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water separation on a gradient nanowire structured surface
Journal of Membrane Science 582 (2019) 246–253
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Unidirectional liquid transportation and selective permeation for oil/water separation on a gradient nanowire structured surface
T
Yufeng Yana, Linlin Hea, Yan Lia, Dongliang Tiana,b,∗, Xiaofang Zhangc,∗∗, Kesong Liua,b,∗∗∗, Lei Jianga,b,d a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, PR China c School of Mathematics and Physics, University of Science & Technology Beijing, Beijing 100083, PR China d Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100191, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Unidirectional motion Liquid transportation Selective permeation Oil/water separation Gradient structure
Liquid transport and permeation on solid surfaces with gradient wettability had attracted tremendous research attention to solve some important problems of interface science and technology. On account of the existing problem of film pore blocking during the oily water separation process, however, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Here we demonstrate a strategy to achieve unidirectional motion and selective permeation of underwater oil droplet and water on the Cu(OH)2 nanowires structured copper foil/mesh surface with a density and length gradient. The gradient Cu(OH)2 nanowires fabricated by anodic oxidation method with anodization time gradient, shows wettability gradient from hydrophilic/oleophobic to superhydrophilic/superoleophobic and adhesion force gradient to underwater oil from high to low. As a result, the underwater oil droplet can only move unidirectionally from the superhydrophilic/superoleophobic scope to the hydrophilic/oleophobic area on the nanowires structured copper foil/mesh surface, meanwhile, water can only permeate on the superhydrophilic area, but not permeate on the hydrophobic area of the nanowires structured copper mesh. Therefore, a new way was presented to provide more contact area of oil/water mixture, owing to the unidirectional motion of underwater oil droplets and selective permeation of water, for rapid, continuous, high flux oil/water mixture separation with different interfacial tension in engineering field, which would also be promising to develop smart interface materials for microfluidic devices.
1. Introduction Directional liquid droplets motion or permeation on a solid surface is significant to solve some challenging problems of interface science and technology and has aroused tremendous research attention [1–4], on account of the promising applications prospect of directional manipulation and separation in various fields, including microfluidic devices [5–7], biomolecular interactions [8–10], heat transfer [11,12], chemical reactions [13,14], etc. In nature, many animals and plants can skillfully rely on their special gradient surface to control the directional movement of droplets. For instance, butterfly wings exhibit directional water rolling capacity under the influence of anisotropic surface
wettability gradient produced by the micropatterns on wings [15]. In addition, desert beetles rely on its back with wettability gradient to transport tiny water droplets directionally from hydrophobic waxy regions to hydrophilic nonwaxy regions of the back [16]. Moreover, Nepenthes alata reveals a unique continuous directional fluid transport system combining multiscale microgrooves gradient accelerated the anti-gravity movement of water [17]. Inspired by these biological examples, in the past twenty years, various methods have been adopted to operate liquid droplets directional motion on a solid surface by fabricating a surface with wettability gradient [18–28]. Nevertheless, once the contact angle (CA) hysteresis is larger than 10°, droplets motion will be hold back. As is reported, surface wettability is mainly up to the
∗ Corresponding author. Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China. ∗∗ Corresponding author. ∗∗∗ Corresponding author. Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China. E-mail addresses: [email protected] (D. Tian), [email protected] (X. Zhang), [email protected] (K. Liu).
https://doi.org/10.1016/j.memsci.2019.04.011 Received 14 March 2019; Received in revised form 7 April 2019; Accepted 8 April 2019 Available online 13 April 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 582 (2019) 246–253
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Fig. 1. Fabrication and gradient wettability of the nanowire surface. (a) Schematic fabrication process of the gradient nanowire structured surface via anodic oxidation method. A copper wire is used as cathode, which bottom is aligned with the anode and the horizontal distance between them is 30 mm. During the anodic oxidation process, a dip coater is used to pull upward anode gradually to form an anodizing oxidation time (AOT) gradient under a constant current intensity. By this way, a gradient change of the nanowire length from short (S) to long (L) on the surface is formed with the increasing AOT, accompanying with the density of nanowires increase. (b–e) From top to bottom, the pictures are SEM images, the water CA photographs and their schematic diagrams, the oil CA in water and their schematic diagrams on the gradient nanowire structured surface under different AOT conditions: (b) AOT = 60 s, (c) AOT = 75 s, (d) AOT = 90 s, (e) AOT = 105 s. The results indicate that the gradient wettability is achieved on the gradient nanowire structured surface.
