help to overcome these constraints. Introducing a liquid into a solid material results in dynamic fluid surfaces with “super-slippery” characteristics.24 The recent development of Nepenthes pitcher plant-inspired slippery liquid-infused porous surfaces (SLIPSs) represents a successful application of LISs.25 In this case, omniphobic and self-cleaning properties were introduced by a porous underlayer with low surface free energy trapping of the 9388

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Figure 1. FE-SEM images (first line), film thicknesses (second line), and refractive indices (third line) of porous underlayers with different numbers of bilayers and the hydrophobic porous underlayer (first line). All red scale bars in the FE-SEM images are 1 μm. The film thickness (D) and refractive index (nl) were measured by ellipsometry over an area of 4 × 4 mm.

first line), the porous structure of the (CHINF/PAA) underlayer is derived from the network of CHINFs and increases with the number of bilayers. In addition, the film thickness increases with greater numbers of bilayers (Figure 1, second line). The ellipsometry analysis also suggested that porosity increased with more bilayers, and the refractive index decreased up to 10 bilayers. However, the refractive index values of the 12- and 14-bilayer films were greater than the minimum value of the 10-bilayer film because of PAA aggregation (Figure 1, third line), which was shown via SEM to occur more extensively in the 12- and 14-bilayer films. Therefore, we used the 10-bilayer film for the following experiments. According to the Fresnel equations, the refractive index of an LbL film can be estimated using a simple mixing rule39

above its melting point. Thus, in this case, water droplets on the composite surface should be repelled or immobilized, depending on the environmental temperature. In particular, mixing solidifiable (i.e., solid/liquid transmutable) and liquid paraffin enables the fine-tuning of both the scattering factor of the paraffin layer and the surface topography of thin films. Therefore, tunable broadband optical transparency and wettability switching can be achieved at ambient temperature. Herein, we demonstrate temperature-activated solidifiable/ liquid paraffin-infused porous surfaces (TA-SLIPSs) with transparency and water droplet control at room temperature that can both be simultaneously manipulated (Scheme 1). The porous surfaces were composed of chitin nanofibers (CHINFs) and poly(acrylic acid) (PAA) and were formed via LbL selfassembly. LbL self-assembly is one of the most attractive methods for fabricating SLIPSs because it is inexpensive, ecofriendly, and simple and can be used to coat large areas of curved surfaces, regardless of the radius of curvature.36−38 After the nanofibrous LbL films were hydrophobized, mixtures of solidifiable and liquid paraffin were introduced into the hydrophobic nanofibrous surfaces as the lubricant oil layer. The solidification of the paraffin changed the surface morphology and the size of the light-transmission inhibitor in the lubricant layer; as a result, the temperature dependence of droplet movement and light transmittance could be controlled by the solidifiable/liquid paraffin mixing ratio. Further studies of temperature-responsive, multifunctional systems would be valuable for, e.g., antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications (e.g., an endoscope with transparent and blood-repellent surfaces for surgical applications and a lens that is scratch resistant when not in use) as well as intelligent windows (e.g., antifouling windows that are optically transparent during the daytime but opaque at night to ensure privacy).

nl = fair nair + fpolyelectrolyte 1 n polyelectrolyte 1 + fpolyelectrolyte 2 n polyelectrolyte 2

(1)

where f x is the volume fraction and nx is the refractive index of component x. According to this equation, decreasing or increasing the refractive index of an LbL film will cause the porosity to increase or decrease, respectively. As shown in the SEM image of the four-bilayer film, the CHINFs were well refined by the process described in the Methods. As shown in a TEM image of chitin nanofibers (Figure S1), aggregation of chin nanofibers should demonstrate stacking of nanofibers with maintaining pores of films, and the material filling the pores of the films was PAA. Therefore, the PAA aggregated on the surface as the number of bilayers increased; thus, the refractive index decreased as the film thickness increased. After the porous (CHINF/PAA) 10-bilayer underlayer was hydrophobized, the film thickness decreased slightly from 79.2 to 76.2 nm because of the film shrinking after being heated to 140 °C for the decyltrimethoxysilane (DTMS) chemical vapor deposition (CVD) process. Simultaneously, the refractive index increased from 1.34 to 1.39 because of the film shrinking and the presence of excess DTMS in the films, resulting in a slight decrease in the porosity.

