Integrative Bioinspired Surface with Wettable Patterns and Gradient for

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Surfaces, Interfaces, and Applications

Integrative bioinspired surface with wettable patterns and gradient for enhancement of fog collection Yan Xing, Weifeng Shang, Qianqian Wang, Shile Feng, Yongping Hou, and Yongmei Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19574 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Applied Materials & Interfaces

Integrative Bioinspired Surface with Wettable Patterns and Gradient for Enhancement of Fog Collection Yan Xing, ‡,§ Weifeng Shang, † Qianqian Wang, † Shile Feng, † Yongping Hou*,† and Yongmei Zheng*,†

†Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, and Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, P. R. China.

‡School of Aeronautics and astronautics, Beihang University, Beijing, 100191, P. R. China.

§ShenYuan Honors College, Beihang University, Beijing, 100191, P. R. China

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KEYWORDS: bioinspired surface, wettable patterns, wettable gradient, fog collection; directional movement

ABSTRACT: A novel integrative bioinspired surface with wettable patterns and gradient (WPGS) is proposed for fog collection via a novel anodic oxidation strategy. We study the water collection behaviors on WPGS with different parameters. Quantitative force analysis is presented putting in evidence the underlying mechanism leading to the directional motion of droplet, which is consistent with the experimental results. Such surface can not only improve the fog droplet capture performance effectively owing to wettable patterns but also accelerate surface regeneration by taking full advantage of the cooperation of multi-driving forces, leading to a further fog collection enhancement.

1. INTRODUCTION

Water as an important substance plays a vital role in all organisms surviving in nature, including human beings. However, with the increase of population and the intensification


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of water pollution, freshwater scarcity is becoming a significant worldwide issue.1-3 How to obtain more available water has been a major problem demanding for prompt solutions.4-6 Collecting water from dew, foggy wind or moisture present in the air is recently recognized as one of the remedies.7-10 In fact, there are lots of creatures exhibiting interesting water-harvesting capability.11 A case in point is that several beetles living in the Namib Desert can survive in such a dry and arid environment due to the outstanding water collection capability induced by their special hydrophilic-hydrophobic patterned surfaces (HHPSs) on the back.12-13 Inspired by these findings, much efforts have been devoted to wettability integrative patterned surfaces for water collection from the fog.12-14 Zheng et al.14 obtained a novel kind of surface with star-shaped wettable patterns via the photomask method, showing an enhanced fog collection efficiency compared to that of the uniform superhydrophilic/superhydrophobic surfaces. Wang et al.15 fabricated a HHPS with high fog collection efficiency by thermal pressing of a hydrophilic polystyrene sheet with a superhydrophobic gauze. Zhang et al.16 constructed a hydrophilic-superhydrophobic patterned surface based on a direct inkjet printing strategy, which shows a great enhancement on the fog collection ability. However, though


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the capture performance of fog droplets on such a wettability integrative surface is improved by the wettable patterns, the surface regeneration rate remains a matter for high collection efficiency. In fact, the most common removal approach for collected water droplet is to use gravity. Despite of the convenience and widespread use, the gravitational removal mechanism is orientation-dependent and only effective for those droplets whose size approaches the millimetric capillary length.17-18 Recently, Wu et al. achieved the removal of droplet independent with gravity through stencil lithography, but the method is too expensive to applying in industry and so limited in the types of substrates.19 Besides, possible mass production is restrained by the complicated methods for wettable patterns. Therefore, increasing removal rate of collected water droplets for rapid regeneration via a simple economical method would be crucial for HHPS to achieve enhanced collection efficiency.

Here, inspired by the unique water collection capability of Namib Desert beetle11 (wettable patterns) and wet-rebuilt cribellate spider silks (wettable gradient),9 we propose a novel kind of integrative bioinspired surface with wettable patterns and gradient (WPGS)


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via novel one-step anodic treatment. The obtained surface can not only improve the fog droplets capture performance due to hydrophilic patterns, but also maintain effective water drainage with hydrophobic substrate during the fog collection process. More importantly, the regeneration of the surface can be further accelerated by transporting the collected water droplets toward more wettable regions via the cooperation of multi-driving forces, leading to continuous water collecting circulation. As a result, this type of surface exhibits enhanced fog collection efficiency compared with the normal HHPS. Such surfaces can be used to enhance the efficiency in water collection or other engineering applications related with liquid harvesting.