manipulate the directional movement of a liquid droplet [36–39]. Meantime, the oily water separation based on the superwetting surface has been widely studied [40–46]. During the gravitational separation process of oily water, film pore blocking is one of the most important problems that limit their application. Despite much progress in this
chemical composition and surface roughness of the solid surface. Owing to the reasonable design of the surface and interface, the motion states of the liquid droplets may be manipulated [29–35]. Subsequently, by creating chemical composition gradient, surface structure gradient, or both to fabricate wettability gradient surfaces is widely used to 247
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water, ethanol and acetone mixture (1:1) for 20 min to remove the grease by ultrasonication on the surface completely, then eliminated the copper oxides with HCl aqueous (1 mol/L) for 5 min also under ultrasonication, finally, which was further rinsed by clean water and dried with a dry nitrogen stream for the following experiments. The gradient nanowire surface was fabricated by electrochemical anodization method. Using the dilute KOH (1 mol/L) aqueous as the electrolyte, the copper foil/copper mesh as the working electrode (anode), and the copper wire as the counter electrode (cathode). The bottom of the anode and cathode is aligned and the horizonal distance between them is 30 mm. The immersion length for copper foil is 24 mm, and for copper mesh is 75 mm. The anodization time change of the copper foil/ copper mesh substrate was controlled by pulling the substrate upward using a dip coater with the rising velocity of 12 mm/min under a constant current intensity 6 mA at room temperature. Then taking down the copper foil/copper mesh from the dip coater, and totally rinsed by deionized water and dried via nitrogen stream. Thus the gradient wettability surfaces on copper foil/copper mesh was achieved. 2.2. Instruments and characterization The model of the dip coater used in the anodizing process of the copper foil/mesh was PTL-MM02. The scanning electron microscope (SEM) images were obtained by JEOL JSM-6700F (3.0 kV) to achieve the morphology of Cu(OH)2 nanowire. Water contact angles (CAs) on the gradient nanowire substrate were obtained by Dataphysics OCA20 CA system. The volume of a droplet for the contact angle measurements is 5 μL and the static CA was measured for five times at the same position on the substrate, then averaging the five measurement results to get a reliable contact angle. The oil droplets (paraffin) adhesion forces on the gradient nanowire surface were tested via Data-Physics DCAT 11, Germany. 3. Results and discussion 3.1. Fabrication and gradient wettability of the nanowire structured surfaces
Fig. 2. Oil adhesion characterization of the as-prepared nanowire copper surfaces. Adhesion force-distance curves (a) and adhesion force values (b) of the underwater oil droplet at different positions of the fabricated copper substrate at the squeezing distance of 0.2 mm. It can be seen that underwater oil-adhesion force of the fabricated surface gradually decreased from 69.4 μN to 0.6 μN from S to L with the AOT increase from 60 s to 105 s.
To realize liquid unidirectional motion/selective permeation, we fabricated gradient wettability surfaces via electrochemical anodic oxidation method. As illustrated in Fig. 1a, taking anodic copper foil for example, by changing the anodic oxidation time (AOT) of the substrate while pulling upward a copper foil from KOH solution at a constant current intensity, the Cu(OH)2 nanowires are deposited increasingly along the latitude of the copper foil/copper mesh substrate from short AOT (S) to long AOT (L), resulting in that chemical composition and nanowires structured gradient in density and length is formed. The electrochemical anodic oxidation reaction process can be described by the following reaction equation:
field, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Herein, we demonstrate the unidirectional motion of underwater oil droplets and selective permeation of water on a Cu(OH)2 nanowires structured copper foil/mesh surface with wettability gradient via electrochemical anodization method. In gradient anodic oxidation procedure, the density and length of grown Cu(OH)2 nanowires are varying gradually, resulting in that the change of water CA, the underwater oil CA and the underwater oil adhesion force. Accordingly, unidirectional motion of an oil droplet and selective permeation of water are realized on the nanowires structured copper foil/mesh surface based on the wettability gradient, which is beneficial to offer more contact areas for oil/water mixture and is suitable for rapid, continuous, high flux oil/ water separation and potential for development of smart interface materials and micro/nano-fluidic devices.
Anode reaction: Cu − 2e− + 2OH− → Cu (OH )2 ↓ Cathode reaction: 2H2 O + 2e− → H2 ↑ + 2OH− The scanning electron microscope (SEM) images indicate that only a few Cu(OH)2 nanowire arrays distribute on the surface sparsely with the average length of 0.5–1 μm and diameter of ∼100 nm when the AOT is 60 s (Fig. 1b–e). Along with the S to L direction, the density and the length of the Cu(OH)2 nanowires grown on copper foil are increasingly larger and longer accompanying with the increase of AOT. When the AOT increases to 105 s, the Cu(OH)2 nanowires arrays aggregate into clusters little by little with the length of 3–3.5 μm. Thus, by controlling the AOT, the oxidation products of Cu(OH)2 nanowires structure with gradient density and length are deposited on the surface and the chemical composition gradient surfaces are formed. With the gradient change of Cu(OH)2 nanowire arrays in density and length from S to L direction, the water CAs vary gradually from
2. Experimental section 2.1. Fabrication of the gradient nanowire surfaces The copper foil and copper mesh were cleaned with deionized 248
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Fig. 3. Dynamic adhesion and movement behavior of underwater oil droplet on the gradient nanowire structured surface. (a–c) Dynamic adhesion behavior of oil droplet in water on different position of the gradient nanowire structured surface from L to S direction. (a) At the high adhesion force area with the AOT of 60 s. (b) At the middle adhesion force area with the AOT of 75–90 s. (c) At the low adhesion force area with the AOT of 105 s. (d–e) The sliding behavior of oil droplet in water on the gradient nanowire surface with different gradient direction. (d) Along with the direction from S to L, an oil droplet is pinned and even slightly moves to left on the S area of the gradient wettability surface at a tilted angle of θ = 1.5°, it is on the surface. (e) Along with the opposite direction, i.e., LS direction, an oil droplet will roll off a certain distance of ∼9.6 mm, and finally pinned on the S area. The results indicate the fabricated gradient wettability surface can realize the unidirectional motion of underwater oil droplet. △ is the initial contact position between an oil droplet and the substrate.
