High-Efficiency Fog Collector: Water Unidirectional Transport on

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High-Efficiency Fog Collector: Water Unidirectional Transport on Heterogeneous Rough Conical Wires Ting Xu, Yucai Lin, Miaoxin Zhang, Weiwei Shi, and Yongmei Zheng* 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 S Supporting Information *

ABSTRACT: An artificial periodic roughness-gradient conical copper wire (PCCW) can be fabricated by inspiration from cactus spines and wet spider silks. PCCW can harvest fog on periodic points of the conical surface from air and transports the drops for a long distance without external force, which is attributed to dynamic asreleased energy generated from drop deformation in drop coalescence, in addition to both gradients of geometric curve (inducing Laplace pressure) and periodic roughness (inducing surface energy difference). It is found that the ability of fog collection can be related to various tilt-angle wires, thus a fog collector with an array system of PCCWs is further designed to achieve a continuous process of efficient water collection. As a result, the effect of water collection on PCCWs is better than previous results. These findings are significant to develop and design materials with water collection and water transport for promising application in fogwater systems to ease the water crisis. KEYWORDS: heterogeneous, periodic wettability gradient, fog collection, unidirectional transportation surfaces.23−25 However, it is still challenging to design a surface that can run the coalescence energy as a driving force to realize droplet transport for high-efficiency fog harvesting processes. Here, we present an artificial periodic roughness-gradient conical copper wire (PCCW) that can be fabricated by the controlled electrochemical corrosion methods with periodic current gradients. It is the structure of PCCW that can harvest fog on periodic points of the conical surface from air and transport the drops a long distance without external force. We reveal that this ability is attributed to dynamic as-released energy generated from drop deformation in drop coalescence, in addition to both gradients of geometric curve (inducing Laplace pressure) and periodic roughness (inducing surface energy difference). We demonstrate further that the ability of fog collection can be related to various tilt-angle wires, thus an array system of PCCWs is further designed to achieve a continuous process of efficient water collection, revealing the effect of water collection on PCCWs is better than previous results. These studies offer insight for the design of fogcollectors, which are significant to develop materials that can be

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iological surfaces (e.g., plants and animals) have been optimized with hierarchical structures to achieve excellent adaptive performances in arid environments, e.g., surfaces of wet spider silk, cactus, and beetles, etc., display the intriguing fog harvesting abilities from air.1−7 Based on the surface model of wet spider silk, artificial bioinspired fibers with water collecting properties can be investigated to integrate the gradients of curvature and roughness by means of designing humps or spindle-knots.4,8,9 Learned from cactus, researchers have designed a conical spine to harvest water, including a cone array to obtain fog harvesting.10,11 Inspired by desert beetles, strategies have been explored to fabricate hydrophobic− hydrophilic patterned surfaces for high-efficient fog collection.12−14 Based on these investigations, it is revealed that the roughness gradient and cone curvature, wettability gradient can be important in aiding the formation of as-condensed droplet and tiny droplet transport during water-collecting processes.15,16 It is known that capillary adhesion or contact angle hysteresis of surface would decrease the efficiency of droplet transport during the process of water collection.7,17 As reported, with the help of the surface-free energy gradient (wettability gradient)18,19 and Laplace pressure,20−22 the droplet can overcome the capillary adhesion to realize the droplet transport in efficiency to extent. Recently, it is found that a kind of energy can result from drop coalescence on © 2016 American Chemical Society

Received: August 19, 2016 Accepted: September 30, 2016 Published: September 30, 2016 10681

DOI: 10.1021/acsnano.6b05595 ACS Nano 2016, 10, 10681−10688

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Figure 1. Surface properties of PCCW. (a) Optical image of a representative periodic roughness-gradient conical copper wire. Scale bar, 1 mm. (b) AFM images of different sites within one roughness period of PCCW. Three squares are used to indicate the HRR (2 site) and its left or right adjacent regions (1 and 3 sites). The average roughness (Ra) of 1, 2, and 3 sites is 80.4, 154.0, and 63.1 nm, respectively.

