A bio-inspired method to fabricate the substrate-independent Janus membranes with outstanding floatability for precise oil/water
separation
ZHECUN WANG1,*, JIANLIN YANG1, SHIYU SONG1, XIAOQIU LIU2and SHENGHAI LI3,4
1College of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, People’s Republic of China
2Department of Prosthodontic Dentistry, Hospital of Stomatology, Jilin University, Changchun, Jilin 130021, People’s Republic of China
3Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
4University of Science and Technology of China, Hefei 230026, People’s Republic of China
*Author for correspondence (wangzhecun12@mails.ucas.ac.cn) MS received 10 February 2020; accepted 20 January 2021
Abstract. The lotus leaf has the Janus wettability and outstanding floatability at multiphase interface. The ingenious design on lotus leaf surfaces enlightens us to fabricate the superhydrophobic/superhydrophilic binary cooperative membrane. Making use of the poor permeability of the candle soot, deposition of a layer of candle soot enables to form the Janus membrane with anisotropic wettability on different surfaces. After fixed by the hydrophobic polymer, the super- hydrophobic/superhydrophilic binary cooperative membranes are prepared successfully on various membranes, such as filter paper, cotton fabric and copper mesh. The Janus membrane exhibits excellent interfacial floatability at the immiscible oil–water interface as a separator, thus affording high-performance immiscible oil/water separation without applied external pressure.
Keywords. Lotus leaf; Janus membrane; floatability; oil/water separation.
1. Introduction
The water repellency of lotus leaves has attracted immense scientific interest in fundamental, biomimetic research [1–6], and wettability model investigation [7–10] recently. The two- level rough microstructures on a waxy epicuticula surface makes the water contact angle (WCA) greater than 150°and the sliding angle (SA) less than 5°for the upper surface of the lotus leaves [11–15]. On the basis of surface property of lotus leaf, diverse superhydrophobic substrates have been suc- cessfully fabricated to apply in antiwetting surfaces [16–19], oil-spill cleanup [20–24] and corrosion prevention [25–29].
Up to now, only the superhydrophobic property of the upper surface has been explored in detail; however, the property of the lower surface is often neglected. Disparate from the upper surface, the lower surface of the lotus leaf shows superhy- drophilic in air and underwater-superoleophobic, exhibiting a Janus property [30,31]. Thanks to the Janus interfacial char- acteristic of lotus leaf, the cooperative effect of water affinity of superhydrophilic surface and water repellency of
superhydrophobic surface results in the lotus leaf attaching itself to and meanwhile keeping itself floating on the air–
water interface naturally. However, the Janus wettability and stable floating property on multiphase interface of the lotus leaf are important for further research, such as, searching the practical applications in real world is still in great challenge.
Nature always enlightens us with infinite wisdom to create various asymmetric microstructure or gradient wet- tability fibres and open surfaces in directional liquids motion [32–35], water harvesting [36–38] and repelling water [39]. Undoubtedly, to explore the ingenious super- hydrophobic/superhydrophilic integrated design on lotus leaf should be valuable and beneficial. Recently, the binary cooperative membranes with Janus wettability, hydrophobic for one surface and hydrophilic for the other, have been explored extensively in directional liquids transportation [40–46], switchable permeation [47] and oil/water separa- tion [48–50], etc. However, such applications require con- quering a physical barrier for rapid separation. The physical barrier is relevant to pore diameter and thickness of the
Electronic supplementary material: The online version of this articlehttps://doi.org/10.1007/s12034-021-02405-6contains supplemen- tarymaterial, which is available to authorized users.
https://doi.org/10.1007/s12034-021-02405-6
advantage of their asymmetric wettability, the Janus sheet, similar with the lotus leaf, exhibits improved interfacial floatability, which not only floats on but also attaches to different oil–water interface naturally. The Janus membrane floats at the oil–water interface that serves as the separator (gate) in the interface, here it can prevent direct contact between oil and water. This novel function may bring new ideas for superior separation performance.
Herein, the lotus leaf with Janus wettability and excellent floatability enlightens us to fabricate a novel asymmetric wettability Janus membrane that integrates superhy- drophobic and superhydrophilic wettability in a membrane.
