Janus Gradient Meshes for Continuous Separation and Collection of

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Janus Gradient Meshes for Continuous Separation and Collection of Flowing Oils under Water Ning Li,†,‡ Cunlong Yu,† Yifan Si,† Meirong Song,§ Zhichao Dong,*,†,‡ and Lei Jiang†,‡ †

Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China ‡ CAS Key Laboratory of Bio-inspired Materials and Interfacial Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § College of Sciences, Henan Agricultural University, Zhengzhou, Henan 450001, P. R. China S Supporting Information *

ABSTRACT: Gradient meshes with Janus wettabilities are fabricated to stably separate and collect spilled oils from a range of flowing oily wastewater. Here, we demonstrate an overflow with separation methodology, which combines selective oil overflow and membrane separation, to separate low content oils from dynamic flowing oil− water mixtures by a curved gradient mesh that covered on a solid edge. The microscaled air−oil−water−solid four-phase wetting state during the oil−water separation process is visualized and demonstrated. The fundamental understanding of this overflow with separation system and the superior gradient mesh materials would enable us to construct a wide variety of separation devices out of traditional designs and advance related applications, such as wastewater treatment and fuel purification. KEYWORDS: superoleophobicity, oil−water separation, overflow, Janus, hydrodynamic

1. INTRODUCTION

water mixtures by utilizing topographic and wettability gradients. Noticeably, every year, more than one-tenth of the world’s total energy consumption is used for separating mixtures into pure forms.30 To reduce energy consumption, mesh materials with particular wettabilities are recognized as promising materials in oil−water separation,2 where selective absorption of oil (or water) with complete water (or oil) repelling has been achieved by the decorating of particular chemical compositions and micro/nanostructured morphologies.31−33 Gravity separation is a typical separation method, and mesh materials including polymer textiles, surface-modified membranes, and hydrogel-coated meshes are used. However, in practice, industrial wastewater is commonly in a flowing state, and the oil content in the wastewater is typically low.34 The buoyancy of oil droplets would force them to flow upward to the top layer of the wastewater. It is therefore desired to directly draw oils from flowing wastewater in a dynamic buoyancy driven method and simultaneously collect separated oil for recycling. Considering the advantage of gradient surface and the urgency of novel designed method to separate oils from flowing wastewater, here we demonstrate a gradient fabrication strategy

Gradient surface, a typical feature of natural creatures, endows biological systems with multiple functions, such as biomolecular interactions, oil collection, cell motility, and predation.1−5 The ability to generate controllable gradient is believed as a royal road to understand the biological systems.5−7 Learning from natural creatures, biomimetic surfaces with physical and chemical properties changing continuously along the materials have been prepared and used to control liquid’s motions.8−10 Wettability gradient, structure gradient, temperature gradient, and even electrostatic gradient have been devised to control liquid motion behavior.9−19 Among these researches, the combination of wettability and topographic gradients, one of the most important behaviors in nature, is of greatest interest to researchers.20−22 Such a kind of combination results from the combination of two or three of the surface morphology gradients, surface chemical gradient, and the topographic gradient and allows liquid’s directional movement; examples include directional transportation of the condensate fog on a gradient cactus needle in a controlled way, unidirectional spreading property of the depositing liquid on the gradient peristome surface of the pitcher plant, and directed bouncing of impacting drop on the gradient butterfly wings.23−29 These biological examples encourage us to attempt to address the next challengethe rapid transportation oil phase from flowing oil− © XXXX American Chemical Society

Received: January 3, 2018 Accepted: February 13, 2018

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DOI: 10.1021/acsami.8b00044 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Flame deposition process for the fabrication of gradient meshes. (a) Diagram of the experimental device. Stainless steel meshes were hung above the flame of paraffin fixed onto a lifting platform. The distance between the mesh and the flame is adjusted by the flame height. The moving speed of the paraffin is controlled by the mobile platform. Gradient surface morphology formed on meshes after tetraethoxysilane (TES) vapor deposition and calcination process. (b, c) Image sequences of the flame deposition process recorded by a high-speed camera. (d, e) Infrared thermography sequences of the flame deposition process recorded by an infrared camera. (b, d) For mesh with large pores sizes, fire flame can overflow wires and penetrate through the mesh. (c, e) For mesh with small pores sizes, fire flame cannot penetrate through the mesh. Soot layer only deposits on the fire-exposed side of the mesh.