RESULTS AND DISCUSSION Porous (CHINF/PAA) Underlayer. First, hydrophilic porous films consisting of CHINFs and PAA were fabricated by LbL self-assembly. As shown in the SEM images (Figure 1, 9389

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Figure 2. C1s core-level spectra of the porous underlayer (a) before and (b) after hydrophobization by DTMS. The black, orange, gray, yellow, blue, and green lines indicate the original intensity, C−C, C−O, O = C−O, sum, and background peaks, respectively. (c) Contact angle (orange bars), sliding angle (green bars), and surface tension (blue diamonds) of water and liquid paraffin oil on a hydrophobized porous (CHINF/PAA) film and on a SLIPS obtained by dropping liquid paraffin oil onto the hydrophobized porous film. Bottom: corresponding images of liquid drops on the different films. (d) E1, E2, and EA are the total interfacial energies per unit area of the wetting configurations 1, 2, and A, respectively. Each Δ indicates an essential criterion to maintain a stable lubricating film on a functional SLIPS under working conditions as expressed in eqs 2 and 3.

Table 1. Contact Angles and Surface Tensions of Water and Liquid Paraffin Oil and Calculated Interfacial Tension and Spreading Coefficient between Water and Liquid Paraffin Oil water γwa (mN/m)

θw (deg)

72.1

108.7

γαwa

liquid paraffin oil (mN/m)

γoa (mN/m)

θo

γαoa (mN/m)

γwo (mN/m)

R

ΔE1

ΔE1

Sow(a)

24.8

17.2

24.8

50.4

2

43.2

140.9

−3.1

21.8

the hydrophobic multilayer in the presence of air. R is the roughness factor of the underlayer, which is defined as the ratio of the solid−liquid area ASL to its projection on a flat plane AF, which is expressed by water contact angle for a rough surface and a smooth surface2,40

The XPS analysis revealed the presence of C−C, C−O, and OC−O bonds, which are major peaks of CHINFs and PAA, indicating that the films contained CHINFs and PAA (Figure 2a,b). These measurements also confirmed that the DTMSmodified films contained abundant C−C bonds before and after DTMS CVD, resulting in a low surface energy. DTMS reacted with hydroxyl groups from the porous (CHINF/PAA) underlayer to form a thin, hydrophobic molecular layer. The CVD step transforms this porous film from hydrophilic to hydrophobic, with a water contact angle of 108.7 ± 0.95°, as illustrated in Figure 2c. Prior to hydrophobization, the contact angles of the hydrophilic porous films were close to zero because of their hydrophilicity and the absorption of the water-soluble polymers. The contact angles of water and liquid paraffin oil on the hydrophobic films were measured to determine whether the hydrophobized porous films met the following SLIPS criteria (Figure 2d)2,25 ΔE1 = R(γoa cos θo − γwa cos θw ) − γwo > 0

(2)

ΔE2 = R(γoa cos θo − γwa cos θw ) + γwa − γoa > 0

(3)

R=

ASL cos θw = AF cos θ

(4)

where θ is 99.4 ± 0.23° on the flat glass substrate covered by DTMS after CVD. γwo was calculated using the Fowkes equation α 1/2 γwo = γoa + γwa − 2(γoaα γww )