Taking advantage of the poor permeability of the candle soot, the Janus membranes with superhydrophobic and superhydrophilic wettability at different surfaces can be fabricated on various substrates, such as filter paper, cotton fabric and copper mesh. Similar with the lotus leaf, the Janus membrane has the outstanding floatability at multi- phase interfaces. Due to the excellent floatability, the Janus membrane could act as a separator at the oil–water interface to separate different oil/water mixtures precisely, including low-density and high-density oil.
2. Experimental 2.1 Materials
Candle soot was purchased from the market. The poly(vinylidene fluoride-co-hexafluoro propylene; PVDF- HFP), Methyl Purple, and Oil Red were purchased from Sigma. The filter paper, cotton fabric and copper mesh (mesh number: 40) were from Beijing North Dawn Mem- brane Separation Technology Corporation. All other sol- vents were in chemically pure grade and used as purchased unless otherwise noted.
2.2 Fabrication of the Janus membranes
The poor permeability of the nanoparticle in comparison with the UV, vapour, and liquids induces it attaching to one surface of the membrane without penetration. Therefore,
drophobic surface is fragile because the particle–particle interactions are only physical and weak. When water rolls off the surface, the drop carries soot particles with it until almost all of the soot deposit is removed and the drop undergoes a wetting transition (supplementary figure S1).
There are two routes for applied hydrophobic polymer to fix the easily damaged candle soot: spraying the polymer solution (1% PVDF-HFP in DMF) to the membrane directly (route 1) and wetted the superhydrophilic membrane by water/ethanol mixture (50/50, v/v) and then spraying the polymer solution (1% PVDF-HFP in DMF) (route 2). The superhydrophobic surface (route 2) possesses a WCA of 156°, while the WCA is about 154° after a finger test, indicating the surface is stable after the fixation process (supplementary figure S2). The ingenious method could be used on various substrates, including cotton fabric, filter paper and copper mesh.
2.3 Characterization
The morphology of the soot particles and the coating were characterized by scanning electron microscope (SEM, Phi- lips XL30 ESEM FEG). Surface chemical characterizations were carried out by X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 280 system with Al/KR radiation as the X-ray source. The contact angles of as- prepared samples were measured by Drop Shape Analysis DSA10 (Kru¨ss Gmbh, Germany) at ambient temperature.
3. Results and discussion 3.1 Lotus leaf
In air system, a lotus leaf exhibits significant Janus wetta- bility between its two surfaces, i.e., one surface shows the superhydrophobic property with a WCA above 150°and the back surface is hydrophilic with a WCA of 18°(figure1a).
This Janus feature of the lotus leaf works together and keeps the lotus leaf floating on the water (figure 1a and b). Fig- ure 1a shows a representative picture of the lotus leaf floating on the water surface. The water droplets retain a
spherical shape on its upper side (dyed by Methyl Purple), and the oil droplets (hexane, dyed by Oil Red) can stay on its lower side in water in the shape of spheres, while the two surface is superoleophilic with an oil (hexane) contact angle of 0° in air (figure 1a). Under the influence of the Janus wettability, the lotus leaf has superior and unique stability at multiphase interfaces (figure 1b–e). Rocking the surface, the lotus leaf could float on the oil (hexane or toluene)–
water interface naturally with the hydrophilic surface downward and it also enables to float on the water–oil (CCl4) interface with the hydrophilic surface upward. In all, the lotus leaf with Janus wettability owns incomparable floating ability.
3.2 WCA, morphology and XPS of the membranes
As can be seen in figure 2, the as-prepared composite membranes exhibit significant wettability difference between its two surfaces. The polymer fixed candle soot surfaces have a WCA above 150°and a sliding angle below 8° for diverse Janus membranes (figure 2a), but the other surfaces of different routes have different WCA (figure 2b and c). Directly spraying the polymer solution results in the
hydrophobic property for other surfaces, 112° for paper, 122°for cotton fabric and 124°for copper mesh (figure2c), indicating that the route 1 forms the superhydrophobic/
hydrophobic wettability Janus membranes. Note that the superhydrophilic surface is pre-wetted by water/ethanol mixture in route 2, and then spraying the polymer solution to the candle soot surface. When the dilute PVDF-HFP solution comes into contact with water/ethanol mixture at the interface, it will be rapidly phase inversion to solidifi- cation and fix it on the interface. After drying in the oven at 120°C for 30 min, the surfaces keep superhydrophilic and under-water superoleophobic (figure 2b). Therefore, the Janus membranes with superhydrophobic/superhydrophilic wettability have been fabricated by route 2.