∼86° to ∼0°. Compared to the original copper foil surface with water CA of ∼89°, the hydrophilicity can be enhanced with both an increase in surface roughness and an increase in surface energy (change from Cu to Cu(OH)2, the polar groups of eOH existed in Cu(OH)2 nanowires with large surface energy) are responsible for this high decrease in CA, according to Wenzel equation (1) [1].
cos θr = r cos θ
3.2. Underwater oil adhesion behavior on the nanowire structured surfaces Owing to the adhesion behavior of a liquid droplet on the solid surface is an important factor affecting the liquid droplets motion behavior on a surface, the gradient adhesion of the gradient wettability surface is expected and promising to realize the unidirectional motion of liquid droplets. Following the underwater oil-adhesion force on the gradient wettability surface is investigated (Fig. 2). During adhesion force measurement of the underwater oil droplet, the nanowires structured surface of the copper foil substrate is firstly moved to contact an oil droplet, squished about 0.2 mm and then leave at 0.01 mm s−1. The adhesion forces between oil droplet (5 μL) and the surface are recorded. It can be observed from the curve of oil adhesion force and distance that underwater oil-adhesion force gradually decreases with the AOT increase from 60 s to 105 s, indicating that the long AOT is beneficial to reduce the oil adhesion force in water gradually from 69.4 μN to 0.6 μN on the gradient wettability surface along with the S to
(1)
where θr are the water CAs on the oxidized surface and θ are the water CAs on the original surface, and r represents the roughness of the surface. For a tri-phase system of oil on a solid surface in water, the oil (liquid paraffin) CAs in water increase from ∼99° to ∼158° on the S to L direction of the gradient surface. The results indicate that the CAs of water and underwater oil are dependent on each other and change conversely. Therefore, a surface with gradient wettability is realized on the copper foil surface with gradient nanowire structure.
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Fig. 4. Unidirectional liquid permeation on the gradient nanowire mesh surface. (a) Schematic illustration of the as-prepared gradient nanowire mesh surface. (b–e) The water droplet CA and their corresponding schematic diagrams on the gradient nanowire structured mesh surface under different anodizing oxidation time conditions: (b) AOT = 96 s, (c) AOT = 156 s, (d) AOT = 198 s, (e) AOT = 228 s. (f–i) The underwater oil droplet CA and their schematic diagrams on the gradient nanowire structured surface under different anodizing oxidation time conditions: (f) AOT = 96 s, (g) AOT = 156 s, (h) AOT = 198 s, (i) AOT = 228 s. (j) The permeation distance of water on the gradient nanowire structured copper meshes with different initial pore size. The results indicate that the permeation distance of water on the meshes is decreased with the pore size increasing from 75 μm to 125 μm, while the permeation distance increases with the increasing pore size from 125 μm to 175 μm. The minimum permeation distance is 55 mm with the pore size of 125 μm owing to suitable roughness by the cooperation of microscale pore, copper wire of mesh and the fabricated nanostructures. Meantime, the meshes surface kept stable underwater superoleophobic from the pore size increasing from 75 μm to 175 μm, although the slightly decrease of oil CA about 2° at 125 μm.
droplet CA on the short nanowire areas; γow is interfacial tension of oil and water, and dl is the droplet length on gradient surface. When an oil droplet (2 μL) hung on the syringe needle moves to contact the substrate and is further squished slightly, then to move the substrate toward right at a certain speed of ∼10 cm/s. The results indicate that there are three different adhesion and stiction behaviors when the oil droplet in water moving along with the gradient wettability surface from S to L direction (Fig. 3a–c). At the high adhesion force area (S area), when the substrate moves toward right, the oil droplet is deformed severely for the stiction between oil and substrate until the deformation force cannot resist the tensile force of oil droplet, i.e. the adhesion force of oil droplet with syringe needle surface (FOS) is bigger than oil droplet on the nanowire structured substrate surface (FOS < SON), then the oil droplet is easier to be pulled down by the substrate and moves toward right together with the substrate (Fig. 3a). At the middle adhesion force area (Fig. 3b), the oil droplet just deforms when the substrate moves toward right, then recovering the initial shape with further moves of the substrate for FOS ≥ SON. It is not
L direction. 3.3. Dynamic adhesion and motion behavior of underwater oil droplet on the gradient nanowire structured surface To make sure the influence of the gradient nanowire structured surface on motion behavior, the dynamic movement behavior of an oil droplet in water along with the fabricated gradient wettability surface was investigated. For a given oil droplet, gravity (G) and water buoyance Fb are invariable, resulting in that stiction on the nanowire structured film (SON) affects the oil droplet motion behavior by adjusting the different directions on the film [47], because the gradient surface may generate an unbalanced force, FG, which is activated for driving the oil droplet motion, as described in equation (2) [18,37]: L
F=
∫ γOW (cosθL − cosθS ) dl S
(2)
where θL is the oil droplet CA on the long nanowire areas; θS is the oil 250
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Fig. 5. Gradient nanowire mesh surface for efficient oil/water separation. The dynamic images of the selective water permeation and unidirectional motion of oil on the gradient nanowire structured copper mesh, which indicates that oil/water separation with the mass ratio of 3:7 is realized.
from 96 s to 228 s (Fig. S1). The surface wettability of the gradient surface changes correspondingly with the water CAs decrease from ∼130° to ∼12° in the S to L direction. At the area with the water CA of ∼12°, water can permeate the nanowire nanostructured copper mesh. Meantime, the underwater oil CA values change oppositely from ∼138° to ∼162° in the S to L direction (Fig. 4f–i), accompanying with the underwater oil droplet adhesion forces decrease from 18.3 to 0.5 μN, and the oil droplet is easier to move along L to S direction than S to L direction with surface wettability and adhesion gradient (Fig. S2). In addition, the permeation distance of water on the gradient nanowire structured copper meshes with pore sizes varying from 75 to 175 μm was investigated (Fig. 4j). The results show the permeation distance of water on the meshes decreases with increasing pore size from 75 μm to 125 μm, while the permeation distance increases with the increasing pore size from 125 μm to 175 μm. The minimum permeation distance is 55 mm on the nanowire structured surface with the pore size of 125 μm, owing to suitable roughness resulting from the cooperation of microscale pores, copper wires of mesh and the fabricated nanostructures. Meantime, the gradient nanowire structured copper meshes surface kept stable underwater superoleophobic from the pore size increasing from 75 μm to 175 μm, although the slightly decrease of oil CA about 2° at 125 μm, which is consistent with the previous result [48,49].