Figure 2. In situ optical images of the drop behaviors on the PCCW surface. (a) Observation of the transport process of the first three drops. Once the first drop increases to the threshold size, it will transport from the tip to base of PCCW, coalescing with the following drops without external force. Scale bar, 1 mm. (b) Optical images of the drop-formation process on HRRs. First, some fog droplets condense on the surface, and then these droplets grow up and merge into a water drop pinned on a specific site (namely, HRR). Scale bar, 5 mm. (c) Optical images of the drop-coalescence process. From 0.040 to 0.080 s, there’s a deformation process. After coalescence, the merged drop will move continuously toward the base of the PCCW. Scale bar, 1 cm.

image of a representative PCCW. Three adjacent regions are marked with squares (as defined as 1, 2, and 3 sites, respectively). Atomic force microscope (AFM) is used to observe the structure properties of 1−3 sites. As shown in Figure 1b, there are the different topologies at these three sites. The high roughness region (HRR) is at the 2 site and low roughness region (LRR) at the 1 and 3 sites, respectively. It

extended to applications for water collection and water transport in fogwater systems.

RESULTS AND DISCUSSION PCCWs are fabricated by a method of periodic gradient electrochemical corrosion (see Experimental Section and the experimental facilities in Figure S1). Figure 1a is the optical 10682

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Figure 3. Illustration of fog collection on PCCW. (a) An overview of the whole harvesting and transportation system processing from “deposition” on the periodic HRRs to “coalescence” and “transportation” along the PCCW. (b) Analysis of the growth of condensed droplets arising from the driving force of roughness gradient. (c) The transport of as-coalesced drop. Drop 1 coalesces with drop 2 forming drop (1 + 2). The surface energy released upon drop coalescence and deformation process overcomes the handicaps and drives the drop to move continuously (Fw, Fc, and FL refer to the effects of wettable gradient, coalescence energy and Laplace pressure, respectively.) (d) Analysis of the driving force arising from the gradient of Laplace pressure. A water drop on a conical spine should move toward the base side with the larger radius (R2) due to the relatively smaller Laplace pressure.

indicates that the periodic roughness appears on PCCW, in addition to the cone geometry. Furthermore, it is analyzed that the roughness of 2 site (Ra2) can be 154.0 nm and roughness of 1 site (Ra1) and 3 site (Ra3) can be 80.4 and 63.1 nm, respectively. The periodic rough structures are also observed by environmental scanning electron microscope (ESEM), where HRR and LRR are continuously arranged along the PCCW (Figure S2b). It indicates fully that the as-desired PCCW has been obtained successfully, which can be used to investigate the fog harvesting ability and is not reported before. To examine the fog harvesting ability of PCCW, we fixed a PCCW horizontally in a chamber with the fog flow velocity of 1.8 m/s at environmental temperature of 15 °C. We can see from Figure 2a, with continuous fog impact, there are several water drops formed on the periodic roughness sites (HRRs) of PCCW, and then the first drop (drop 1) starts to move when it grows to the critical size (about 1.28 mm) of the self-propelled movement, (drop 1 grows faster than others because of high curvature of the tip and high roughness) and triggers a sequence of drop coalescence. It merges with drop 2 forming drop 1 + 2, and then drop 1 + 2 coalesces with drop 3 generating drop 1 + 2 + 3, and afterward, the merged drop moves toward the base of the PCCW very quickly (see Movie S1). With the drop volume increases, it will ultimately form a large drop at the base of the PCCW. To get more details on the fog harvesting process, a series of images are obtained by a

