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Hao Bai, Lin Wang, Jie Ju, Ruize Sun, Yongmei Zheng,* and Lei Jiang Surfaces with controllable wettability are of great significance in chemistry, biology, materials science and nanotechnology,[1–11] especially in microfluidics,[1–3] bioarrays,[4,5] and the fabrication of nanostructures.[6–9] Biological organisms smartly control their surface wettability on multiscale structures to help them survive in harsh environments. For example, certain beetle species in the Namib Desert have evolved to collect water from fog on their backs by way of wettability patterns (i.e., hydrophilic waxfree bumps surrounded by hydrophobic waxy background),[12] which attracted an ongoing interest in biomimetic studies.[13–16] Besides, cribellate spiders also collect water from fog,[17–19] taking advantage of the “periodic spindle-knot” structures on their wetted capture silks, where both wettability[20–22] and shape[23–26] gradients are integrated to collect tiny water droplets directionally.[19] Thereafter, bioinspired fibers with unique wettability have been developed for efficient water collection.[22] However, bioinspired materials related to water collection have not been investigated in depth so far, especially how to improve the water collection efficiency with control over the surface wettability. Here, a novel kind of surface with star-shaped wettability patterns is designed inspired by the water-collecting strategies from desert beetles and spider silk. By integrating a surface energy gradient and Laplace pressure gradient, surfaces with star-shaped wettability patterns can quickly drive tiny water droplets toward more wettable regions to avoid being lost in the wind. As a result, this type of surface is more efficient in water collection than uniform superhydrophilic, uniform superhydrophobic, or even circle-patterned surfaces. Moreover, this enhancement is more obvious when reducing the pattern size (from 1000, 500, to 250 µm). The study provides insights into design of novel surfaces with complex wettability that can be used to enhance the efficiency in water collection or other engineering applications related with liquid harvesting. Dr. L. Wang, Dr. R. Sun, Prof. Y. Zheng, Prof. L. Jiang Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment Beihang University (BUAA) Beijing 100191, P. R. China E-mail: [email protected] Dr. H. Bai National Center for Nanoscience and Technology Beijing 100190, P. R. China Dr. J. Ju, Prof. L. Jiang Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences (ICCAS) Beijing 100190, P. R. China

DOI: 10.1002/adma.201400262

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Surfaces with star-shaped wettability patterns can be fabricated following the procedures in Figure 1. At first, a superhydrophilic surface can be fabricated by depositing TiO2 slurry onto a bare glass substrate via a spin-coating method, where the fog droplets spread easily (Figure 1a). Then the film is treated with heptadecafluorodecyl-trimethoxysilane (FAS)[10] to change the wettability from superhydrophilic (Figure 1a) to superhydrophobic (Figure 1b), where fog droplets hardly wet the surface and remain in a spherical shape (Figure 1b). In addition, photomasks with circle-shaped pattern or 4-, 5-, 6-, and 8-pointed star-shaped pattern are used to obtain features of wettability patterns via selective illumination of UV light (Figure 1c). This is because, after exposure of UV light, the FAS-modified superhydrophobic TiO2 surface becomes superhydrophilic again due to the photocatalytic decomposition of the FAS monolayer (Figure 2e).[27] On the surfaces with star-shaped wettability patterns, fog droplets can be directionally collected toward more wettable regions (Figure 1c). In this way, various kinds of surfaces with different wettability patterns can be successfully obtained and tested for their water collection properties. Scanning electron microscopy (SEM) was used to observe microstructures of the surfaces, as shown in Figure 2a and b. A highly porous film (∼19.2 µm in thickness) composed of clusters of TiO2 nanoparticles (∼50 nm in size) was obtained after spin-coating. Observation on its wettability shows that the unmodified TiO2 surface is superhydrophilic with a water contact angle (CA) close to zero (Figure 2c), whereas the FASmodified surface is superhydrophobic with a CA above 150° (Figure 2d). However, wettability of the FAS-modified surface can be changed from > 150° to < 5° after illumination under a UV light (365 nm, ∼25 mW/cm2) for ∼60 min (Figure 2e). It indicates that surfaces with various kinds of wettability patterns (e.g., circle-pattern or 4-, 5-, 6-, and 8-pointed star-pattern) can be designed effectively to improve water collection efficiency by integrating both surface energy gradient and Laplace pressure gradient. The processes of water collection were observed by an optical microscopy, and recorded by a charge coupled device camera (CCD). A fog flow generated by an ultrasonic humidifier (relative humidity of more than 95%) was used to examine the water collection properties on four kinds of surfaces, i.e., uniformly superhydrophilic, uniformly superhydrophobic, circle-patterned, 8-pointed star-patterned surface, respectively. As shown in Figure 3, top-view optical images show the behavior of captured water droplets on these surfaces. As for uniformly superhydrophilic surface shown in Figure 3a, the droplets spread immediately when they are captured by the surface, owing to the low CAs (not more than 5°). The solid-liquid contact lines (see arrows) would eventually propagate to cover the whole surface. On the other hand, the uniformly superhydrophobic