difficult to speculate the following behavior of oil droplet in low adhesion area in which the oil droplet neither is deformed nor moves toward right together with the substrate owing to FOS > SON (Fig. 3c). Accordingly, the surface shows gradient adhesion on the surface with gradient wettability along with the single direction, which may have an important role in the directional liquid motion. To verify the feasibility of liquid motion unidirectionally on the gradient wettability surface, the sliding behavior of underwater an oil droplet on the tilted gradient nanowire surface with different gradient directions was investigated. When an oil droplet is dropped on the S area of the gradient wettability surface at a tilted angle of 1.5°, it is pinned on the surface owing to the high adhesion and stiction, FG + SON ≥ (G-Fb)sinθ, and even slightly moves to left (Fig. 3d). While along with the opposite direction, i.e., LS direction, an oil droplet will roll off a certain distance of ∼9.6 mm with the force balance of SON ≤ FG + (G-Fb)sinθ, and finally pinned on the S area due to the increasing underwater oil adhesion force along with the direction from L to S (Fig. 3e). The results indicate that the dynamic wetting behaviors of oil droplet (in water) on the surface produce preferred directions of liquid motion, thus the fabricated surface can realize the unidirectional oil droplet motion in water, which may be beneficial to the research and development of the micro-reactors and microfluidic devices.
3.4. Unidirectional permeation of water on the gradient nanowire structured mesh surface
3.5. Oil/water separation on the gradient nanowire structured mesh surface
Based on the unidirectional underwater oil droplets motion on the gradient nanowire surface, the gradient wettability copper mesh fabricated using the anodic oxidation method is expected to realize the unidirectional permeation of water (Fig. 4a–e). The SEM images results show the Cu(OH)2 nanowire gradient increases in density and length along the S to L direction of the surface with the gradient AOT ranging
From the different surface wettability, motion behavior and selective permeation of water on the gradient mesh surface with gradient nanowire, it can be conceived that the gradient wettability copper mesh would be potential for smart interface materials to realize the selective liquid separation. As is shown in Fig. 5, when the oil/water mixture, e.g., liquid paraffin and water mixture with the mass ratio of 3:7, is 251
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added onto the gradient nanowire structured copper mesh, water can move and selectively permeate the mesh film at the hydrophilic side, while the oil droplets move on the mesh to the opposite side on the mesh film and provide more contact area of oil/water mixture for further rapid and continuous separation, which indicates that the oil/ water mixture can be effectively separated with high flux. To quantify the efficiency of oil-water separation, the oil content in the oil/water mixture is measured prior to separation and the result is defined as V0. The oil content after oil/water separation is also measured and defined as V. The efficiency of separating oil can be calculated by formula (3):
efficiency =
V0 − V V0
× 100%
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(3)
Meanwhile, other oils such as diesel oil, vegetable oil, heptane, dibromomethane and 1,2-dichloroethane with different interfacial tension from 20 to 40 mN m−1 with water can also be separated with high efficiency more than 98.5%. Therefore, the gradient nanowire structured copper mesh is beneficial to avoid the problem of film pore blocking during the oily water separation process, and is promising in the rapid, continuous oil/water separation with high efficiency owing to the selective permeation of water and unidirectional motion of underwater oil. 4. Conclusion In conclusion, we have presented a strategy to realize the unidirectional oil motion in water and selective permeation of water based on the surface wettability gradient on the nanowire nanostructured copper foil/copper mesh substrate fabricated by anodic oxidation method. The different wettability and adhesion force for oil droplet in water on the two sides of a copper foil show two different oil droplets motion states in water on the nanowire nanostructured substrate, and underwater oil droplets can be transported to the desired direction, which is beneficial to the research and development of micro-reactors and microfluidic devices. More importantly, based on the different gradient wettability change of water and oil droplets in water along the gradient copper mesh, selective permeation of water through the mesh and the oil droplets unidirectional motion can provide more contact area of oil/water mixture for rapid, continuous, high flux oil/water separation can be realized, which is promising to broaden its application on selective separation of different liquids with different interfacial tension in the following work and. This work also provides a new way for development of smart interface materials and micro/nano-fluidic devices. Acknowledgements Y. Y., and L. H. contributed equally to this work. The authors are grateful for financial support from the Chinese National Natural Science Foundation (21671012, 21601013, 21561017), Beijing Natural Science Foundation (2172033, L160003), the Fundamental Research Funds for the Central Universities, Beijing Young Talent Support Program, Beijing Nova Program, and the 111 Project (B14009). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.04.011. References [1] M.J. Liu, S.T. Wang, L. Jiang, Nature-inspired superwettability systems, Nat. Rev. Mater. 2 (2017) 17036. [2] Y. Cui, D.W. Li, H. Bai, Bioinspired smart materials for directional liquid transport, Ind. Eng. Chem. Res. 56 (2017) 4887–4897. [3] J. Ju, Y.M. Zheng, L. Jiang, Bioinspired one-dimensional materials for directional liquid transport, Acc. Chem. Res. 47 (2014) 2342–2352.