reaction, silica formed on the soot template by hydrolysis of the condensate TES that catalyzed by ammonia. After calcination at 600 °C, the gradient mesh surface is achieved (Figure 2c).35 Figure 2d,e shows the scanning electron microscopy (SEM) images of the as-prepared mesh surface in a top-sectional view, showing the silica coating covered on the fire deposited side (Figure 2d,e). Subsequent fluorination of the as-prepared gradient mesh led to the gradient surface with a high water repellency. Surface wettability is characterized by the oil contact angle (OCA) at either side of the mesh comprehensively. Stereoscopic microscope image in Figure 2f shows that liquid droplets deposited on the fire-exposed mesh surface keep sphere shapes. Beside surface superhydrophobicity with a contact angle (CA) of 160.5 ± 2.9°, in air superoleophobicity is also achieved on the fire-exposed side, and the OCAs of which are 153.8 ± 2.6° for olive oil, 153.6 ± 1.6° for hexadecane, 151.6 ± 3.3° for dodecane, and 150.1 ± 4.2° for gasoline (Figure 2f). In contrast to the surface wetting properties in air, when immersing the mesh into water, underwater oleophilicity is measured with an average underwater OCA of 73.8° (Figure 2g). Although oil could repel the air layer and wet the micropores of the mesh, dry mesh is still achieved when pulling the mesh out of the oily water, indicating that air layer is stable entrapped in the nanostructures of the gradient mesh. This kind of wetting property is quite important for the directional oil penetration and will be discussed below. When applying an alternating current on the mesh, the fluorosilanes that grafted on the bald steel side can be decomposed. Soon after the electrical corrosion process, the OCA on the unexposed mesh surface changes from above 90° to an average value of below 10° (Figure 2f), and the underwater OCA of the unexposed surface is above 160° (Figure 2g). In order to confirm that superoleophobicity still exists on the fire-exposed surface after electrochemical corrosion, a 100 nL olive oil droplet is dipped onto the surface

to introduce gradient nanodendritic silica onto the steel mesh surface. The fabricated surface can rapidly establish both wettability and topographic gradients. This kind of mesh can be coated onto the curved tank, where flowing oil phase overflows the curved mesh and is selectively separated from the mixture without the need of energy input. This overflow with separation device can treat a large volume of oil/water mixture in a continuous process. Through laser confocal microscopy, for the first time, the microscaled air−oil−water−solid four-phase wetting state during the oil−water separation process is visualized and demonstrated. A fundamental understanding of this overflow with separation system and the superior gradient mesh materials would enable us to construct a wide variety of separation devices out of traditional designs.

2. RESULTS AND DISCUSSION 2.1. Fabrication of the Gradient Mesh Surfaces. Figure 1a shows the schematic diagram of the fabrication process. The flame-deposition method is used for the fabircation of gradient mesh surface. During the flame deposition process, there is a threshold mesh size for the fabrication of the gradient mesh surfaces. To find the threshold value, in the experiment, meshes with sizes range from 50 to 400 are used. As shown in Figure 1b, when the mesh size is 50, fire flame can penetrate through mesh holes during the flame deposition process (Figure S1). Infrared camera records the deposition process, where hot temperatures are detected above and below the mesh (Figure 1d). In contrast, a cool temperature still holds above the mesh with a mesh size ranges between 100 and 400 (Figure 1c,e). Even the flame is in a moving state, the soot can still be deposited onto the mesh surface properly (Figure S2), which ensures the uniform formation of gradient soot layer on the mesh surface. After the flame deposition process, a uniform soot layer is covered on the fire-exposed side but is rarely deposited on the unexposed mesh surface (Figure 2a,b). According to the Stöber B