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Efficient Water Collection on Integrative Bioinspired Surfaces with Star-Shaped Wettability Patterns

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after the droplets release the original regions (see the blue dotted line, Figure 3d). It displays a water collection behavior similar to the beetle’s back. It can be understood that the circle-shaped pattern provides an imbalanced force at the two sides of a droplet which acts as a driving force (Fwet-grad) to collect the droplet directionally toward the more wettable region.[20] It can be described by Fwet − grad ≈ γ water (cosθ inside − cosθ outside ), where γwater is the surface tension of water, θinside and θoutside are CAs inside and outside of the pattern, respectively. In fact, that θinside < 5° Figure 1. Schematic illustration of the fabrication process of bioinspired surfaces with star- and θoutside > 150° induces a large driving shaped wettability patterns. a) Superhydrophilic surface composed of TiO2 nanoparticles, force (Fwet-grad) to propel water droplets into where fog droplets spread (the bottom). b) Superhydrophobic surface modified with FAS more wettable circular region (Figure 3c). showing non-wetting property to fog droplets (the bottom). c) Bioinspired gradient surface It is considered that three main steps are with a star-shaped wettability pattern. It is realized by illuminating the FAS-modified film under followed to affect fog collection efficiency, i.e., UV light with a photomask. The fog droplets would be collected directionally toward the star“capture”, “collect”, and “transport”. During shape region, which is more wettable (the bottom). the “capture” step tiny fog droplets (1−40 µm in diameter[12,28]) need to be captured from air.[16,29–31] The fog droplets are propelled by the wind and capsurface (Figure 3b) allows droplets to maintain in spherical shape with high CAs (more than 150°). These droplets are tured onto the solid surface by direct contact,[14,17,19,25] where unstable to roll randomly on the surface. Although coalescence the capturing efficiency is related to wind flow velocity, wettafrequently happens between neighboring droplets (e.g., dropbility[30,32] and structures on the as-designed surfaces.[16] Even lets 1, 2 and 3 coalesce to a larger droplet 4), the moving directhough the tiny water droplets can be captured by the surface, tion is not regulated effectively. Figure 3c shows a surface with they might be lost into the air again due to evaporation or wind. circle-shaped wettability pattern (marked with red dotted line) Therefore, the “collection” step is crucial for a well-designed similar to the desert beetle’s back. The circular region is more surface to achieve a high efficiency. For example, spider silks wettable and has higher surface energy (superhydrophilicity), are capable of collecting tiny droplets in a directional manner where the droplets spread easily (CA ∼ 0°). On the other hand, to form large droplets on more wettable regions, taking advanthe region outside the circle has lower surface energy (superhytage of the gradients in wettability and Laplace pressure.[12,17] drophobicity) to allow the droplet have high CAs and maintain Finally, the “transport” step also needs to be carried out with in spherical shape. The droplets are easily coalesced to move high efficiency. The droplets will roll off the surface by gravity toward the circular, more wettable region (see the arrows) from when they grow large enough,[15,33] where the hydrophilicity outside. More importantly, a new collecting-cycle starts right of the collecting site and its area, as well as the tilted angle of the surface are crucial factors.[15] During the whole process of water collection, tiny water droplets would be collected and then coalesce into larger ones as fast as possible to avoid evaporation, and subsequently be transported into the reservoir of the water-collecting system. Based on the above discussions, we designed surfaces with star-shaped wettability patterns, where structural features of both the beetle’s back and spider silks were integrated. As shown in Figure 3e, an 8-pointed starshaped pattern (depicted by the red dotted line) was used to observe the water collection process. The fog droplets are initially captured everywhere on the patterned surface. Figure 2. The characteristics of a wettability-patterned surface. a) Top view SEM image of the Since there is a surface energy gradient, tiny surface structures on the TiO2 film, where clusters of TiO2 nanoparticles can be observed. droplets on the superhydrophobic region are b) Side view SEM image of the spin-coated TiO2 film with a thickness of ∼19.2 µm. c) Optical propelled into the superhydrophilic region image showing the superhydrophilic property on an unmodified TiO2 film with CA < 5°. to form a larger droplet (see arrows). In fact, d) Optical image showing the superhydrophobic property on an FAS-modified TiO2 film with CA > 150°. e) The superhydrophobic FAS-modified TiO2 film becomes superhydrophilic again the tips of the star generate a Laplace pressure gradient from the shape gradient, which after being illuminated by UV light for 60 min, owing to the photocatalytic decomposition of further enhances this directional movement the FAS monolayer.