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Unidirectional liquid transportation and selective permeation for oil/water separation on a gradient nanowire structured surface
T
Yufeng Yana, Linlin Hea, Yan Lia, Dongliang Tiana,b,∗, Xiaofang Zhangc,∗∗, Kesong Liua,b,∗∗∗, Lei Jianga,b,d a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, PR China c School of Mathematics and Physics, University of Science & Technology Beijing, Beijing 100083, PR China d Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100191, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Unidirectional motion Liquid transportation Selective permeation Oil/water separation Gradient structure
Liquid transport and permeation on solid surfaces with gradient wettability had attracted tremendous research attention to solve some important problems of interface science and technology. On account of the existing problem of film pore blocking during the oily water separation process, however, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Here we demonstrate a strategy to achieve unidirectional motion and selective permeation of underwater oil droplet and water on the Cu(OH)2 nanowires structured copper foil/mesh surface with a density and length gradient. The gradient Cu(OH)2 nanowires fabricated by anodic oxidation method with anodization time gradient, shows wettability gradient from hydrophilic/oleophobic to superhydrophilic/superoleophobic and adhesion force gradient to underwater oil from high to low. As a result, the underwater oil droplet can only move unidirectionally from the superhydrophilic/superoleophobic scope to the hydrophilic/oleophobic area on the nanowires structured copper foil/mesh surface, meanwhile, water can only permeate on the superhydrophilic area, but not permeate on the hydrophobic area of the nanowires structured copper mesh. Therefore, a new way was presented to provide more contact area of oil/water mixture, owing to the unidirectional motion of underwater oil droplets and selective permeation of water, for rapid, continuous, high flux oil/water mixture separation with different interfacial tension in engineering field, which would also be promising to develop smart interface materials for microfluidic devices.
1. Introduction Directional liquid droplets motion or permeation on a solid surface is significant to solve some challenging problems of interface science and technology and has aroused tremendous research attention [1–4], on account of the promising applications prospect of directional manipulation and separation in various fields, including microfluidic devices [5–7], biomolecular interactions [8–10], heat transfer [11,12], chemical reactions [13,14], etc. In nature, many animals and plants can skillfully rely on their special gradient surface to control the directional movement of droplets. For instance, butterfly wings exhibit directional water rolling capacity under the influence of anisotropic surface
wettability gradient produced by the micropatterns on wings [15]. In addition, desert beetles rely on its back with wettability gradient to transport tiny water droplets directionally from hydrophobic waxy regions to hydrophilic nonwaxy regions of the back [16]. Moreover, Nepenthes alata reveals a unique continuous directional fluid transport system combining multiscale microgrooves gradient accelerated the anti-gravity movement of water [17]. Inspired by these biological examples, in the past twenty years, various methods have been adopted to operate liquid droplets directional motion on a solid surface by fabricating a surface with wettability gradient [18–28]. Nevertheless, once the contact angle (CA) hysteresis is larger than 10°, droplets motion will be hold back. As is reported, surface wettability is mainly up to the
∗ Corresponding author. Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China. ∗∗ Corresponding author. ∗∗∗ Corresponding author. Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, PR China. E-mail addresses: [email protected] (D. Tian), [email protected] (X. Zhang), [email protected] (K. Liu).
https://doi.org/10.1016/j.memsci.2019.04.011 Received 14 March 2019; Received in revised form 7 April 2019; Accepted 8 April 2019 Available online 13 April 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Fabrication and gradient wettability of the nanowire surface. (a) Schematic fabrication process of the gradient nanowire structured surface via anodic oxidation method. A copper wire is used as cathode, which bottom is aligned with the anode and the horizontal distance between them is 30 mm. During the anodic oxidation process, a dip coater is used to pull upward anode gradually to form an anodizing oxidation time (AOT) gradient under a constant current intensity. By this way, a gradient change of the nanowire length from short (S) to long (L) on the surface is formed with the increasing AOT, accompanying with the density of nanowires increase. (b–e) From top to bottom, the pictures are SEM images, the water CA photographs and their schematic diagrams, the oil CA in water and their schematic diagrams on the gradient nanowire structured surface under different AOT conditions: (b) AOT = 60 s, (c) AOT = 75 s, (d) AOT = 90 s, (e) AOT = 105 s. The results indicate that the gradient wettability is achieved on the gradient nanowire structured surface.