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Figure 2. Gradient morphology and Janus wetting property of the mesh surface. (a, b) Optical images of the mesh at fire-exposed side and unexposed side. A dark surface is achieved on the fire-exposed side, while a light-colored surface is achieved on the unexposed side. (c) Cross-section view of stainless steel meshes after the flame deposition process; a soot layer is deposited on the fire-exposed surface but rarely deposited on the unexposed surface. (d, e) Top views scanning electron microscope (SEM) images of the gradient mesh after calcination. (f) In air superoleophobicity of the fire-exposed mesh surface and oleophilicity of the unexposed surface. (inset) From left to right: 2 μL of olive oil, n-hexadecane, n-dodecane, and gasoline, respectively. The gradient mesh shows superoleophobicity on the fire-exposed surface and superoleophilicity on the unexposed surface. (g) Underwater oleophilicity of the fire-exposed surface and superoleophobicity of the unexposed surface.

by a superoleophobic nozzle.36,37 The oil drop keeps a sphere shape with a CA larger than 150° (Figure S3). The antiabrasion ability is crucial for practical applications. We also perform sandpaper abrasion test to measure the antiabrasion ability. Meshes with pore sizes ranging from 50 to 400 are used. The relationship between mesh size and pore size is shown in Figure S4. After 20 measurements, the surface superoleophobicity and superhydrophobicity still exist on the fire-exposed side (Figure S5). Stable Janus mesh with gradient surface wettability is thus successfully fabricated. 2.2. Liquid Gating on the Gradient Mesh Surfaces. In recent years, since increasingly environmental recognition of the worldwide oil pollution, the treatment of oily polluted water, caused by the industries as well as the frequent oil-spill accidents, has become ever increasingly urgent to be solved.38 A facile method is greatly needed to effectively collect and remove these organic pollutants with reduced energy consumption.10,39 Considering the density of most nonpolar oil contaminants is lower than that of water,40 the ideal separation method, buoyancy driven method, is expected to directly “draw” oil out of the oil−water mixture, rather than the gravity driven method. In this part, a series of proof-of-concept studies are then carried out to test the liquid gating capability of gradient mesh.

High-speed images are used to demonstrate the buoyancy driven method with oil-gating behavior at the gradient mesh surface in real time. Figure 3a shows high-speed images of the oil directional penetration behavior at the air−solid−water interface, where the gradient mesh floats on the water surface with a 10 μL n-hexadecane droplet contacting the fire-exposed mesh below. When the underwater oil drop contacts the mesh, the droplet first shows a hemisphere shape below the mesh, then rapidly penetrates through the mesh, and finally spreads on the other side of the mesh. The total time for the oil penetration is within 100 ms (Figure 3a). In contrast, as shown in Figure 3c, when the mesh is turned over, the oil droplet is blocked and keeps a sphere shape on the unexposed side. Besides oil droplets, the gradient mesh also shows directional gating property for continuous oil flow (Figure 3b,d). A highspeed oily water flow can be separated within 10 s, and the flux of the gradient with 100 mesh size is above 25 000 L m−2 h for hexadecane−water mixture. The flux decreases as the increase of the mesh size, but the minimum flux is still above 5000 L m−2 h for the gradient mesh with 400 mesh size. These observations clearly show that oil can directionally transport from the fire-exposed side to the unexposed side of the gradient mesh, indicating a potential material for the dynamic oil−water separation. C


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Figure 4. Directional oil penetration at air−mesh−water interface. (a) Confocal microscopy images with top cross-section and side crosssection views of the double size exposed superoleophobic mesh floating on the water surface with a 10 μL oil drop below. No oil penetrates through the mesh. (b) The gradient mesh at the air−oil− mesh−water interface with top cross-section and side cross-section views, where oil passes through the gradient mesh. (c) Schematic diagrams demonstrate that oil rapidly penetrates through the mesh and spreads on the other side of the mesh, when floating the gradient mesh with fire-exposed side (FE) down. Increasing the immersion time, the wetting state of oil can be transferred from state S1 to S3. (d, e) Confocal microscopy images confirm the wetting state of the mesh when floating on the water surface and floating the mesh on the water surface with a 10 μL oil drop below, respectively. A thin layer of water covered on the bottom side of the mesh prevents oil from wetting or penetration across the mesh. (f) Schematic diagrams demonstrate that oil phase is repelled from wetting the micro/nanostructured mesh by a thin layer of water, when floating the gradient mesh with unexposed side (UE) down. Fluorescent emission from water and oil is shown in blue and green, respectively.