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COMMUNICATION Figure 3. Different fog-collecting processes on surfaces with various wettability features. a) On uniformly superhydrophilic surface, water droplets spread over surface, see arrows. b) On uniformly superhydrophobic surface, individual water droplets coalesce randomly (e.g., droplet 1+2+3 to 4). c-f) On bioinspired surfaces with wettability patterns, tiny water droplets are collected directionally toward the more wettable region, see arrows. d) and f) Detailed directional collecting processes on the border of c) circle and e) star-shaped wettability patterns, respectively. b-f) The collecting processes are continuous because new droplets appear right after the previous ones move away, see the dotted circles and arrows in the rightmost row. The directional motion of tiny water droplets enhances the fog-collecting efficiency.

of water droplets.[23,24,34] As observed in details at a single tip (Figure 3f), the droplets move and spread toward the center of the star pattern owing to the restriction of the “wedge-shaped” structure. After the droplets release their original places, a new cycle of collection begins immediately (see the arrows). In the same period of time, the surface with a star-shaped pattern (Figure 3e,f) is more efficient at collecting water than that with a circle-shaped pattern (Figure 3c,d). As known, asymmetric surface structures induce asymmetric spreading and moving of water droplets, which may introduce different water collection mechanisms compared to circle-shaped patterns.[24,35–37] Star-shaped patterns generate a Laplace pressure gradient from the tip to the base, similar to the wetting on a wedge-shaped structure,[38–40] which reflects on the arm of the star pattern. In more detail, because of the confined “tip” (shape-gradient) on the star-patterned surface, the water droplet has asymmetric CAs, i.e., a CA toward the center (θinside) and a CA toward the tip (θ’inside). Thus, a driving

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force (Fshape-wet-grad) arises from the shape-gradient to move the droplet directionally toward the center of the pattern, i.e.,[24,35]: Fshape− wet − grad ≈ γ water ⎡⎣(cosθ inside − cosθ inside ′ ) + (cosθ inside − cosθ outside )] > Fwet − grad

(1)