XTL-500 optical microscope and CCD camera. To know the drop formation, we focus on the location of PCCW to observe the beginning case at fog harvesting. The drop formation (e.g., drop 1) can be shown in Figure 2b. From Figure 2b, we can observe the initial, simultaneous condensation of fog droplets on the surface. As the condensation proceeds, some fog droplets converge on HRRs, and then water drops on HRRs are formed (see vertical arrows and dashed line circle) at 0.02 s, and these drops grow to coalesce together into drops 1 or 2 at 0.03 s. Subsequently drops form a larger drop 1 + 2 at 0.04 s (the dynamic process can be seen in Supporting Movie S2). The drops grow continuously during the fog-flowing action (at 0.05 s). When drops coalesce to surround the PCCW, directional transports of drops start. Figure 2c shows the details of the merging−transporting process, as observed when a drop transports from the former HRR and moves close to the next drop at 0.02 s (see arrow). Then these two drops merge into one drop observed in 0.04−0.06 s. Subsequently, the coalescence process triggers drop deforming (at 0.08 s) and produces a higher gradient of Laplace pressure which is generated from the conical shape of PCCW, further aiding the directional transport of water drops (at 0.098 s), which is considerably faster and is completed in 1 s (see Movie S3). The coalescence energy of two big drops will increase the speed of the drop moving toward the base of the PCCW. These observations demonstrate that the water drops could move 10683

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Figure 4. The fogwater unidirectional transport properties of PCCW at different tilt angles. (a−e) A wire was placed in tilt angles: 15° (a), 5° (b), 0° (c), − 15° (d), and −35° (e), respectively. The horizontal one is defined as tilt-angle of 0°; the up-tilted angle is a positive value, e.g., in (a) and (b), the down-tilted angleis a negative value, e.g., in (d) and (e). It is observed that drops transported to the end of wire take different times of ∼0.81 s in (a), ∼ 0.49 s in (b), ∼ 0.30 s in (c), ∼0.50 s in (d), and ∼14.77 s in (e). It indicates that drop transport can be effective in (c) with a tilt-angle of 0°. Scale bar, 10 mm. (f) The average velocity of a drop motion on PCCW placed at different tilt-angles. It increases as the tilt-angle decreases (from 15° to 0°), and when the tilt angle is below zero, the velocity begins to decrease, namely, the average velocity is the highest at 11.67 cm/s at 0°. (g) The fog collection rate of a single PCCW. It reaches the highest 0.36 mg/s at 0°. The error bars were obtained from four repeated measurements. The experiment is done in humidity (H) of 90%, temperature (T) of 15 °C, and fog flow velocity of 1.8 m/s.

surface tension of water, θ1 and θ2 are the advancing and receding angles of water droplet on PCCW respectively, and dl is the integrating variable along the length from the low roughness site (LL) to the high roughness site (LH). The surface energy gradient arising from differences in roughness will thus drive water droplets to move from the less hydrophilic region (LRRs with relative lower surface energy) to the more hydrophilic region (HRRs with high surface energy) (see Figures 3b and 2b and Movie S2). The gradient of roughness generates a gradient of wettability, i.e., a gradient of surface-free energy.28,29,31 The driving force for water drops moving from tip to base of the spine directionally arises from the conicalshaped geometry of the wire, which will generate a difference in Laplace pressure between the two opposite sides of the drop6,32

from the tip to the base of PCCW with the driving forces, i.e., the coalescence energy generated from drops merging and the gradient of Laplace pressure. This can be understood by the wettability and energy law. A schematic diagram (Figure 3a) is used to illustrate the whole harvesting-transport system, including processes from “deposition” on the HRRs to “coalescence” and “transportation” along the PCCW. It is analyzed in three sections: wettability gradient, coalescence energy, and Laplace pressure. Surface energy gradients can be derived from differences in either surface chemical composition18,19 or surface roughness,26−28 and such gradients will drive water droplets toward the more wettable region with a higher surface energy. According to Wenzel’s law: cos θw = r cos θ,29 where r is surface roughness and θw and θ are the apparent and intrinsic contact angles on rough and smooth surfaces, respectively. As for hydrophilic copper wire, its chemical composition does not change much along the fiber, but the periodic HRRs, which are cultivated by periodic current, have a higher roughness and hence smaller water contact angle than the LRRs (see Figures 1b and S2d). That is, the HRR is more hydrophilic and has a higher apparent surface energy than the LRR. The force generated by surface energy gradient that arises from differences in surface roughness is L given by Fw = ∫ LHL γ(cos θ1 − cos θ2)dl,18,19,30 where γ is the