manipulate the directional movement of a liquid droplet [36–39]. Meantime, the oily water separation based on the superwetting surface has been widely studied [40–46]. During the gravitational separation process of oily water, film pore blocking is one of the most important problems that limit their application. Despite much progress in this
chemical composition and surface roughness of the solid surface. Owing to the reasonable design of the surface and interface, the motion states of the liquid droplets may be manipulated [29–35]. Subsequently, by creating chemical composition gradient, surface structure gradient, or both to fabricate wettability gradient surfaces is widely used to 247
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water, ethanol and acetone mixture (1:1) for 20 min to remove the grease by ultrasonication on the surface completely, then eliminated the copper oxides with HCl aqueous (1 mol/L) for 5 min also under ultrasonication, finally, which was further rinsed by clean water and dried with a dry nitrogen stream for the following experiments. The gradient nanowire surface was fabricated by electrochemical anodization method. Using the dilute KOH (1 mol/L) aqueous as the electrolyte, the copper foil/copper mesh as the working electrode (anode), and the copper wire as the counter electrode (cathode). The bottom of the anode and cathode is aligned and the horizonal distance between them is 30 mm. The immersion length for copper foil is 24 mm, and for copper mesh is 75 mm. The anodization time change of the copper foil/ copper mesh substrate was controlled by pulling the substrate upward using a dip coater with the rising velocity of 12 mm/min under a constant current intensity 6 mA at room temperature. Then taking down the copper foil/copper mesh from the dip coater, and totally rinsed by deionized water and dried via nitrogen stream. Thus the gradient wettability surfaces on copper foil/copper mesh was achieved. 2.2. Instruments and characterization The model of the dip coater used in the anodizing process of the copper foil/mesh was PTL-MM02. The scanning electron microscope (SEM) images were obtained by JEOL JSM-6700F (3.0 kV) to achieve the morphology of Cu(OH)2 nanowire. Water contact angles (CAs) on the gradient nanowire substrate were obtained by Dataphysics OCA20 CA system. The volume of a droplet for the contact angle measurements is 5 μL and the static CA was measured for five times at the same position on the substrate, then averaging the five measurement results to get a reliable contact angle. The oil droplets (paraffin) adhesion forces on the gradient nanowire surface were tested via Data-Physics DCAT 11, Germany. 3. Results and discussion 3.1. Fabrication and gradient wettability of the nanowire structured surfaces
Fig. 2. Oil adhesion characterization of the as-prepared nanowire copper surfaces. Adhesion force-distance curves (a) and adhesion force values (b) of the underwater oil droplet at different positions of the fabricated copper substrate at the squeezing distance of 0.2 mm. It can be seen that underwater oil-adhesion force of the fabricated surface gradually decreased from 69.4 μN to 0.6 μN from S to L with the AOT increase from 60 s to 105 s.
To realize liquid unidirectional motion/selective permeation, we fabricated gradient wettability surfaces via electrochemical anodic oxidation method. As illustrated in Fig. 1a, taking anodic copper foil for example, by changing the anodic oxidation time (AOT) of the substrate while pulling upward a copper foil from KOH solution at a constant current intensity, the Cu(OH)2 nanowires are deposited increasingly along the latitude of the copper foil/copper mesh substrate from short AOT (S) to long AOT (L), resulting in that chemical composition and nanowires structured gradient in density and length is formed. The electrochemical anodic oxidation reaction process can be described by the following reaction equation:
field, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Herein, we demonstrate the unidirectional motion of underwater oil droplets and selective permeation of water on a Cu(OH)2 nanowires structured copper foil/mesh surface with wettability gradient via electrochemical anodization method. In gradient anodic oxidation procedure, the density and length of grown Cu(OH)2 nanowires are varying gradually, resulting in that the change of water CA, the underwater oil CA and the underwater oil adhesion force. Accordingly, unidirectional motion of an oil droplet and selective permeation of water are realized on the nanowires structured copper foil/mesh surface based on the wettability gradient, which is beneficial to offer more contact areas for oil/water mixture and is suitable for rapid, continuous, high flux oil/ water separation and potential for development of smart interface materials and micro/nano-fluidic devices.
Anode reaction: Cu − 2e− + 2OH− → Cu (OH )2 ↓ Cathode reaction: 2H2 O + 2e− → H2 ↑ + 2OH− The scanning electron microscope (SEM) images indicate that only a few Cu(OH)2 nanowire arrays distribute on the surface sparsely with the average length of 0.5–1 μm and diameter of ∼100 nm when the AOT is 60 s (Fig. 1b–e). Along with the S to L direction, the density and the length of the Cu(OH)2 nanowires grown on copper foil are increasingly larger and longer accompanying with the increase of AOT. When the AOT increases to 105 s, the Cu(OH)2 nanowires arrays aggregate into clusters little by little with the length of 3–3.5 μm. Thus, by controlling the AOT, the oxidation products of Cu(OH)2 nanowires structure with gradient density and length are deposited on the surface and the chemical composition gradient surfaces are formed. With the gradient change of Cu(OH)2 nanowire arrays in density and length from S to L direction, the water CAs vary gradually from
2. Experimental section 2.1. Fabrication of the gradient nanowire surfaces The copper foil and copper mesh were cleaned with deionized 248
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Fig. 3. Dynamic adhesion and movement behavior of underwater oil droplet on the gradient nanowire structured surface. (a–c) Dynamic adhesion behavior of oil droplet in water on different position of the gradient nanowire structured surface from L to S direction. (a) At the high adhesion force area with the AOT of 60 s. (b) At the middle adhesion force area with the AOT of 75–90 s. (c) At the low adhesion force area with the AOT of 105 s. (d–e) The sliding behavior of oil droplet in water on the gradient nanowire surface with different gradient direction. (d) Along with the direction from S to L, an oil droplet is pinned and even slightly moves to left on the S area of the gradient wettability surface at a tilted angle of θ = 1.5°, it is on the surface. (e) Along with the opposite direction, i.e., LS direction, an oil droplet will roll off a certain distance of ∼9.6 mm, and finally pinned on the S area. The results indicate the fabricated gradient wettability surface can realize the unidirectional motion of underwater oil droplet. △ is the initial contact position between an oil droplet and the substrate.