It indicates that Fshape-wet-grad combines a wettability gradient and a shape gradient. In other words, a star-patterned surface might have a higher water collection efficiency than a circle-patterned surface, because a larger driving force (Fshape-wet-grad > Fwet-grad) might move the tiny droplets faster toward the collecting site. To characterize the water collection efficiency quantitatively, seven types of surfaces, i.e., uniformly superhydrophilic, uniformly superhydrophobic, circle-patterned, 4-, 5-, 6-, and 8-pointed star-patterned surfaces, are compared with the weight of water collected per unit time and unit area (see

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Figure 4. a) Schematic illustration of the method used to quantitatively measure the fog collection efficiency of different surfaces. The substrate is inclined by 45° from the horizontal plane. A fog flow (∼75 cm/s in velocity) containing numerous tiny water droplets is generated by a humidifier. Water droplets are collected on the substrates at first and then roll into a collector. The total weight of the collected water is measured after 2 h to evaluate fog-collecting efficiency. b) Fog-collecting efficiency of different wettability surfaces. Bioinspired surfaces with wettability patterns are more efficient than surfaces with uniform wettability. Star-shaped pattern are more efficient than that with circle-shaped pattern. c) Comparison of efficiency between circular and 5-pointed star-shaped patterns when the substrate is inclined at 15°, 45°, 90°, respectively. d) Ratios of star-shaped patterns to circle-shaped pattern (in terms of their hydrophilic area, as well as collected water). Star-shaped patterns have smaller hydrophilic area (< 1, the dashed line) but higher fog-collecting efficiency (> 1).

Figure 4). These surfaces are fixed on a sample stage with an inclined angle of 15°, 45° or 90° to the horizontal plane, and exposed under a fog flow (∼ 75 cm/s in velocity) to mimic the way beetles collect water (Figure 4a).[6,14,15,19,32] All surfaces are 20 mm × 20 mm in size, and all the patterns have a periodicity of ∼1000 µm to mimic the beetle’s back[12] (see also Figure S2 and Table S1 in Supporting Information for a detailed calculation of the hydrophilic area of the different surfaces). As shown in Figure 4b, the uniformly superhydrophobic surface collects more water than the uniformly superhydrophilic surface

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(∼1.33 and ∼0.55 g/cm2·h, respectively), because the superhydrophobic surface allows droplets to roll down into the watercollecting container more easily instead of adhering and evaporating on the superhydrophilic surface. In agreement with the results in the literature,[12,14] the surface with a circle-shaped pattern is more efficient (∼1.65 g/cm2·h) than either uniformly superhydrophilic or superhydrophobic surfaces because of the cooperation between these two regions. As expected, in particular surfaces with star-shaped patterns collect more water (ranging from about 2.11 to 2.78 g/cm2·h) than those with

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though the pattern shape is the same, because the Laplace pressure gradient, which is involved in the “collection” step, is highly sensitive to the length scale. Overall, our results show that the shape and size of the pattern is crucial for enhancing the water collection efficiency on patterned surfaces. These investigations may provide insights in designing and developing materials with controllable wettability for highly efficient water- or liquid-collecting technology.