R2 2γ sin R1 (R + R 0)2

as follows: ΔPcurvature = −∫

θdz , where R is the

local radius of the spine (R1 and R2 are the local radii of the conical wire at the two opposite sides of the drop), γ is the surface tension of water, R0 is the drop radius, θ is the half apex angle of the conical copper wire, and dz is the incremental radius of the wire. The Laplace pressure on the region face the wire’s tip (small radius R1) is larger than that toward the base (large radius R2). The difference (ΔPcurvature) within the water drop initiates a driving force FL that makes the drop overcome 10684

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Figure 5. Estimation of water collector with PCCW arrays. (a) Illustration of the PCCWs array. The array of PCCWs is fixed on the square frame with a groove; the groove connects the ends of PCCWs, and the bottom of frame stores the water. The arrows represent the moving directions of water drops. (b) Estimation of water collection rates versus fog flowing velocities. The different velocity (0.8, 1.2, 1.6, 2.0, and 2.4 m/s) of fog flow can act on arrays of PCCWs at different impact angles (30°, 45°, 60°, 75°, and 90°). Generally, a collecting rate of 90° impact angle is higher than others. The maximal water collection rate can reach up to 0.618 g/cm2/h at an impact angle of 90° and fog velocity of 2.4 m/s. Experiment is done in humidity (H) of 90% and temperature (T) of 15 °C.

the resistance and move from the tip to the base along the conical copper wire (see Figure 3d). In addition to the gradient of the Laplace pressure, the coalescence energy of drops merging along the conical wire is another enhancing force for drop’s directional movement. Specifically, the coalescing process of drops causes a deformation of the merged drop. In the process of coalescence, the area of the solid−liquid interface reduced is equal to the area of the solid−vapor interface increased. Then the surface energy released in the coalescence process is ΔEs = γlvΔAlv + γslΔAsl + γsvΔAsv = γlvΔAlv − (γsv − γsl)ΔAsl, where γlv, γsl, and γsv are the interfacial tension of liquid−vapor, solid−liquid, and solid−vapor, respectively. ΔAlv, ΔAsl, and ΔAsv refer to the variations of area of the liquid−vapor, the solid−liquid, and the solid−vapor interfaces. Because the copper wire is hydrophilic, ΔAsl is approximate to 0. Therefore, ΔEs = γlvΔAlv = γlv[4πr12 + 4πr22 − 4π(r13 + r23)2/3] = 4πγlv (r12 + r22 − (r13 + r23)2/3), where r1 and r2 refer to the radius of two drops before coalescing. Therefore, the coalescence energy is higher when the coalescing drops are bigger (further analyses are contained in the Supporting Information, analysis I and Figure S6). The as-released energy favors the liquid viscous dissipation inside the drops and to overcome the surface adhesion as well as the intermolecular attraction of the solid−liquid interface. The releasing of energy continues with the drop deformation in the transport process. Resulting from the surface energy released upon drop coalescence, the coalesced drops can jump from superhydrophobic surfaces33−36 to a hydrophobic fiber37 without any external forces, or viscous drop motion on a hydrophobic fiber.38 During the drop jumping process, the coalesced drop will endure a shape deformation process, after the first stage in which a necking of the liquid bridge between the merged drop (when the drops start to jump off from surfaces). In this paper, the PCCW is hydrophilic as well as the shapes of drops on the wire before coalescence are barrel, thus when two of them are coalescing, the droplets are unable to overcome the drop-surface adhesion, so the merged drop will deform along the wire as shown in Figure 3c (see Figure 2c and Movie S3). As the drop deforms, the triple line of the drop on surface will change and so will the Laplace pressure which propels the merged drop to move forward. In other words, the hydrophilic copper wire applied the coalescence energy into