2. EXPERIMENTAL SECTION

2.1. Preparation of WPGS. Firstly, the copper plate (purity 99.9%) with the size of 30 × 10 × 0.2 mm was immersed in a 15 wt% nitric acid (HNO3) solution under ultrasonic cleaning for 15 min to remove the native oxide and impurities on the surface. After that, the copper plate was rinsed with deionized water and ethanol successively, and dried at room temperature under vacuum. Secondly, in order to improve the hydrophobicity of the


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substrate, the cleaned copper plate was immersed in a dodecanethiol solution (volume ratio, dodecanethiol : alcohol = 1 : 4) for 3 hours. Afterwards, the modified substrate was rinsed with deionized water and dried at room temperature under vacuum. Thirdly, the copper substrate was covered by a porous polytetrafluoroethylene (PTEF) mask tightly with different parameters and perpendicularly immersed into 0.05 M electrolyte solution sodium hydrate (NaOH) as the anode. A platinum plate was adopted as the cathode facing the masked substrate. During the mask-based gradient anodic oxidation (MGAO) process, the masked surface cannot be oxidized except for the areas exposed to electrolyte. Accordingly, a hydrophilic-hydrophobic patterned surface was obtained. Besides, a wettable gradient was also formed due to the gradient of current density and oxidation time induced by the discharge of electrolyte via the valve attached to the bottom of the chamber. By adjusting the flow velocity of electrolyte, the wettable gradient can be adjusted easily. For great wettable gradient, the volume flow rate of electrolyte here is controlled at 1.8 L/h. After MGAO, the obtained plate was rinsed thoroughly with freshly deionized water and dried at room temperature under vacuum.


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2.2. Characterization. Water contact angles (CAs) were measured by the optical contact angle meter system (OCA40Micro, Dataphysics Instruments GmbH, Germany). A droplet of 0.5 μl was dripped onto the samples and the static CA was determined by the average of at least five measurements. The dynamic transport process of water droplets between different hydrophilic patterns was also recorded by the system. Scanning electron microscopy (SEM) was employed for the microstructures of copper plate surface. The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS) (AXISUltra instrument from Kratos Analytical) and X-ray diffraction (XRD) patterns (XRD-6000 SHIMADZU).

2.3. Measurement of the fog collection properties. We fixed the prepared sample on a holder with an angle of 45o to the horizontal plane in open environment. A simulated fog flow was generated and introduced to the sample by a commercial humidifier (JSQ107, Guangdong CHIGO Air Conditioning Co., Ltd.) equipped with an electric fan, where the fog impacting velocity can be controlled by the fan. The distance between the fog generator and the sample was kept constant (7 cm). A temperature and humidity sensor


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was employed to test. The temperature and relative humidity around the sample were 20 oC

and 90 - 95%, respectively. The liquid water droplets collected by the surface were

drained by gravity into a container placed on top of a digital analytical balance under the sample. The collection efficiency is calculated based on both the collection time and surface area. Each test was carried out for five times.

3. RESULTS AND DISCUSSION

3.1. Fabrication and Characterization of WPGS

WPGS is fabricated by a mask-based gradient anodic oxidation (MGAO) strategy (see Scheme 1 and Experimental section). In this fabrication, a copper plate is used as the substrate, owing to excellent electrical conductivity, thermal conductivity and mechanical properties.20 In order to improve the hydrophobicity of the substrate, the copper plate is pre-modified by dodecanethiol firstly (see Experimental section). The results demonstrate that the water contact angle (CA) of the substrate increases from 92.9o to 130.8o with decreased water adhesion (see Figure S1), which is suggested beneficial to fog collection enhancement of surface for good drainage performance.21 During the oxidation process,