Figure 3. Directional oil penetration in an air−mesh−water system. (a) Gradient mesh with Janus wettabilities allows penetration of underwater n-hexadecane droplet when the fire-exposed side (FE) is toward water. (c) When the unexposed side (UE) of the mesh is toward water, the mesh prevents the n-hexadecane drop’s penetration. (b) Meanwhile, oils were ejected through the underwater syringe’s nozzle by a pump and penetrated across the mesh from fire-exposed side (b) to unexposed side (a). The oil/water separation speed is quite fast, and a height of 10 cm oil can be acquired within 15 s (the ejection speed is 2.0 mL/s). (d) Gradient prevents underwater oil flow’s penetration across the mesh when the unexposed side is toward water. A height of 10 cm oil can be held below the gradient mesh.

To explain the detail mechanism, microscaled air−oil− water−solid four-phase wetting state during the oil−water separation process is visualized and demonstrated for the first time through laser confocal microscopy. Figures 4a and 4b compare the final states of the penetrating oil on the double treated superoleophobic mesh (Figure 4a) and the as-prepared gradient mesh surface (Figure 4b). For the gradient mesh, as shown in Figure 4b, oil covers on the whole mesh, indicating that oil can penetrate the mesh. In contrast, without surface gradient, oil cannot penetrate through a double fire-exposed mesh surface with convex heads pining at the pores of the gradient mesh (Figure 4a). The morphology gradient and the wettability gradient are therefore both important in controlling oil penetration behavior. During the oil penetration, a large Young−Laplace pressure, Pp ∼ 4γoil cos θmicro/Dpore, exists on the fire-exposed mesh surface, and a buoyancy force, Fb ∼ (ρwater − ρoil)gV, acts on the oil drop.41 Here, γoil is the oil surface tension, θmicro is the contact angle, Dpore is the mesh pore length, V is the oil volume, and ρwater, ρoil are the water and oil densities, respectively (Figure 4c). We need to mention that θmicro is the microscopic contact angle between oil and silica covered pore rather than the apparent contact angle, as θ(h) shown in Figure 4c, right. In

detail, θmicro is larger than 150° when using a confocal microscopy to analyze the wetting state of the mesh in a cross section, although the apparent contact angle θ is 71.1 ± 2.4°. Thus, the driving force for oil penetration is quite large. In addition, the gradient mesh can withstand the penetrated oil on the mesh surface with a maximum pressure of 2000, 5000, and 7800 N/m2 for 100, 200, and 300 mesh sizes, respectively (Figure S6). In contrast, no oil can penetrate the gradient mesh when the unexposed surface is toward water, where no liquid is observed at the top surface (Figure 4d). As demonstrated in Figures 2g and 2h, similar wettability exists on either side of the mesh, i.e., in air superoleophobicity on the fire-exposed surface and underwater superoleophobicity on the unexposed surface. As oil droplet contacts the unexposed surface, a negligible Laplace pressure, which is close to zero, acts on the mesh surface (Figure 4f). The buoyancy force is thus the only driving force for oil penetration. However, as shown in the confocal microscopy images in Figure 4e, a thin water film is adsorbed D


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Figure 5. Oil overflow and absorption at a gradient mesh covered solid edge. (a) Schematic of flow dynamics of underwater−oil/water mixture at superoleophobic round edge. (b) Time sequences of underwater−oil/water mixture (Voil/Vwater = 1:3) at round edge. Red dyed oils flow upward along the edge and gather on the top surface while blue dyed water separates from curved edge into multi-streamlines. (c) Schematic diagram of the experimental setup. A gradient mesh (300 mesh) is held vertically to the glass plate and mounted in the channel with the fire-exposed side outside. (d) Optical images depicting experimental setup in side and top views. (e) Schematic diagrams and time sequence images of the overflow and collection ability. Within 180 s, a full tank of 110 mL of dyed n-hexadecane can be separated and collected from a 220 mL oil−water mixture (Voil/ Vwater = 1:1). (f) The diagram of separation and collection abilities of the device for n-hexadecane/water mixtures at various concentrations and injection flow velocities. (g−j) Besides n-hexadecane, dyed n-dodecane, gasoline, and olive oil can be separated from the oil/water mixture and collected in the tank.