kinetic energy of merged drop. When the horizontal long axis of the deformed drop reaches its maximum, the Laplace pressure between two opposite sides of the coalesced drop is largest,21 therefore, driven by the coalescence energy emitted (Fc) and Laplace pressure (FL), the coalesced drop can overcome the wettability gradient and moves toward the next HRR. To compare effects of orientations of PCCW for the fog harvesting ability, PCCW was fixed, respectively, at tilt angles (15°, 10°, 5°, 0°, −10°, −15°, and −35°) in a chamber with a fog flow velocity of 1.8 m/s at an environmental temperature of 15 °C to examine how the drops responded to Laplace pressure, coalescence energy, and wettability gradient during the fog harvesting process. The tilt angle is defined on a basis of the horizontal one (defined as tilt-angle of 0°); the up-tilted angle is regarded as a positive value, e.g., 15°, 10°, and 5°, while the down-tilted angle is a negative value, e.g., −10°, −15°, and −35°. Figure 4a shows the case of the tilt angle at 15°. The first as-formed drop (diameter of ∼0.88 mm) starts to move along PCCW and sweeps the other drops for one whole transport process in 0.81 s. When PCCW is placed at a tilt angle of 5° (Figure 4b), the first as-formed drop (diameter of ∼1.05 mm) sweeps other drops for one whole transport process in 0.49 s. As for the horizontal one (PCCW at tilt-angle 0°), as shown in Figure 4c, the first as-formed drop (diameter of ∼1.28 mm) sweeps other drops for the complete process in 0.30 s. The drop size is decreasing with the tilt angle declining from 15° to 0°. When PCCW is fixed at a tilt-angle of −15°, the first asformed drop (diameter of ∼1.43 mm) performs the whole transport process in 0.50 s. As for PCCW with tilt angle of −35° (Figure 4e), the drop transport on the PCCW is in a critical situation, where the transport direction is different from those recently mentioned, at the time of 14.77 s; the last drop moves from the base to tip in 2.45 mm. These details of drop transport above can be seen in Movie S4. In drop transport on PCCW above proper tilt-angles (e.g., −15° to 15°), it indicates that the periodic roughness sites (HRRs) with alternate joint (LRR) propel effectively the continuous drop transport in a long distance from one roughness site (HRR) to another roughness site (HRR), favoring the development of excellent fog harvesting ability. The above phenomena would not take place on a uniformly rough wire (URCW, see Movie S5; 10685

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achieved continuous fog collection and efficient drop transportation by mimicking the fog collection principle of wetrebuilt spider silk and cacti. We have also presented that coalescing energy from two drops can be applied on a hydrophilic surface by generating higher Laplace pressure when the coalesced drop deforms during the merging process along the conical copper wire. The combination of the gradient of Laplace pressure arising from the conical shape and wettability arising from the gradient of roughness as well as the coalescence energy guarantees quick collection and longrange unidirectional transportation of drops. The investigation of this bioinspired PCCW and further designed water collection device may provide ideas for future fog collection projects.