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the masked substrate surface cannot be oxidized except for the areas exposed to electrolyte. Accordingly, hydrophilic patterns are fabricated on the hydrophobic substrate. On the other hand, a wettable gradient can be also formed at the same time because of the current density and oxidation time gradient induced by emptying of the electrolyte gradually (Scheme 1a). More importantly, the patterned surfaces with different parameters (D: diameter of the circle-shaped hydrophilic pattern, S: separation distance between patterns center-to-center) can be adjusted precisely via simply adopting different pre-designed masks. In this work, we could control the diameter D from 0.5 mm to 1.5 mm and separation distance S from 0.8 mm to 2.3 mm easily. In addition, the wettable gradient of WPGS can be adjusted via controlling the volume flow rate of electrolyte. For such WPGS, the hydrophilic patterns can capture fog droplets effectively at first. Besides, the removal of the collected water droplets can be promoted by the cooperation of multidriving forces for continuous water collecting circulation. As a result, with improved capture performance of fog droplets and surface regeneration, the fog collection efficiency may be enhanced effectively (Scheme 1b).


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After MGAO, orderly circle-shaped patterns with clear edge are obtained on the substrate (Figure 1a, b). As shown in Figure 1c, CA on the hydrophilic pattern surface of Sample-A (D/S = 1.5 mm /1.8 mm, current: 0.12 A) decreases gradually from 113.20o to 36.43o with a wettable gradient 3.07o/mm along the direction from T to B (T: top area and B: bottom area of copper plate during the MGAO process), while CA on the hydrophobic area is 120o ± 4.49o, showing little change due to the protection of the mask. The variation of CA on the hydrophilic area demonstrates that a wettable gradient is formed on the substrate as design. To confirm the chemical composition of the Sample-A, the X-ray diffraction (XRD) was employed. The results indicate the generation of Copper (II) (Cu2+) (Figure S2), which is well-known hydrophilic as a borderline Lewis acid.22-24 To make a further investigation, the X-ray photo electron spectroscopy (XPS) experiment was conducted. The shake-up satellite peaks indicate the formation of Cu2+ on the copper surfaces after MGAO (Figure 1d).25 For detailed surface chemical composition, we investigated the Cu 2p3/2 core level spectrum envelope curve-fitted into three components centered at 932.5, 933.8 and 935.1 eV (Figure 1e). The peak at 932.5 eV is attributed to either metallic Cu or Cu2O. The component at 933.8 and 935.1 eV arises


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from the presence of CuO and Cu(OH)2.25 The XPS spectra results show that with the increase of oxidation time, the content of metallic copper or Cu2O decrease obviously, while that of CuO and Cu(OH)2 increase sharply. The results of XPS imply the formation of chemical gradient with the increase of the oxidation extent. The scanning electron microscopy (SEM) images of Sample-A indicate that some rough structures are formed at the patterned sites. However, different patterns show little difference between each other (Figure S3). Based on the results above, we can demonstrate successfully that wettable gradient is achieved on the Sample-A, and the area ratio of the hydrophilic patterns can be easily adjusted via adopting different oxidation conditions and predesigned masks.

3.2. Water collection behaviors on the WPGS

A simulated fog flow generated by a commercial humidifier (relative humidity of 90 95%) is used to examine the fog collection properties of different samples. To reduce observation interference, we only focus a row of the patterns to observe the collection process, shown in Figure 2, Movie S1-S2. On Sample-A, at the beginning, several tiny


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condensed droplets appear randomly on the surface (Figure 2a, 10 s). Then, the droplets are collected on the hydrophilic patterns, forming humps separately and growing in situ (Figure 2a, 27 s), just like on the Namib beetle’s back. At 37 s, the two collected droplets on the hydrophilic spot 2 and 3 meet each other and coalesce in situ. Differently, as the collection process goes on, the merged droplets covered the hydrophilic spot 2, 3 and 4 would move along the direction of wettable gradient (see orange arrows), and coalesce with droplet at the spot 5, leading to the release of less hydrophilic patterns at the left side (the hydrophilic spot 3 and 4, Figure 2a, 143 s). Then, a new collecting circulation starts and similar collection process is observed, i.e., when the merged droplets covered more than two spots, the droplet would move to more wettable region (Figure 2a, 161 s - 234 s). As a result, the regeneration of the hydrophilic regions is accelerated, which is favors to increase water collection efficiency.