(5)

where γαoa and γαwa are the dispersion force contributions of the liquid surface tensions. The dispersion force contribution of the water surface tension was 21.8 mN/m. For nonpolar materials, γαoa ≈ γoa.2,25 After the CVD step, the films did not show superhydrophobicity because they had nanotopography derived from CHINFs rather than microtopography. The hydrophobic porous surface met the SLIPS criteria and displayed a stable liquid surface, as described in Table 1, indicating that this film had a SLIPS consisting of a porous nanofibrous underlayer and a lubricant layer (i.e., liquid paraffin oil). The criterion for cloaking is determined by the spreading coefficient as follows:41

where E1 and E2 represent the total interfacial energies per unit area of the wetting morphologies 1 and 2, respectively, as shown in Figure 2d, and each Δ indicates the working conditions for maintaining a stable liquid-infused film, γxy is the interfacial tension between two phases, designated by the subscripts w (water), o (liquid paraffin oil), and a (air), and θo and θw are the contact angles of liquid paraffin oil and water on

Sow(a) = γwa − γwo − γoa 9390

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Figure 3. Dependence of the (a) contact angle and (b) sliding angle of a 10 μL water droplet on the volume ratio of the paraffin mixture (solidifiable/liquid paraffin = 1:10, lime-green line; 1:15, yellow line; 1:20, pink line; 1:25, purple line; 1:30, blue line; 1:35, green line; 1:40, brown line; 1:45, red line; and 1:50, black line) at different temperatures. (c) Images of a 10 μL water droplet on a TA-SLIPS (volume ratio 1:25) at different temperatures obtained using a charge-coupled device camera. (d) Color 2D laser scanning microscopy images of a TASLIPS (volume ratio 1:25) at different temperatures. Bottom: corresponding roughness scale bars. Black scale bars: 100 μm. Larger 3D images are presented in the Supporting Information.

53 ± 3.98° near the transition temperature, which was approximately 28 °C (Figure 3a). Examining the sample images revealed that the transition of the paraffin mixture from the solid to the liquid phase at the surface occurred as the surface temperature increased (Figure 3c). As shown in Figure 3 b, the sliding angle of water droplets with a fixed volume of 10 μL decreased as the temperature increased from 22 to 30 °C such that the droplets were no longer immobilized but were free sliding; this behavior was attributed to the change of surface tension on the TA-SLIPSs, which improved the mobility of the droplets, especially near the transition temperature. For temperatures below 22 °C, no water droplet sliding could be observed, even when the solidified paraffin surface was tilted at 90°; this finding was supported by the microtopography of the paraffin mixture that formed as the temperature decreased (Figure 3d). The paraffin surface changed from the Wenzel state to the SLIPS state at temperatures near the transition temperature. Thus, in the current experimental setup, the adhesive behavior of TASLIPSs toward water droplets can be controlled by tuning the temperature to be either above or below the transition temperature. Optical Properties of TA-SLIPSs. Similarly, the optical properties of the TA-SLIPSs were investigated, as shown in Figure 4. The volume ratio of the paraffin mixture altered the trends observed in the transmittance measured with changing temperature (Figure 4a). All data showed nonlinear (e.g., quadratic) behavior caused by a reduction in the cross-sectional area when going from solidified to liquefied paraffin, assuming that the solidified paraffin became macroscopically spherical. Specifically, the transmittance of the TA-SLIPS with a volume

Sow(a) > 0 implies that the water droplet will be cloaked by the lubricant fluid and that the lubricant can be easily lost through evaporation, whereas Sow(a) < 0 implies that the water droplet will not be cloaked by the lubricant fluid and that the lubricant will remain in the porous underlayer and will not evaporate. For SLIPSs with liquid paraffin oil, Sow(a) = −3.1 (Table 1); thus, the lubricant fluid encapsulating the porous multilayer was stable. The above-mentioned criterion was determined at room temperature. Here, the TA-SLIPSs were composed of mixed paraffin oil consisting of solidifiable and liquid paraffin, and the melting point depended on the volume ratio of the paraffin mixture (solidifiable/liquid paraffin). Therefore, it was difficult to strictly apply this approach to TA-SLIPSs. However, by investigating SLIPSs designed based on the assumption that the lubricating oil of the liquid phase should be liquid paraffin, TASLIPSs may be considered to meet the criterion and become stable when the surface changes from the solid to the liquid phase. Surface Wettability of TA-SLIPSs. After the paraffin mixture (i.e., solidifiable/liquid paraffin) was introduced to the hydrophobic porous underlayer, the surface wettability of the TA-SLIPSs was investigated at different temperatures. As shown in Figure 3a,b, the contact and sliding angles sharply decreased at certain temperatures depending on the volume ratio of the paraffin mixture; these results indicated that the temperature of the liquid-to-solid transition of the paraffin mixture at the surface depended on the ratio of solidifiable to liquid paraffin. Here, we focus on the surface wettability of TASLIPSs with a volume ratio of 1:25 at different temperatures. The water contact angle decreased rapidly from 99 ± 4.82° to 9391