upward curved superoleophobic surface in a streamline. The path of separated oil streamline is like a parabolic curve. In contrast, for lyophilic solid edge, liquid flow has the tendency to overflow the solid edge and, then, flows in the opposite direction. Different from the surface wettability shown in air, when immersing the plate into water, the in air superoleophobic solid edge shows underwater oleophilicity. In comparsion to the flow dynamics in air, the path of the underwater ejected oil is instead along the underwater solid edge without separation (Figure S7b). We are just curious about what phenomena will occur when an oil−water mixture flows along the above-mentioned solid edge underwater. An experiment is performed. Figure 5a shows the schematic cartoon of the flow behavior of water−oil mixture on the superoleophobic curved edge. Figure 5b shows the experimental results, where oil phase and water phase of the mixture could flow in different directions. Time sequences show that red dyed oils round the curved edge, and flow back along the top surface, while blue dyed water could continue its path away from the solid surface with multiple streamlines. Increasing the injection time, more oil is gathered on the top surface. As a hypothesis, as the schematic demonstrates in Figure 5c, if we cover a hollow solid edge with the Janus mesh, which allows oil directional penetration, but forbids water’s penetration, a novel overflow with separation apparatus would

on the unexposed superhydrophilic side and acts as an energy barrier for oil’s penetration. This is the first time for us to directly observe the microcontacts among solid, air, water, and oil in a four-phase wetting system. Since this thin water layer, the act of buoyancy is limited, and as a result, the unexposed mesh surface forbids the further transport of the oil droplet. 2.3. Continuous Separation and Collection of Flowing Oils under Water. In practice, industrial wastewater is commonly in a flowing state, and the oil content in the wastewater is typically low.34 The buoyancy of oil droplets would force them to flow upward to the top layer of the wastewater. It is therefore desired to directly draw oils from flowing wastewater in a dynamic buoyancy driven method and simultaneously collect separated oil for recycling. Superwettability controlled overflow systems,42 such as driven thin films or direct liquid flows on curved edges,43 constitute attractive alternatives owing to their advantages in manipulating flow hydrodynamics. However, no overflow mechanism to direct underwater−oil dynamic flow that leads to controlled absorption of oil and blockage of water has yet been devised. Liquid trickling along a teapot’s spot, commonly named as “teapot effect”, is a kind of overflow behavior.44 Superliquidrepellent material has the ability to reduce overflow.42 Just as shown in Figure S7a, an oil flow, which is ejected from a nozzle that mounted under the plate, tends to separate from the E


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treatment, oil spill cleanup, emulsions separation, and oil recovery.

be devised to continuously separate and collect spilled oil from oil contained wastewater. Having shown the working principle for oil directional penetration, here we investigate the overflow with separation ability in a dynamic state, using dyed n-hexadecane as the model oil. The overflow with separation apparatus consists of a gradient mesh, which is covered on a hollow and curve-edged plate with a radius of curvature of 2.5 cm (Figure 5d) and a nozzle that is fixed horizontally below the bottom surface of the plate. A duct is mounted on the top surface of the plate and contacts with the atmosphere to balance the pressure during the overflow with separation process. The as-prepared plate is then immersed into water at a depth of 10.0 cm with the fireexposed side of the gradient mesh toward the water phase. A gear pump is utilized to mix the oil/water mixture. Oil/water mixture flow is subsequently ejected from the nozzle at a linear flow velocity of v and flows along the bottom surface before encountering the gradient mesh surface at the plate’s solid edge. Figure 5e shows the continuous separation and collection ability of the under water flowing oils on the gradient mesh surfaces. As the ejected n-hexadecane/water mixture encountering the gradient mesh at the solid’s curved edge, it flows upward along the edge (Figure 5e1). In the cross section, dyed n-hexadecane penetrates through the mesh surface rapidly while water is blocked outside. Inside the plate, the penetrated nhexadecane flows downward along the inner curved mesh (Figure 5e2). Oil flow convections exist on the either side of the mesh during the overflow with separation process (Figure 5e3). The convection increases the separation efficiency. Finally, oil fills the tank, where 110 mL of hexadecane is separated from a 220 mL mixture solution. The whole separation duration is only 180 s. Overflow with separation device can treat a large volume of n-hexadecane/water mixture at various concentrations and injection flow velocities in a continuous process, as shown in Figure 5f. Threshold values exist in the diagram. For a low flow velocity and a large water concentration, the ejected oil can be directly absorbed into the gradient mesh at the bottom side without overflow behavior. Whereas at high flow speed, concentrated hexadecane can overflow the solid edge with partial liquid penetration across the gradient. Besides nhexadecane, in order to emulate the oil−water separation in practical applications, other three kinds of dyed oils, including n-dodecane, gasoline, and olive oil, are used (Figure 5g−j). Significantly, nearly no visible oil exists in the water tank when using laser scanning to examine the residual oil in the water tank. Furthermore, chromatography−mass spectrometry and micro-MR demonstrate that the water content in the separated oil is lower than 0.5%. Therefore, the separation efficiencies are both above 99.5% for these two oil−water mixtures (Figures S8 and S9).