However, such phenomena are hardly observed on Sample-B (D/S = 1.5 mm/2.3 mm, 2.08 o/mm). As shown in Figure 2b, at the beginning, the droplets are also collected on the hydrophilic patterns as humps (Figure 2b, 104 s). As the collection process goes on,


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the droplets coalesce one after another to form a layer of water film finally without any directional transport (Figure 2b, 192 s - 401 s), similar as that on normal HHPS (SampleC, D/S = 1.5 mm/1.8 mm, current: 0.12 A, time: 3 minutes, volume flow rate of electrolyte: 0, Figure S4, Movie S3). We also studied the collection process on the samples with other values of S (D = 1.5 mm). The results indicate that the directional motion could be observed only when the value of S is below 2.3 mm. In addition, with the decrease of the value of S, the average velocity of the droplet increases obviously, as shown in Figure S5.

To thoroughly understand the unique phenomenon, we take a force analysis on the system to describe the motion dependence on the pattern distance. During the coalescence process, there are two driving forces, i.e., the coalescence driving force (FD) and the wettability gradient force (FW). The resistance is the hysteresis force (FH).26 The wettable different force (FWD), related to the difference of wettability (between inside and outside of circle-pattern), should not be considered because of its almost negligible effect on the self-propelled moving of droplet, while the main role of FWD is to improve the fog


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capture ability. Thus, there seems to be three forces in this system: i.e., the coalescence force (FD), wettable gradient force (FW), and hysteresis force (FH), which can be related to the coalescence process, the surface wettable gradient, and the CA hysteresis, respectively.

During the water coalescence process, the coalesced droplet initially is usually in an unstable state with its IFE greater than the corresponding equilibrium value. The droplet will tend to transform itself toward its equilibrium state by reducing its IFE through base radius diminution. The direction of FD on the both side of droplet is different (on the left side, the driving force (FLD) points to right end; on the right side, the driving force (FRD) points to left end). On a wettable surface, under effect of the Laplace pressure within the drop, merged droplet tends to create nearly equal contact angles both edges during coalescence process,27 but the equilibrium contact angle on the left side (less wettable) is greater than that on the right side (more wettable). Therefore, FLD is larger than FRD and the direction of FD (FD = FLD - FRD) points to the direction of wettable gradient.


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Therefore, the coalescence driving force (FD) can be described as (more details can be found in the Supplementary Analysis I):28-30

FD  2  L  2 R  Rk  sin 

(1)

where L is the length of O1O2 (O1O2 is the center line of the two circles), R is the radius of the circle pattern, k is the average wettable gradient, γ is the surface tension of water, θ is the center position-responsive sessile CA of coalesced droplet.

It is known to all that the wettable gradient force (FW) induced by wettable gradient would drive droplet towards the more wettable region of surface, which can be described as (Supplementary Analysis I):

FW =  2 L + R  R k sin 

(2)

The hysteresis force (FH) rises from the resistance on three-phase contact line. It is hard to measure the exact hysteresis force directly on WPGS so that we use adhesion force to take the place of hysteresis force. We tested the adhesion forces on different hydrophilic patterns, and get the linear relationship between adhesion forces and water


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contact angles, as shown in Figure S6. To simplify the calculation, we shall assume that the adhesion force is proportional to the length change of three phase contact line. Finally, the hysteresis force of coalesced droplet on WPGS can be described as：

FH =

R 197  0.96 

(3)

rC

Where rC is the radius of the contact area of the droplet during the adhesive force test.

The total force exerted on the coalesced droplet during the coalescence process can be written as: F  FD  FW  FH =  4 L +4 R + R  R k sin  

R 197  0.96 

(4)

rC

In order to visually illustrate the influence of pattern distance, we plot the total force function in MATLAB. We draw the 3D surface plot of FT with different k in Figure 3, and we find that when k = 2.08o/mm, the F is always below zero, while when k= 3.07 o/mm, the total force is greater than zero when the CA and L are in a certain condition. When the L = 1.8 mm (two patterns are covered), the total force is below zero. For the L = 3.6 mm (three patterns are covered), the total force is great than zero when the θ is between


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[92.4o, 110o]. The results of force analyses are consistent with the experimental results. i.e., the directional transport cannot be observed on surface with the parameter D/S=1.5 mm/2.3 mm (k= 2.08 o/mm). We could use the Figure 3 to predict the movement behavior of coalesced droplet via the two parameters, i.e., contact angle θ and the length L.