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Figure 4. (a) Measured transmittance changes of TA-SLIPSs at 550 nm depending on the volume ratio of the paraffin mixture (solidifiable/ liquid paraffin = 1:10, lime-green dots; 1:15, yellow dots; 1:20, pink dots; 1:25, purple dots; 1:30, blue dots; 1:35, green dots; 1:40, brown dots; 1:45, red dots; 1:50, black dots) at different temperatures. (b) Measured transmittance changes of TA-SLIPSs (volume ratio 1:25) depending on the temperature (0 °C, black dots; 5 °C, gray dots; 10 °C, purple dots; 15 °C, magenta dots; 20 °C, pink dots; 25 °C, sky-blue dots; 26 °C, blue dots; 27 °C, dark-blue dots; 27.5 °C, dark-green dots; 28 °C, green dots; 28.5 °C, lime-green dots; 29 °C, yellow dots; 29.5 °C, orange dots; and 30 °C, red dots). (c) TT (orange bars), HAZE (blue bars), PT (green bars), and DIF (purple bars) of TA-SLIPSs (volume ratio: 1:25) depending on the temperature. Each optical parameter is explained in the Supporting Information. The HAZE, which describes the amount of light scattering that occurs as the light passes through the film, was calculated by dividing DIF by TT (d) Schematics of the TA-SLIPS scattering models according to temperature and corresponding digital microscopy images of a TA-SLIPS at different temperatures. Red scale bars: 100 μm. (e) Images showing the temperature-dependent optical performance of a TA-SLIPS (volume ratio 1:25) as the temperature was increased from 0 to 30 °C at 2 °C intervals.

(Figure S8) and the TA-SLIPSs demonstrated high transparency with transmittance values exceeding 90%. Each optical parameter provided information regarding the optical performance of the TA-SLIPSs with a volume ratio of 1:25, as shown in Figure 4c. The parallel transmittance (PT) showed trends similar to those observed in Figure 4a,b. The diffusion (DIF) continued to increase from 0 to 6 °C but tended to plateau at its peak at 6 °C because of backward scattering from the microtopography of the solidified TASLIPSs. The total transmittance (TT) increased substantially at temperatures under 10 °C, but its growth was limited above 10

ratio of 1:25 changed from 4% to 91% at 550 nm. According to the UV−vis transmittance results (Figure 4b), the TA-SLIPS with a volume ratio of 1:25 showed temperature-dependent broadband transmittance. For surface temperatures ranging from 0 to 10 °C, the transmittance depended on the wavelength because of an increase in the wavelength-dependent Rayleigh scattering; this phenomenon occurs because lower surface temperatures give rise to larger root-mean-square (RMS) surface roughness values of TA-SLIPSs.42 As the temperature increased, the wavelength-dependent transmittance at the wavelength of more than 400 nm decreased 9392

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Figure 5. Multiple cycles of switching the (a) transparency and (b) adhesion of a 10-μL water droplet on a TA-SLIPS (volume ratio: 1:25) with variations in the temperature (high temperature, 30 °C; low temperature, 20 °C).