∼86° to ∼0°. Compared to the original copper foil surface with water CA of ∼89°, the hydrophilicity can be enhanced with both an increase in surface roughness and an increase in surface energy (change from Cu to Cu(OH)2, the polar groups of eOH existed in Cu(OH)2 nanowires with large surface energy) are responsible for this high decrease in CA, according to Wenzel equation (1) [1].
cos θr = r cos θ
3.2. Underwater oil adhesion behavior on the nanowire structured surfaces Owing to the adhesion behavior of a liquid droplet on the solid surface is an important factor affecting the liquid droplets motion behavior on a surface, the gradient adhesion of the gradient wettability surface is expected and promising to realize the unidirectional motion of liquid droplets. Following the underwater oil-adhesion force on the gradient wettability surface is investigated (Fig. 2). During adhesion force measurement of the underwater oil droplet, the nanowires structured surface of the copper foil substrate is firstly moved to contact an oil droplet, squished about 0.2 mm and then leave at 0.01 mm s−1. The adhesion forces between oil droplet (5 μL) and the surface are recorded. It can be observed from the curve of oil adhesion force and distance that underwater oil-adhesion force gradually decreases with the AOT increase from 60 s to 105 s, indicating that the long AOT is beneficial to reduce the oil adhesion force in water gradually from 69.4 μN to 0.6 μN on the gradient wettability surface along with the S to
(1)
where θr are the water CAs on the oxidized surface and θ are the water CAs on the original surface, and r represents the roughness of the surface. For a tri-phase system of oil on a solid surface in water, the oil (liquid paraffin) CAs in water increase from ∼99° to ∼158° on the S to L direction of the gradient surface. The results indicate that the CAs of water and underwater oil are dependent on each other and change conversely. Therefore, a surface with gradient wettability is realized on the copper foil surface with gradient nanowire structure.
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Fig. 4. Unidirectional liquid permeation on the gradient nanowire mesh surface. (a) Schematic illustration of the as-prepared gradient nanowire mesh surface. (b–e) The water droplet CA and their corresponding schematic diagrams on the gradient nanowire structured mesh surface under different anodizing oxidation time conditions: (b) AOT = 96 s, (c) AOT = 156 s, (d) AOT = 198 s, (e) AOT = 228 s. (f–i) The underwater oil droplet CA and their schematic diagrams on the gradient nanowire structured surface under different anodizing oxidation time conditions: (f) AOT = 96 s, (g) AOT = 156 s, (h) AOT = 198 s, (i) AOT = 228 s. (j) The permeation distance of water on the gradient nanowire structured copper meshes with different initial pore size. The results indicate that the permeation distance of water on the meshes is decreased with the pore size increasing from 75 μm to 125 μm, while the permeation distance increases with the increasing pore size from 125 μm to 175 μm. The minimum permeation distance is 55 mm with the pore size of 125 μm owing to suitable roughness by the cooperation of microscale pore, copper wire of mesh and the fabricated nanostructures. Meantime, the meshes surface kept stable underwater superoleophobic from the pore size increasing from 75 μm to 175 μm, although the slightly decrease of oil CA about 2° at 125 μm.
droplet CA on the short nanowire areas; γow is interfacial tension of oil and water, and dl is the droplet length on gradient surface. When an oil droplet (2 μL) hung on the syringe needle moves to contact the substrate and is further squished slightly, then to move the substrate toward right at a certain speed of ∼10 cm/s. The results indicate that there are three different adhesion and stiction behaviors when the oil droplet in water moving along with the gradient wettability surface from S to L direction (Fig. 3a–c). At the high adhesion force area (S area), when the substrate moves toward right, the oil droplet is deformed severely for the stiction between oil and substrate until the deformation force cannot resist the tensile force of oil droplet, i.e. the adhesion force of oil droplet with syringe needle surface (FOS) is bigger than oil droplet on the nanowire structured substrate surface (FOS < SON), then the oil droplet is easier to be pulled down by the substrate and moves toward right together with the substrate (Fig. 3a). At the middle adhesion force area (Fig. 3b), the oil droplet just deforms when the substrate moves toward right, then recovering the initial shape with further moves of the substrate for FOS ≥ SON. It is not
L direction. 3.3. Dynamic adhesion and motion behavior of underwater oil droplet on the gradient nanowire structured surface To make sure the influence of the gradient nanowire structured surface on motion behavior, the dynamic movement behavior of an oil droplet in water along with the fabricated gradient wettability surface was investigated. For a given oil droplet, gravity (G) and water buoyance Fb are invariable, resulting in that stiction on the nanowire structured film (SON) affects the oil droplet motion behavior by adjusting the different directions on the film [47], because the gradient surface may generate an unbalanced force, FG, which is activated for driving the oil droplet motion, as described in equation (2) [18,37]: L
F=
∫ γOW (cosθL − cosθS ) dl S
(2)
where θL is the oil droplet CA on the long nanowire areas; θS is the oil 250
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Fig. 5. Gradient nanowire mesh surface for efficient oil/water separation. The dynamic images of the selective water permeation and unidirectional motion of oil on the gradient nanowire structured copper mesh, which indicates that oil/water separation with the mass ratio of 3:7 is realized.
from 96 s to 228 s (Fig. S1). The surface wettability of the gradient surface changes correspondingly with the water CAs decrease from ∼130° to ∼12° in the S to L direction. At the area with the water CA of ∼12°, water can permeate the nanowire nanostructured copper mesh. Meantime, the underwater oil CA values change oppositely from ∼138° to ∼162° in the S to L direction (Fig. 4f–i), accompanying with the underwater oil droplet adhesion forces decrease from 18.3 to 0.5 μN, and the oil droplet is easier to move along L to S direction than S to L direction with surface wettability and adhesion gradient (Fig. S2). In addition, the permeation distance of water on the gradient nanowire structured copper meshes with pore sizes varying from 75 to 175 μm was investigated (Fig. 4j). The results show the permeation distance of water on the meshes decreases with increasing pore size from 75 μm to 125 μm, while the permeation distance increases with the increasing pore size from 125 μm to 175 μm. The minimum permeation distance is 55 mm on the nanowire structured surface with the pore size of 125 μm, owing to suitable roughness resulting from the cooperation of microscale pores, copper wires of mesh and the fabricated nanostructures. Meantime, the gradient nanowire structured copper meshes surface kept stable underwater superoleophobic from the pore size increasing from 75 μm to 175 μm, although the slightly decrease of oil CA about 2° at 125 μm, which is consistent with the previous result [48,49].