Having known about the forces exerted on the droplet, we further analyze the moving behavior of droplet. For the surface with normal HHPS, the water droplets captured near the circle-patterned boundary move toward the hydrophilic regions under the effect of FWD (Figure S4, 65 s; Figure 4a, 1), forming bigger water humps on the hydrophilic patterns separately. As a result, the circular directional self-clearing of the droplets from the hydrophobic regions to hydrophilic regions allows continuous nucleation and growth of new droplets, leading to a certain fog collection enhancement.31-33 However, as the collection process goes on, the growing water humps would occupy the whole hydrophilic regions (Figure S4, 136 s; Figure 4a, 2), and then coalesce in situ to form a bigger water droplet. Although the coalescence driving forces (FLD, FRD) generated during the coalescence process try to decrease base radius from both sides, no directional


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spreading or movement occurs due to high and symmetrical hysteresis forces (FLH, FRH) on both sides (Figure 4a, 3). Finally, a water film is formed (Figure S4, 648 s; Figure 4a, 4, 5) and continual water collection is prevented unless the water film is removed by tilting the substrate with some angle. For such a normal wettability integrative surface, during the fog collection process, the water collection efficiency is high at the beginning due to significant wettable difference between the hydrophilic patterns and the hydrophobic background. However, as the collection process goes on, without effective drainage, the hydrophilic patterns will be occupied by the formed water film, leading to limited water collecting circulation numbers. When the collected water on the surface reaches saturation, the collection efficiency of the surface will be sharply decreased.

For WPGS (Figure 4b, c), at the beginning, similar to the normal patterned surface, the droplets are collected on the hydrophilic patterns, forming humps separately (Figure 2a, 10 - 27 s; Figure 2b, 10 - 104 s; Figure 4b, 1; Figure 4c, 1). Although two new driving forces (FW, FD) are introduced into this system during the first coalescence, no directional movement is observed during the first coalescence process due to high hysteresis force


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(Figure 2a, 37 s; Figure 2b, 192 s; Figure 4b, 2; Figure 4c, 2). As the collection process goes on, more interestingly phenomena appear. If the separation distance between two adjacent hydrophilic patterns is small (For D = 1.5 mm, S is below 2.3 mm), the water droplets collected on the hydrophilic patterns could meet each other in a smaller volume and the merged droplets move directionally toward the more wettable region (Figure 2a, 39 s; Figure 4b, 3, 4, 5), which is different from the movement behavior of droplet after the first coalescence process. The reason may be attributed to: (1) Compared with the first coalescence, the length of merged droplet along the wettable gradient increases obviously during the second coalescence process (covered more than two spots), leading to larger wettable gradient force (FW) (the larger difference of CA on both sides). (2) During the second coalescence process, the as-released energy becomes higher as the merged drops are bigger,29 which favors to increase the coalescence driving force (FD) (the direction of FD points to the direction of wettable gradient). (3) During the coalescence process, as-released energy favors the liquid viscous dissipation inside the drops to reduce the surface adhesion as well as the intermolecular attraction of the solid-liquid interface.29 Therefore, under effect of larger coalescence driving force (FD), wettable


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gradient force (FW) and smaller hysteresis force, the merged droplets would move along the direction of wettable gradient (Figure 2a, 39 s - 234 s; Figure 4b, 4, 5). After successive coalescence, the less hydrophilic patterns at the left side are released (Figure 2a, 143 s, 234 s; Figure 4b, 5). A new collecting circulation starts right after the release of the hydrophilic patterns. As a result, the regeneration of the hydrophilic regions is accelerated, leading to further water collection enhancement of the surface. However, when the separation distance between two adjacent spots exceeds a threshold value (For D = 1.5 mm, S is equal or above 2.3 mm) (Figure 4c), similar to the normal patterned surface, a water film is formed after several coalescences and no directional movement is observed (Figure 2b). The different phenomena may be due to that: (1) with the increase of the separation distance, the proportion of hydrophilic region along the direction of wettable gradient decreases markedly, leading to the smaller wettable gradient force (FW) for merged droplet (wettable gradient reduces according to Equation (2)). (2) The collected droplets need to grow larger for coalescence, which would induce the high hysteresis force (FH) (larger solid−liquid contact area, Figure 4c, 3). Therefore, during the coalescence process of two bigger droplets, under effect of smaller wettable