°C, indicating that the wavelength-dependent scattering mentioned above decreased. On the basis of these results, we developed optical scattering models representing the temperature responsiveness of the TASLIPSs, as shown in Figure 4d. At lower temperatures, incident light was scattered forward and backward with low DIF because of Mie scattering. As the temperature increased, the backward scattering and DIF decreased. After the paraffin mixture changed from the solid to the liquid phase, the DIF almost disappeared, and the transmittance mainly consisted of PT. As shown in Figure 4d, the aggregation of the paraffin mixture occurred at the surface. Increases in nano-/microtopography and surface roughness cause the transmittance of a surface to decrease by increasing scattering, as predicted by the optical theory expressed in the following equation43−45 ln(T /T0) = −{2π (nair − ncoating ) cos θ}2 (σ /λ)2

used as innovative smart coatings for medical devices, such as endoscopes, to repel blood while maintaining high transparency and device protection via the solidified paraffin surface.

CONCLUSIONS In conclusion, multifunctional smart surfaces were produced using TA-SLIPSs consisting of a hydrophobized porous underlayer and a mixed paraffin lubricant layer. The porous underlayer was fabricated using LbL self-assembly to form a nanofibrous membrane, and the mixed paraffin consisted of solidifiable and liquid paraffin. The scattering factor of the paraffin layer and the surface topography of the thin films could be finely tuned, and these materials exhibited desirably tunable broadband optical transparency and wettability switching at ambient temperature. When solidified, the TA-SLIPS immobilized water droplets and became opaque due to abundant light scattering. As the temperature increased, the TA-SLIPS changed from the solid to the liquid phase. When liquefied, the TA-SLIPS readily allowed the movement of water droplets and exhibited decreased light scattering. The transition temperature depended on the solidifiable:liquid paraffin mixing ratio. The solidification of the paraffin changed the surface morphology and the size of the light-transmission inhibitor in the lubricant layer. As a result, the droplet movement and light transmittance at different temperatures could be controlled by altering the solidifiable/liquid paraffin mixing ratio. Although our TA-SLIPSs can maintain the properties for long periods by utilizing the transition from solid state to liquid state, the sustained liquid state of TA-SLIPSs leads to their disruption. Therefore, the mechanical properties and stability, such as weatherability and mechanical robustness, remain to be determined as future challenges. Further investigations into such temperature-responsive, multifunctional systems would be valuable for antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications and smart windows, as well as other devices.

(7)

where T and T0 are the transmittance of the coating with and without surface roughness, respectively; nx is the refractive index of x; cos θ is the optical incidence angle of the outermost surface layer; σ is the surface roughness RMS; and λ is the wavelength. According to this equation, increasing the roughness σ should decrease the transmittance, with higher transmittance values obtained at longer wavelengths if each variable is set as follows: nair = 1.00, ncoating = 1.48, and θ = 0 (Supporting Information). This relationship supports the discussion above. In addition, the images demonstrating the temperature responsiveness of TA-SLIPSs (volume ratio 1:25) from 0 to 30 °C also provided information about the optical properties, as discussed above and shown in Figure 4e. Reversible Transparency and Adhesion Switching of TA-SLIPSs. Continuously switchable optical and surface wettability properties are crucial for enabling the wide application of these types of materials.46−48 As shown in Figure 5, the TA-SLIPSs demonstrated excellent transparency and water droplet adhesion switching with good reversibility. In addition, the transition temperature of this system can be readily changed by using different paraffin mixture volume ratios. Thus, the properties of TA-SLIPSs can be adjusted according to the specific requirements of practical applications. For a TA-SLIPS with a volume ratio of 1:25, the transition temperature is near 28 °C. Therefore, TA-SLIPSs can be applied in unique smart windows with controllable transmittance, such as one that is highly transparent during the daytime but opaque at night. Alternatively, TA-SLIPSs could be