4. EXPERIMENTAL SECTION 4.1. Fabrication of Janus Gradient Meshes. Stainless steel meshes with mesh sizes ranging from 100 to 400 were held horizontally above the flame of paraffin. A 2D mobile platform (Zolix SC300-3A motion controller) was utilized to control the moving speed of the paraffin in order to achieve large scale modified meshes with uniform depositions. The soot coated meshes were placed in a vacuum dryer together with two open small beaker containing 1 mL of tetraethoxysilane (TES) and aqueous ammonia solution, respectively. Then chemical vapor deposition (CVD) of TES was carried out for 48 h. The gradient mesh was then annealed at 600 °C for 4 h. The as-prepared gradient mesh was treated by O2 plasma at 150 W for 10 min and immersed into (heptadecafluoro-1,1,2,2tetradecyl)trimethoxysilane solution in hexane (1 mg/mL) for 2 h. After the electrochemical corrosion, the fluorination on the unexposed side mesh surface is damaged, leading to the superhydrophilicity on the unexposed side, while superoleophobicity still exists on the fireexposed surface. 4.2. Characterization. High-speed movies and infrared thermography images were taken with a high-speed camera (Fastcam Mini UX100, Japan) and infrared camera (FLIR A655sc, America). Scanning electron microscope (SEM) images and transmission electron microscope (TEM) images were captured by a field-emission scanning electron microscope (Hitachi S-4800) and transmission electron microscope (JEM-1011F). The element ratio of silicon (Si) and ferrum (Fe) along the mesh was detected by SEM energydispersive X-rays. Contact angles were measured on an OCA 20 machine (Germany) and were obtained by measuring more than five different positions on the coating surfaces. Optical images of oil overflow and absorption experiment were captured by a Nikon D90 digital camera with illumination from an LED lamp. The microscaled air−oil−water−solid four-phase wetting state during the oil−water separation process is visualized and demonstrated through laser confocal microscopy (OLYMPUS FV1000-IX81). Separation efficiency is qualitatively characterized by the titration method. The sandpaper abrasion tests were carried out to test the stability of the coated surface.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00044. Optical images of fabrication process for gradient meshes; stability test of Janus gradient meshes; the directional oil penetration in an air−mesh−water system; separation efficiency characterization as noted in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Z.D.) E-mail [email protected]; Fax +86-1082621396.

3. CONCLUSIONS In summary, we present a facile method for constructing a thermally stable, gradient mesh with Janus wettability that exhibits oil gating property and overflow with selective absorption ability when equipped onto a hollow and curved solid edge. The gradient mesh can selectively draw oils from flowing oil−water mixtures in preference to water at high speeds, through a combination of selective oil overflow behavior and oil gating property. Our results suggest an innovative material that should find practical applications in the removal of organics, particularly in the field of wastewater

ORCID

Zhichao Dong: 0000-0003-0729-5756 Author Contributions

N.L. and C.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge project funding provided by the National Key R&D Program of China (2017YFA0206901), the National F


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Natural Science Foundation (21703270, 21431009, 91127025), the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01), and Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191 and the 111 Project (B14009).

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Janus Gradient Meshes for Continuous Separation and Collection of

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