Experimental Section

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circular patterns. This can be attributed to the integration of both the wettability gradient and shape gradient on the surface, which together speed up the process by directionally collecting tiny water droplets toward more wettable regions. When a water droplet grows to a critical size, it will roll off the surface because of gravity (“transport”), following the relationship of ρ gV sin α ≥ wγ water (cosθ upper − cosθ lower ),[15] where ρ and V are the density and volume of the droplet, g is the gravitational acceleration, α is the surface tilted angle, w is the length of the air-liquid-solid contact line, θupper and θlower are the CAs at the upper and lower side respectively. It is worth pointed out that the droplet gathered on a single pattern usually coalesces with droplets on its adjacent patterns at first before it reaches the critical size. As a result, we tested the CA hysteresis on the surfaces used in our experiments, Table S2. The results show that there is no significant difference in CA hysteresis comparing with circle- and star-shaped patterns. This can be attributed to the fact that the superhydrophobic areas on these surfaces are still over 93% (Table S1). We further compared the water collection efficiency of circle- and 5-pointed star-shaped patterns at different tilting angles (15°, 45°, and 90°, Figure 4c). It shows that 5-pointed star-shaped pattern always has higher efficiency than circle-shaped pattern. In fact, superhydrophilic area plays two opposite roles in the whole process of water collection. Its larger adhesion to water droplets helps in droplet “capture”, whereas hinders droplet “transport”. For example, the overlap between neighboring tips of the 8-pointed star shortens its shape gradient as well as enlarges the adhesion area, and in turn pin the collected droplets stronger and slow down the “transport” step (see also Figure S1b and c in Supporting Information). It indicates that the optimal points in star-shapes are necessary to rise up water collection efficiency. To clarify the effect of the hydrophilic area, ratios of starshaped wettability patterns to the circle-shaped wettability pattern (in terms of their hydrophilic area, as well as collected water) are calculated and compared (see Table S1 in Supporting Information). As shown in Figure 4d, surfaces with 4-, 5-, and 6-pointed star-shaped patterns are more efficient than the circle-shaped pattern (ratio of collected water > 1), although they have smaller hydrophilic areas (ratio of hydrophilic area < 1). These data indicate that not only the area of the hydrophilic pattern but also its shape influences the water collection efficiency. It is the integration of the wettability gradient and shape gradient that gives star-shaped patterns the enhanced water collection efficiency. Since the Laplace pressure plays a key role in our observations, we further tested the water collection efficiency on surfaces with different pattern sizes (1000, 500, and 250 µm), but have the same star shape and total hydrophilic area. As shown in Figure S2, smaller patterns have higher efficiency, which indicates that the efficiency is highly sensitive to the actual size of the pattern besides shape. In conclusion, we have successfully designed and fabricated surfaces with star-shaped wettability patterns to integrate water-collecting strategies of both desert beetles and spider silk. Such surfaces with star-shaped patterns realize more efficient water collection compared to other surfaces that are uniformly superhydrophilic, uniformly superhydrophobic, or even circle patterned. In addition, surfaces with a smaller pattern size are more efficient than those with a larger pattern size, even

Fabrication of bioinspired surfaces with star-shaped wettability patterns: The glass substrate is coated with TiO2 particles from a typical spincoating process of TiO2 slurry. Then, the substrate is heated at 450 °C for 2.5 hours, and this superhydrophilic surface becomes superhydrophobic after being modified by FAS molecules through chemical vapor deposition. Finally, the bioinspired surfaces with star-shaped wettability patterns are obtained by selective UV light (365 nm, ∼25 mW/cm2) illumination on FAS-modified surfaces with the photomasks. Characterization of the bioinspired surfaces: The SEM images of the bioinspired surfaces are obtained using a field-emission scanning electron microscopy (JEOL-6700F, Japan). Water contact angles are measured on an OCA20 machine (Data-Physics, Germany). Measurement of the water collection properties: The surface is carefully fixed on a sample frame with an inclined angle of 15°, 45°, or 90° from the horizontal plane (See also Figure S1 in Supporting Information for detailed experimental setup). Numerous tiny water droplets are generated by an ultrasonic humidifier (YC-E350, Beijing YADU Science and Technology Co., Ltd.) as a fog flow and captured on the surface. The whole process is recorded by an optical-microscopy and its CCD components. The weight of collected water is measured after 2 hours. The collecting-efficiency is calculated based on both the collecting time and surface area (See also Figure S1 and S2, and Table S1 in Supporting Information).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work is supported by the State Basic Research Program of China (2013CB933001), the National Natural Science Foundation of China (21234001), and the Doctoral Fund of Ministry of Education of China (20121102110035). The Chinese Academy of Sciences is gratefully acknowledged. Received: January 17, 2014 Published online: May 22, 2014

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Adv. Mater. 2014, 26, 5025–5030

Efficient water collection on integrative bioinspired surfaces with star-shaped wettability patterns.

Inspired by the water-collecting strategies of desert beetles and spider silk, a novel kind of surface with star-shaped wettablity patterns has been d...
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