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gradient force (FW) and high hysteresis force (FH), a water film with unequal CA at both sides is formed and no directional movement is observed (Figure 2b, 372 s, 401s; Figure 4c, 4, 5). Obviously, for better water droplet transport performance, proper size is also an important factor for a WPGS. Here, for a surface with circle-shaped patterns with diameter 1.5 mm, in order to accelerate the regeneration of the hydrophilic regions, the separation distance should be controlled between 1.8 mm - 2.3 mm (For D = 1.5 mm, S = 1.8 mm is minimum spacing we could obtain). According to the results mentioned above, it can be induced that successive regeneration on the WPGS is realizable, which may enhance the water collection efficiently.

To characterize the fog collection performance of surface with wettable patterns and gradient, different types of surfaces, i.e., uniformly superhydrophobic (CA = 150o), uniformly hydrophilic (CA = 30o), normal HHPS (no wettable gradient, patterns with average CA = 40o) and WPGS with different parameters, are compared with weight of water collected per unit time and unit area (Figure 5). Figure 5a shows the schematic setup for the fog-collecting system. The prepared sample is carefully fixed on a holder


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with an angle of 45o to the horizontal plane. As shown in Figure 5b, the uniformly superhydrophobic surface collects more water than the uniformly hydrophilic surface (~ 0.1245 and ~ 0.0766 g/cm2·h, respectively), because of better drainage ratio (high CA and low adhesion for superhydrophobic surface).22 Obviously, the patterned samples with wettable gradient show enhanced fog collection efficiency compared to the normal ones with the same parameter or superhydrophobic surface. For example, when the D/S=0.75 (D = 1.5 mm, k=2.75 o/mm) shows the highest water collection efficiency ~ 0.2568 g/cm2·h, ~ 1.7 times as that of the normal patterned one (~ 0.145 g/cm2·h). In addition, consistent with motion analyses above, the fog collection efficiency improve obviously when the value of S is between 1.8 mm and 2.3 mm (directional motion could be realized on such surface), and the efficiency would decrease with the increase of the separation distance. Generally, improved water droplets capture combined with accelerated surface regeneration, the fog collection efficiency on a WPGS is enhanced remarkably.

There is no doubt that gravity has a great influence on the fog collection efficiency, especially to the droplet whose radius is up to capillary length. A fog collection test is


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carried out to describe the role of the gravity. The prepared sample is carefully fixed on a holder with an angle of 0o and 90o to the horizontal plane, the results is shown in Figure S7. According to the results, we know that with the increase of the tilt angle, gravity play an important role gradually. WPGS has higher water collection efficiency than HHPS when the tilt angle is 0o and 45o. However, when the tilt angle is up to 90o, gravity playing a major role in the removal of collected water, WPGS shows no significant advantages on the water collection efficiency.

4. CONCLUSIONS

In conclusion, a novel kind of WPGS is proposed and fabricated via the novel one-step MGAO method. The obtained surface can not only improve the fog droplets capture performance due to hydrophilic patterns, but also maintain effective water drainage with hydrophobic substrate during the fog collection process. More importantly, the regeneration of the surface can be further accelerated by driving the collected water droplets toward more wettable region taking advantage of the cooperation of multi-driving forces, leading to continuous water collecting circulation. The method adopted here is


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scalable and could be broadened to some other substrate materials (aluminum, carbon, etc.). The results suggest potential applications in water collection and other engineering related with liquid harvesting.33


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ASSOCIATED CONTENT

Supporting Information. CA and adhesion force on original and dodecanethiol solution modified copper substrate. X-ray diffraction (XRD) patterns of the copper plates. SEM images of different oxidized areas of the patterned copper surface. Motion velocity of droplet on the different samples.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. (Yongping Hou)

*E-mail: [email protected]. (Yongmei Zheng)

Author Contributions ‡Yan Xing, ‡Weifeng Shang contributed equally. All authors contributed to the manuscript preparation. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (21673015, 51203006, 21473007, 21771015), and Fundamental Research Funds for Central Universities and Aeronautical Science Foundation of China (2015ZF51060) and 111 Project (B14009)