The SEM images and digital photos verify that the sur- face morphologies of the two surfaces of the membranes have remarkable differences. After deposition of the soot layer, all the membranes’ one surface turn black (figure3a0– c0), while the other surfaces are same with the original appearance without changeable (figure3a and b). Revealed from the SEM images, as shown in figure3a and b (candle soot), the surface is smooth and flat without changeable. It is surprise that the copper mesh’s superhydrophilic surface is fully covered by a uniform oxide layer with roughness Scheme 1. Schematic illustrating the procedure of fabricating different Janus interfacial materials.
Figure 1. (a) Digital photographs of a lotus leaf floating on the water surface showing different wettability for water and oil droplets at air–water system, and the WCA and oil contact angle of different surfaces at air system are also listed in the picture. The outstanding floatability property of the lotus leaf at multiphase interfaces: (b) air–water interface, (c) hexane–water interface: (d) toluene–water interface, (e) water–CCl4interface.
wrinkles after calcination in the deposition process (fig- ure3c, candle soot). The colour of the surface is darkened in comparison with the original copper mesh (figure 3c).
However, SEM reveals that the black surface, deposition by candle soot, consists of carbon particles with a typical diameter of 30–40 nm, forming closely packed, fractal-like networks (figure3a0, b0and c0, candle soot process), which is similar to the published report [18]. Through the fixing step, the colour of diverse membranes’ different surface is same with the candle soot process for route 1 and route 2, but the superhydrophobic surface’s roughness changed (figure3). Fixed by the polymer solution via different route (route 1 and route 2), the hydrophobic surface for route 1
and superhydrophilic surface for route 2 are flat and smooth similar with the candle soot process (figure3a and b). Fixed by the polymer, the candle soot is agglomerated together with some holes forming the irregular roughness for the superhydrophobic surface of different routes, where the air likely entrapped in the holes space below the water droplets reduces the surface contact area forming the surperhy- drophobic surface.
To further prove different Janus membranes, we used the XPS to examine it. Two peaks at 285 and 689 eV are labelled, representing C1s and F1s, respectively. The candle soot, incomplete combustion, forms short carbon chains, inducing more carbon contents (C1s) for the of route 1’s hydrophobic surface.
Figure 3. The morphology and digital pictures of different membranes’ different surface: without deposition candle soot surface [(a) filter paper, (b) cotton fabric, (c) copper mesh] and deposition candle soot surface [(a0) filter paper, (b0) cotton fabric, (c0) copper mesh].
superhydrophobic surface than the other surface (figure 4a and b). Directly spraying the hydrophobic polymer solution (route 1), as shown by figure4c and d, the two surfaces have the F content (F1s). However, the F content for superhy- drophobic surface is larger than hydrophobic surface. As to the Janus membrane by route 2, the superhydrophobic surface has the F content, but it disappears in the superhy- drophilic surface.
3.3 The floatability of the Janus membrane at multiphase interfaces
Similar to the lotus leaf, the Janus membrane has extraor- dinary interfacial floating stability arising from the Janus wettability. However, the floating stability is unlike the reported floatable of hydrophobic and superhydrophilic membranes. The superhydrophilic membrane contacted the hexane (or toluene) and passed through the hexane (or toluene)/water interface, sinking at the bottom of water, but it was wetted by water forming underwater-superoleophobic and floated at the water/CCl4 interface (figure 5a). The hydrophobic membrane first contacted with hexane (or toluene) phase, and it was able to stay at the hexane/water interface after rocking the water (figure 5b). Additionally, the hydrophobic membrane floated at the water surface of water–CCl4multiphase system all the time. A Janus mem- brane put into the hexane (toluene)–water solution, the superhydrophobic surface could effectively capture a layer of hexane (toluene), which cannot be infused by water.
However, the organic liquid adhering to the superhy- drophilic surface would be replaced by water after entering the middle water phase after rocking syringe, resulting in the floating at the hexane (toluene)–water interface (fig- ure5c). In addition, the Janus membrane could also float at
the water–CCl4interface after violent rocking. Additionally, different from superhydrophilic and hydrophobic mem- branes, the Janus membrane not only floats but also sta- bles at the interface.