difference of water collection rate between URCW and PCCW is shown in Figure S3). A quantitative analysis on average transport velocity or water collection rate on PCCW is shown in Figure 4f,e. Figure 4f shows relationship of average velocity versus tilt-angle (−15° to 15°). When PCCW has tilt angle of 0°, the average velocity is the highest 11.67 cm/s. The other PCCW tilt-angles of drop transport have a relatively low average velocity (see Table S1 and velocity distribution on a PCCW in Figure S4). Figure 4g shows the relationship of fog collection rate versus PCCW tiltangles. The water collection rate reaches the highest 0.36 mg/s at tilt-angle of 0°, greater than others. This phenomenon is mainly related to the mutual effect of the coalescence energy, Laplace pressure, and periodic roughness. In this work, it predicts that the drop transport is mainly propelled by the coalescence. The surface energy released is higher when the difference in the surface area (before and after coalescing) is higher, i.e., the coalescence energy is higher when the coalescing drops are bigger in volume. As we demonstrated above, the drop size decreases with the diminution of tilt angle (from 15° to 0°). This phenomenon is related to the mutual effect of the coalescence energy and gravity (among which the coalescence energy plays a major role). As for PCCW at up-tilt angles (e.g., 15°, 10° and 5°), the drops would slide at a smaller size due to the influence of gravity, accordingly, retard the growth of the drop in size. As for PCCW at tilt angle of 0°, drops would grow quickly at a rough site to a much bigger size, which has a larger coalescence energy for efficient transport with high velocity. As for PCCW at down-tilt angles (e.g., −10°, −15°, and −35°), drops in growth at rough sites need to overcome the gravity to move up, thus the drop transports slowly with low velocity. According to the experimental phenomena and analysis of coalescence energy releasing, the transport velocity of drop is highest when the PCCW is placed horizontally, and so does the fog collecting rate. This means that coalescence energy is related to the drop size formation at periodic rough sites, which would be predominant in the fog collection process. We especially designed a water collecting array of PCCWs (see schematic diagrams in Figure 5a). In our experiment, an array of 18 PCCWs is fixed on a Teflon frame (6.5 × 6.5 × 0.5 cm, optical images are provided in Figure S5). The inner side of the Teflon frame is notched as a drainage channel, and a small hole is drilled at the bottom to collect water drops which flow through the drainage channel into the tube as a storage collector. Based on the analysis of Figure 4f, we have chosen to place the PCCWs horizontally. In order to see whether this array can catch droplets from different directions, a series of fog collection measurements were done by setting this array at different fog impacting angles, i.e., 30°, 45°, 60°, 75°, and 90°, respectively, with different velocities (0.8, 1.2, 1.6, 2.0, and 2.4 m/s) of fog flow. Due to the differences of the fog receiving area and fog impacting velocity,39 the water collection rate varies. When the fog flow blows vertically at this array, the receiving area is the largest and so is the water collection rate. Thus, when we put this array in an environment of room temperature 15 °C, 90% relative humidity, and fog impacting velocity of 2.4 m/s with an impact angle of 90°, it can reach the highest water collection rate of 0.618 g/cm2/h (see Figure 5b and Table S2), which is very promising for practical application.

EXPERIMENTAL SECTION Preparation of PCCW. A commercial copper wire with diameter of approximately 800 μm was polished carefully with sandpaper and rinsed with ethanol and deionized water. Afterward it was dried with nitrogen to remove the peripheral insulating paint before use. As shown in Figure S1, the wire was then fixed vertically and connected to the anode of a 10 V DC power cooperating with a curled copper sheet connected to the cathode and CuSO4 solution (0.5 mol/L) working as the electrolyte. A container was filled with CuSO4 solution which pumps from a syringe pump (Smith WZ-50F6) at a constant rate of 5 mL/min. By increasing the liquid level of electrolyte, the copper wire was subject to a gradient of electrochemical corrosion along its height, thus producing a conical shape. As to the periodic roughness gradient, it was controlled by the periodic current generated from the DC power supply which is manipulated by a programmed stepping motor. The periodic current increased from 0.05 to 0.8 A in a line and pondered for 5 s and then declined to 0.05 A (as shown in Figure S2c). Thus, these HRR and LRR are periodically arranged along the entire wire, i.e., PCCW can be fabricated successfully. Characterization. SEM images of PCCW were obtained by an environmental scanning electron microscope (Quata FEG 250, FEI, U.S.A.) with a high-vacuum mode and an accelerating voltage of 10 kV. The AFM images were obtained by atomic force microscope (Bruker Dimension Icon, U.S.A.). The optical images were captured from the video obtained by a Nikon D60 camera, and the optical micrographs in Figure 2 were obtained by a set of XTL-500 optical microscope (Gui Guang, China) and CCD (UI-2220SE-C-HQ, IDS) with the frame rate of 170. Water contact angles (CAs) on different segments of the PCCW were tested with the optical contact angle meter system (Dataphysics SCA20, Germany). A PCCW was carefully fixed on a sample stage horizontally. Droplets (4 μL in volume, pure water) were placed gently onto different sites of the surfaces. When the droplets were steady, the CAs were tested by an optical contact angle system. The room temperature and relative humidity were ≈15 °C and ≈40 ± 10%, respectively. Fog Collection Performance of PCCW. PCCW was carefully fixed on a sample stage at different tilt angles (from −35° to 15°). A fog flow with relative humidity of 90% at room temperature ≈15 °C and velocity of ca. 1.8 m/s (0.438 g tiny liquid droplets and 0.023 g water vapor per liter saturated fog flow, data from the illustrations of the humidifier) was generated by an ultrasonic humidifier (YC-E350, China). The humidifier was set to blow vertically at the PCCW. The whole process was recorded by a Nikon D60 camera. Estimation on Array of PCCWs. The device was carefully fixed on the retort stand by a ring clamp. A fog flow with relative humidity of 90% at room temperature ≈15 °C was set to blow at the device with different velocities (from ca. 0.8 to 2.4 m/s). A series of fog collection measurements were done by setting this device at different fog impacting angles, i.e. 30°, 45°, 60°, 75°, and 90°, respectively. The whole process was recorded by a Nikon D60 camera.