METHODS Materials. Crab shells (Kawai Hiryo, Iwata, Japan), PAA (Mw ∼ 100 kg/mol, 35 wt % aqueous solution, Sigma-Aldrich, St. Louis, MO), DTMS (Shin-Etsu Chemical Co., Ltd., Tokyo, Japan), liquid paraffin (Kanto Chemical Co., Inc., Tokyo, Japan), paraffin oil (melting point: 58−60 °C, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and glass substrates (76 × 26 mm, thickness: 1.0 mm, refractive index: 1.52, Matsunami Glass Ind., Ltd., Kishiwada, Japan) were used to produce the films. All LbL dipping suspensions were fabricated using 9393

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ACS Nano ultrapure water (Aquarius GS-500.CPW, Advantec, Japan), and the suspension pH was adjusted using CH3COOH (Kanto Chemical Co., Inc., Tokyo, Japan). Glass substrates were cleaned in KOH solution (1:120:60 wt % KOH/H2O/IPA) for 2 min and then rinsed thoroughly with ultrapure water before use. HCl, NaOH, NaClO2, and NaBH4 were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Refinement of CHINFs. CHINFs were fabricated using the following method. The crab shells were purified as previously described.49,50 First, crab shell powder was treated in 2 M HCl for 2 days at room temperature to remove mineral salts. After a thorough rinsing with distilled water, the treated chitin powder was heated under reflux in 2 M NaOH for 2 days to remove proteins. The pigment was then removed using 1.7 wt % NaClO2 in buffer solution for 6 h at 80 °C. After the sample was rinsed thoroughly with distilled water, it was suspended in 33 wt % NaOH containing NaBH4. The suspension containing the CHINFs, NaOH, and NaBH4 was washed several times with pure water via centrifugation (5000 rpm, 5 min). Then, the CHINF suspension was diluted to a concentration of 0.025 wt % and dispersed via ultrasonication. Preparation of Paraffin Oil/Liquid Paraffin Mixture. Solidified paraffin oil was placed in a thermal incubator for 3 h at 90 °C to be liquefied. The liquefied paraffin oil and liquid paraffin were combined at different ratios and stirred in the same thermal incubator for 12 h at 90 °C (solidifiable/liquid paraffin = volume ratio of 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45 or 1:50). Fabrication of TA-SLIPSs. The porous films used as the underlayer were fabricated by the LbL method using suspensions of refined CHINFs and PAA. A glass substrate was alternately immersed in the cationic CHINF suspension (0.025 wt %, pH 3) and the anionic PAA suspension (1 mM, pH 3) for 1 min, rinsed with pure water for 3 min, and air-dried from a distance of 10 mm at 0.05 MPa after the deposition of each layer. The films covered approximately two-thirds of each glass substrate, and the remaining one-third was bare glass. The porous films were hydrophobized using CVD. A porous film with the lowest refractive index was placed in a 100 mL glass bottle together with a 2 mL glass bottle containing 200 μL of DTMS. This system was then inserted into a thermal incubator for 2.5 h at 140 °C. After hydrophobization, the solidifiable paraffin oil/liquid paraffin mixture (1 μL cm−2) was added dropwise onto the hydrophobized film at 60 °C. Excess paraffin oil on the films was removed by blowing the surface with nitrogen gas. Characterization. Field emission scanning electron microscopy (FE-SEM) images were captured (S-4700, Hitachi, Japan) at an accelerating voltage of 3 kV to characterize the surface morphology of the films. X-ray photoelectron spectroscopy (XPS, JPS-9010TR, JEOL, Tokyo, Japan) with an MGKα laser was performed to investigate the chemical composition of the porous surfaces. Transmittance measurements in the spectral range of 300−900 nm were conducted using a spectrophotometer (UVmini-1240, Shimadzu, Kyoto, Japan). The topography of the films was analyzed with a color 3D laser scanning microscope (VK-9710, Keyence, Osaka, Japan). The film thickness and the refractive index of the porous underlayer coated on the glass substrate were determined by ellipsometry (MARY-102, Five Lab, Saitama, Japan) (measuring model: rotating retarder method; light source: 0.8 mW HeNe laser at 632.8 nm; beam diameter: 0.8 mm; fixed incident angle: 45°; sample time: 0.05 s, refractive index of glass: 1.52, absorption index of glass: 0.0203). The contact and sliding angles of a 10-μL droplet at room temperature were measured using a contact angle meter (CA-DT, Kyowa, Saitama, Japan) at different surface temperatures. TT, PT, DIF, and haze values (HAZE) of the films were measured by a haze meter (NDH-5000, Nippon Denshoku Industries, Tokyo, Japan) with a white light-emitting diode (5 V, 3 W) as an optical source.