difficult to speculate the following behavior of oil droplet in low adhesion area in which the oil droplet neither is deformed nor moves toward right together with the substrate owing to FOS > SON (Fig. 3c). Accordingly, the surface shows gradient adhesion on the surface with gradient wettability along with the single direction, which may have an important role in the directional liquid motion. To verify the feasibility of liquid motion unidirectionally on the gradient wettability surface, the sliding behavior of underwater an oil droplet on the tilted gradient nanowire surface with different gradient directions was investigated. When an oil droplet is dropped on the S area of the gradient wettability surface at a tilted angle of 1.5°, it is pinned on the surface owing to the high adhesion and stiction, FG + SON ≥ (G-Fb)sinθ, and even slightly moves to left (Fig. 3d). While along with the opposite direction, i.e., LS direction, an oil droplet will roll off a certain distance of ∼9.6 mm with the force balance of SON ≤ FG + (G-Fb)sinθ, and finally pinned on the S area due to the increasing underwater oil adhesion force along with the direction from L to S (Fig. 3e). The results indicate that the dynamic wetting behaviors of oil droplet (in water) on the surface produce preferred directions of liquid motion, thus the fabricated surface can realize the unidirectional oil droplet motion in water, which may be beneficial to the research and development of the micro-reactors and microfluidic devices.
3.4. Unidirectional permeation of water on the gradient nanowire structured mesh surface
3.5. Oil/water separation on the gradient nanowire structured mesh surface
Based on the unidirectional underwater oil droplets motion on the gradient nanowire surface, the gradient wettability copper mesh fabricated using the anodic oxidation method is expected to realize the unidirectional permeation of water (Fig. 4a–e). The SEM images results show the Cu(OH)2 nanowire gradient increases in density and length along the S to L direction of the surface with the gradient AOT ranging
From the different surface wettability, motion behavior and selective permeation of water on the gradient mesh surface with gradient nanowire, it can be conceived that the gradient wettability copper mesh would be potential for smart interface materials to realize the selective liquid separation. As is shown in Fig. 5, when the oil/water mixture, e.g., liquid paraffin and water mixture with the mass ratio of 3:7, is 251
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added onto the gradient nanowire structured copper mesh, water can move and selectively permeate the mesh film at the hydrophilic side, while the oil droplets move on the mesh to the opposite side on the mesh film and provide more contact area of oil/water mixture for further rapid and continuous separation, which indicates that the oil/ water mixture can be effectively separated with high flux. To quantify the efficiency of oil-water separation, the oil content in the oil/water mixture is measured prior to separation and the result is defined as V0. The oil content after oil/water separation is also measured and defined as V. The efficiency of separating oil can be calculated by formula (3):
efficiency =
V0 − V V0
× 100%
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Meanwhile, other oils such as diesel oil, vegetable oil, heptane, dibromomethane and 1,2-dichloroethane with different interfacial tension from 20 to 40 mN m−1 with water can also be separated with high efficiency more than 98.5%. Therefore, the gradient nanowire structured copper mesh is beneficial to avoid the problem of film pore blocking during the oily water separation process, and is promising in the rapid, continuous oil/water separation with high efficiency owing to the selective permeation of water and unidirectional motion of underwater oil. 4. Conclusion In conclusion, we have presented a strategy to realize the unidirectional oil motion in water and selective permeation of water based on the surface wettability gradient on the nanowire nanostructured copper foil/copper mesh substrate fabricated by anodic oxidation method. The different wettability and adhesion force for oil droplet in water on the two sides of a copper foil show two different oil droplets motion states in water on the nanowire nanostructured substrate, and underwater oil droplets can be transported to the desired direction, which is beneficial to the research and development of micro-reactors and microfluidic devices. More importantly, based on the different gradient wettability change of water and oil droplets in water along the gradient copper mesh, selective permeation of water through the mesh and the oil droplets unidirectional motion can provide more contact area of oil/water mixture for rapid, continuous, high flux oil/water separation can be realized, which is promising to broaden its application on selective separation of different liquids with different interfacial tension in the following work and. This work also provides a new way for development of smart interface materials and micro/nano-fluidic devices. Acknowledgements Y. Y., and L. H. contributed equally to this work. The authors are grateful for financial support from the Chinese National Natural Science Foundation (21671012, 21601013, 21561017), Beijing Natural Science Foundation (2172033, L160003), the Fundamental Research Funds for the Central Universities, Beijing Young Talent Support Program, Beijing Nova Program, and the 111 Project (B14009). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.04.011. References [1] M.J. Liu, S.T. Wang, L. Jiang, Nature-inspired superwettability systems, Nat. Rev. Mater. 2 (2017) 17036. [2] Y. Cui, D.W. Li, H. Bai, Bioinspired smart materials for directional liquid transport, Ind. Eng. Chem. Res. 56 (2017) 4887–4897. [3] J. Ju, Y.M. Zheng, L. Jiang, Bioinspired one-dimensional materials for directional liquid transport, Acc. Chem. Res. 47 (2014) 2342–2352.
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