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Scheme 1. Schematic illustration of the fabrication process of surface with wettable patterns and gradient (WPGS) and the fog collection process on it. (a) The fabrication process of WPGS by a mask-based gradient anodic oxidation (MGAO). During the oxidation process, the masked surface cannot be oxidized except for the areas exposed to electrolyte. Wettable patterns are obtained. Besides, a wettable gradient is formed on the surface from T to B due to current density and oxidation time gradient induced by emptying of the electrolyte. (b) The three main steps of the fog collection process, capture, collect and transport. The droplets captured from the foggy wind coalesce into larger ones


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on the surface. Then the droplets are transported directionally toward more wettable regions taking advantage of the wettable gradient, which could accelerate the regeneration of the surface, leading to enhanced water collection efficiency (T: top area and B: bottom area of copper plate during the MGAO process, D: diameter of the circleshaped hydrophilic pattern, S: separation distance between patterns center-to-center).


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Figure 1. (a) Optical images of WPGS with D/S =1.5 mm/1.8 mm and (b) D/S = 0.5 mm/1.3 mm. Scale bars 2 mm. (c) CAs on different areas of Sample-A along the wettable gradient direction. (d) Cu 2p core-level XPS spectra of different samples fabricated at different oxidation time under current 0.12 A. (e) Corresponding Cu 2p3/2 XPS spectra. Clearly, we could fabricate a WPGS with different parameters.


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Figure 2. In situ optical images (top view) of the droplet behaviors on WPGS during the fog collection process. (a) Sample-A with D/S = 1.5 mm/1.8 mm and a wettable gradient 3.07o/mm. In the first circle (see blue square), tiny condensed droplets appear on the surface randomly (10 s), and gather in the hydrophilic patterns successively (27 s). Then, the merged droplet on the two adjacent spots coalesce with each other without any directional movement (37 s). When the merged droplets cover more than two hydrophilic spots, the droplet would move directionally towards the more wettable regions (see


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orange arrows) and the less hydrophilic patterns at the left are regenerated rapidly (39 s - 143 s). The second circle repeats the process at a faster rate (161 s - 234 s) (see yellow square). (b) Sample-B with D/S = 1.5 mm/2.3 mm and a wettable gradient 2.08o/mm. Initially, similar movement behaviors are observed (0 s - 192 s). But no distinct directional transport or movement of the droplets is observed in the subsequent collection process even the merged droplets cover four hydrophilic spots. Finally, a water film is formed. Scale bars, 1 mm.


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Figure 3. Calculated total (FT) at different conditions. a) When wettable gradient k = 2.08 o/mm,

total force is always below zero. b) k = 3.07 o/mm, the total force is greater than

zero when the CA and L are in a certain condition. When the L = 3.6 mm (three patterns are covered), the total force is great than zero when the θ is between [92.4o, 110o]. When the L = 1.8 mm (two patterns are covered), the maximum total force is below zero.


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Figure 4. (a-c) Illustration and force analysis of the collected droplet behaviors on patterned surfaces. (a) hydrophilic-hydrophobic patterned surface (HHPS). The merged droplets gather in the hydrophilic spots due to the wettable difference force (FWD). But no directional movement is observed during the coalescence process due to high hysteresis force (FH) and only a water film is formed. (b) WPGS with narrow hydrophobic separation gaps. The water droplets collected on the patterns could meet each other in a small


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volume. When the merged droplets cover more than two hydrophilic spots, it would move directionally towards the more wettable regions under effect of larger coalescence driving force (FD) and wettable gradient force (FW). (c) WPGS with wide hydrophobic separation gaps. Because of the decrease of wettable gradient force (FW) and the increase of hysteresis force (FH), there is no directional movement can be observed during the coalescence process.


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Figure 5. Water collection performance characterization of surfaces with different wettability. (a) Illustration of experimental setup for fog collection. (b) Water collection efficiency comparison of hydrophobic surface (brown bars), hydrophilic surface (light blue bars), HHPS (hydrophilic-hydrophobic patterned surface) (blue bars) and WPGS (red bars). Obviously, with the WPGS, the fog collection can be enhanced a lot compared to the other samples. For these HHPS and WPGS, D is a constant (D=1.5 mm), while S is a variable.


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Integrative Bioinspired Surface with Wettable Patterns and Gradient for

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