TEM image of chitin nanofibers; DLS image with the average particle diameter; measured transmittance changes of porous LbL underlayer composed of CHINFs and PAA; color 3D laser scanning microscopy images of TA-SLIPS surface roughness (Ra and RMS) at different temperatures; an explanation of each optical parameter; a relation between surface roughness and transmittance; time-resolved images of TA-SLIPSs at different temperatures observed by a high-speed camera; 2D image of measured transmittance changes of TA-SLIPSs; 2D and 3D measured absorbance changes of a paraffin mixture in a quartz cell at different temperatures; measured melting point depending on the volume ratio of the paraffin mixture; dependence of the surface tensions of the paraffin mixture at different temperatures; thickness and refractive indices of porous underlayers with different numbers of bilayers and the hydrophobic porous underlayer (PDF) Movie of the optical performance of TA-SLIPSs (volume ratio 1:25) as the temperature increased from 0 to 30 °C (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

K.M. conceived, designed and performed the experiments, analyzed the data, and wrote the paper. K.M. and K.-H.K. designed the equipment. T. Matsubayashi, T. Moriya, Y.T., M.T., and K.-H.K. provided experimental support and contributed to the data analysis. S.S. supervised the project, provided scientific advice, and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are deeply grateful to Dr. Kouji Fujimoto, Ms. Nanae Fukao, and Ms. Chie Tanaka, whose insightful comments and suggestions were of inestimable value for our study. We are indebted to Dr. Yoshio Hotta, whose meticulous comments were also enormously helpful. We are also grateful to all members of the Shiratori Laboratory for their helpful discussions. We received funding from the Keio Leading-Edge Laboratory of Science and Technology (KLL) (KLL Research Grant No. 60 in 2015 and No. 34 in 2016, awarded to K.M.). This work was supported by JSPS KAKENHI (Grant No. JP26420710). REFERENCES (1) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus, or Escape from Contamination in Biological Surfaces. Planta 1997, 202, 1−8. (2) Dai, X.; Stogin, B. B.; Yang, S.; Wong, T. S. Slippery Wenzel State. ACS Nano 2015, 9, 9260−9267. (3) Yu, L.; Chen, G. Y.; Xu, H.; Liu, X. Substrate-Independent, Transparent Oil-Repellent Coatings with Self-Healing and Persistent Easy-Sliding Oil-Repellency. ACS Nano 2016, 10, 1076−1085. (4) Vogel, N.; Belisle, R. A.; Hatton, B.; Wong, T.-S.; Aizenberg, J. Transparency and Damage Tolerance of Patternable Omniphobic Lubricated Surfaces Based on Inverse Colloidal Monolayers. Nat. Commun. 2013, 4, 2167. (5) Huang, Y.-F.; Chattopadhyay, S.; Jen, Y.-J.; Peng, C.-Y.; Liu, T.A.; Hsu, Y.-K.; Pan, C.-L.; Lo, H.-C.; Hsu, C.-H.; Chang, Y.-H.; Lee, C.-S.; Chen, K.-H.; Chen, L.-C. Improved Broadband and Quasi-

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04333. 9394

DOI: 10.1021/acsnano.6b04333 ACS Nano 2016, 10, 9387−9396

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