CONCLUSION In conclusion, we have successfully prepared the desired periodic roughness-gradient conical copper wire, which has 10686

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05595. Experimental details and data (PDF) Transportational process of the first three drops on the PCCW (periodic roughness gradient conical copper wire) surface (MPG) Forming process of the first drop on the PCCW surface (MPG) Coalescence process of two drops on the PCCW surface (MPG) Water collecting process of single PCCW at different tilt angles (MPG) Water collecting process of a uniformly rough conical wire (URCW) at different tilt angles (MPG)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is supported by Key Program of National Natural Science Foundation of China (no. 21234001), National Research Fund for Fundamental Key Project (no. 2013CB933001), National Natural Science Foundation of China (no. 21473007), and Fundamental Research Funds for Central Universities (no. YWF-16-JCTD-A-01). REFERENCES (1) Malik, F. T.; Clement, R. M.; Gethin, D. T.; Krawszik, W.; Parker, A. R. Nature’s Moisture Harvesters: A Comparative Review. Bioinspiration Biomimetics 2014, 9, 031002. (2) Xue, Y.; Wang, T.; Shi, W.; Sun, L.; Zheng, Y. Water Collection Abilities of Green Bristlegrass Bristle. RSC Adv. 2014, 4, 40837− 40840. (3) Tracy, C. R.; Laurence, N.; Christian, K. A. Condensation onto the Skin as A Means for Water Gain by Tree Frogs in Tropical Australia. Am. Nat. 2011, 178, 553−558. (4) Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F. Q.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640−643. (5) Ju, J.; Bai, H.; Zheng, Y.; Zhao, T.; Fang, R.; Jiang, L. A MultiStructural and Multi-Functional Integrated Fog Collection System in Cactus. Nat. Commun. 2012, 3, 1247−1247. (6) Comanns, P.; Buchberger, G.; Buchsbaum, A.; Baumgartner, R.; Kogler, A.; Bauer, S.; Baumgartner, W. Directional, Passive Liquid Transport: the Texas Horned Lizard as A Model for A Biomimetic ’Liquid Diode’. J. R. Soc., Interface 2015, 12, 0415. (7) Parker, A. R.; Lawrence, C. R. Water Capture by A Desert Beetle. Nature 2001, 414, 33−34. (8) Chen, Y.; Wang, L.; Xue, Y.; Jiang, L.; Zheng, Y. Bioinspired TiltAngle Fabricated Structure Gradient Fibers: Micro-Drops Fast Transport in A Long-Distance. Sci. Rep. 2013, 3, 2927. (9) Xue, Y.; Chen, Y.; Wang, T.; Jiang, L.; Zheng, Y. Directional SizeTriggered Microdroplet Target Transport on Gradient-Step Fibers. J. Mater. Chem. A 2014, 2, 7156−7160. (10) Ju, J.; Xiao, K.; Yao, X.; Bai, H.; Jiang, L. Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection. Adv. Mater. 2013, 25, 5937−5942. 10687

DOI: 10.1021/acsnano.6b05595 ACS Nano 2016, 10, 10681−10688

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DOI: 10.1021/acsnano.6b05595 ACS Nano 2016, 10, 10681−10688