3.4 Oil/water separation of the Janus membrane
The Janus porous membrane with superior interfacial floatability can be developed to a movable gate for ultrafast immiscible oil/water separation. The Janus membrane can accommodate the superhydrophilic and superhydrophobic wettability into one system, inducing it is co-infused of oil and water within superhydrophilic and superhydrophobic surfaces forming the liquids infusion interface that blocks the immiscible organic solvents and water. Moreover, as shown in figure6a and c, the water and oil co-infused Janus porous membrane can be fixed to float at the immiscible oil–water interface as a separator, thus affording high-per- formance immiscible oil/water separation without applied external pressure. We used a glass tube (diameter: 4 cm) with a small hole (diameter: 0.5 cm) at one end to separate different oil/water mixtures. As in figure 6b, a pierce of hexane and water co-filled Janus membrane (diameter:
3 cm) put into the device, and it would float at the hexane–
water (100–100 ml) interface. Pulling out the plug to start the separation, the water would be separated from the mixture quickly, and the water in superhydrophilic surface of the Janus membrane could block the hole and the upper Figure 4. The XPS of the filter paper Janus membrane’s
different surfaces from different fabrication process: (a) superhy- drophobic and (b) superhydrophilic surfaces of candle soot;
(c) superhydrophobic and (d) hydrophobic surfaces of route 1;
(e) superhydrophobic and (f) superhydrophilic surfaces of route 2.
Figure 5. (a) The state of superhydrophilic filter paper at different oil–water system: immersion in hexane–water or toluene–water system, floating at the water–CCl4 interface.
(b) The state of hydrophobic PP membrane at different oil–water system: floating at hexane–water or toluene–water interface, floating on the surface of water at water–CCl4 system. (c) The state of filter paper Janus membrane: fixed floating at the hexane–
water, toluene–water and water–CCl4interface.
layer oil perfectly to stop the process at the end (figure 6b and supplementary Video S1). The separation flux is about 80,000 l (m-2h-1), which is far greater than the flux by traditional filtration method (figure 6e). The separation efficiency is high, nearly no visible oil existed in the per- meated water, and the oil content (hexane) in the filtrates after one-time separation was about 65 ppm (figure 6f). In addition, the Janus membrane can ultrafast separate differ- ent immiscible low-density oil/water mixtures, including petroleum ether–water and soybean oil–water with high efficiency (figure6e). As for the water–CCl4co-filled Janus membrane, it enables to separate the CCl4 from the water with a flux of about 80,000 l (m-2h-1) (figure 6d and e, supplementary Video S2). The separation is also with high efficiency, where the water content in the filtrates after one- time separation is below 90 ppm (figure6f).
4. Conclusion
The lotus leaf has the Janus wettability on different sur- faces. Thanks to the Janus wettability, the lotus leaf floats at multiphase interfaces with high stability. Learning from the lotus leaf and taking advantage of the poor permeability of
the candle soot, coating a layer of candle soot on one sur- face of the membrane forms the Janus membrane. Fixed by the hydrophobic polymer, the Janus membrane with superhydrophilic/superhydrophobic wettability on different surfaces is fabricated successfully. Due to the poor perme- ability of the candle soot, various membranes could be fabricated to the Janus membranes. Similar with the lotus leaf, the Janus membrane not only floats at multiphase interfaces, but also attaches to the interface, showing out- standing interfacial floatability. The water and oil co-in- fused Janus membrane can be fixed to float at the immiscible oil–water interface like a separator, thus it enables to ultrafast separate different immiscible oil/water mixtures where the separation flux is up to 80,000 l (m-2h-1), which is far greater than the flux by traditional filtration method.
Acknowledgements
This work was supported by the Scientific Research Funding Project of the Education Department of Liaoning Province (LJ2020QNL002), the Development of Scientific and Technological Project of the Jilin Province (No.
Figure 6. (a) Schematic of Janus membrane separating the water from the low-density oil at the immiscible low-density oil and water system. (b) The filter paper Janus membrane separates the water from immiscible hexane–water system. (c) The schematic of Janus membrane separating the high-density oil from the water at the immiscible water–high-density oil system. (d) The filter paper Janus membrane separates the CCl4from immiscible water–CCl4system. (e) The separation flux of different immiscible oil–water system.
(f) Oil or water contents in different system after separation.
20160519017JH), and Chinese Academy of Sciences—
Wego Group High-Tech Research & Development Program.
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