Flexible Microfluidic Fabrication of Anisotropic Polymer Microfibers

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Flexible Microfluidic Fabrication of Anisotropic Polymer Microfibers Wenjie Lan,*,† Yinjie Du,† Xuqiang Guo,† Aixian Liu,† Shan Jing,‡ and Shaowei Li*,‡,§ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China § State Key Laboratory of Chemical Engineering, Tsinghua University, Beijing 100084, China ‡

ABSTRACT: This article describes a microfluidic method for fabricating anisotropic polymer microfibers. Janus colaminar flows were realized in a microfluidic device. Microfibers with a noncircular cross-section were fabricated by an in situ photopolymerization method. The microflows experienced different evolutionary states in the microchannel. Simple control of the curing time resulted in the microfibers forming different shapes. The morphology of the crosssection could be readily manipulated by the interfacial tensions and flow rates of the fluids. Theoretical analysis and computational fluid dynamics simulation were used to predict the effect of these factors on the fiber morphology. The theoretical and simulation results were in good agreement with the experimental results. Microfibers with nonuniform surface properties could also be produced by adding modification reagents into the different fluids. The selective coating of micro/nanoparticles on the convex or concave surfaces of the microfibers was realized.

1. INTRODUCTION In the past two decades, microstructures such as microfibers, microbeads, and microstrips have received increasing attention in areas including tissue engineering, biotechnology, and analytical chemistry.1 Fiber materials are effective microcarriers for many industrial applications such as in chemical reactions, energy conversion, and pollutant degradation. Fiber materials exhibit various functions owing to their large surface area to volume ratio, varied architectures, and tunable mechanical properties.2−5 Microfibers with flexible complex configurations such as fiber−sphere hybrid materials and anisotropic fibers can potentially exhibit multiple functionalities and therefore have a wide range of potential applications.6,7 Synthesizing such microfibers has become a topic of great academic and industrial interest. Various methods, including electrospinning,8,9 wet spinning,10,11 melt spinning,12−14 and self-assembly15 have been used to fabricate microfibers. However, most of these have difficulties with the flexible control of the fiber dimensions, morphologies, and structures. This is typically owing to insufficient control of the flow of the fluid. Most of these methods are limited to creating simple cylindrical fibers. Some of them are also subject to high cost, low reproducibility of the plate form set up, and poor compatibility with biological materials. Microfluidics is a promising alternative approach for synthesizing microfibers.16 Microfluidic approaches are particularly suitable for synthesizing microfibers with geometric and chemical complexity because of their excellent control of flow. Microfluidic approaches are also convenient, cost-effective, and reproducible. Microfibers with various morphologies, chemical compositions, and functionalities have been fabricated using © XXXX American Chemical Society

microfluidic approaches over the past decade. For example, polymeric, biocompatible, inorganic, and hollow fibers were fabricated for applications such as mixture separation, cell culture, and catalysis.17−21 Bamboo-like fiber−sphere hybrid materials were synthesized by a combination of laminar and droplet flows in a microfluidic device.7,22 In comparison, the fabrication of anisotropic microfibers has been far less reported. This is despite the absence of centrosymmetry in synthetic anisotropic systems having led to the discovery of materials with properties that cannot be obtained using homogeneous or symmetric materials. Fiber anisotropy may arise from their noncircular cross sections or their nonuniform intrinsic properties. Fibers with nonuniform intrinsic properties can be divided into two classes: fibers composed of multicompartments and fibers with varying surface patches (patchy fibers). Microfibers with anisotropic shapes have been reported, such as double hollow and belt-like poly(ethylene glycol) diacrylate microfibers23 and Janus polyurethane microfibers.24 Despite these reports, much work is required to increase the geometric diversity. Further research is also necessary on fabricating fibers with nonuniform intrinsic properties. In the current study, we use a microfluidic approach to fabricate anisotropic microfibers with both anisotropic shape and surface properties. Anisotropic microfibers with various morphologies can be prepared by simply adjusting the curing Received: Revised: Accepted: Published: A

September 8, 2017 December 3, 2017 December 12, 2017 December 12, 2017 DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research time. The effect of the interfacial tensions and flow rates on the fiber morphology was investigated. Finally, patchy microfibers with particles selectively coated on the convex and concave surfaces of the microfibers were fabricated.

then droplets pinched off from the thread. The thread was photopolymerized to form microfiber by exposing to the UV light. The horizontal distance between the UV light and tip was varied to control the fiber morphology. To fabricate patchy microfibers with microspheres coated on the convex surface, 2 wt % hydrophilic SiO2 microspheres were added to the outer phase. To fabricate patchy microfibers with microspheres coated on the concave surface, 0.5 wt % of hydrophobic SiO2 microspheres were added to the noncurable phase. Microparticles were uniformly dispersed in the fluids by sonication for more than 60 min. The obtained microfibers were washed with ethanol and collected. 2.4. Characterization. The microflows were observed with an optical microscope (MZ101102, ViewSolutions Co., P. R. China) coupled with a high-speed camera (SP-5000C-PMCL, JAI Co., Denmark). The detailed structures of the microfibers were observed using scanning electron microscopy (SEM, FEI Quanta 200 FEG, United States). Energy spectrum analysis (EDS) was performed on the SEM samples using the EDS function of the SEM apparatus. The dynamic viscosities of the liquids used in the experiments were measured by a Ubbelohde viscometer. The results are shown in Table 1. The interfacial tensions between the liquids were measured by a pendent drop interfacial tension meter (OCAH200, DataPhysics Instruments GmbH, Germany)

2. EXPERIMENTS 2.1. Materials. 1,6-Hexanediol diacrylate (HDDA; SigmaAldrich, United States) and silicone oil (VAS Chemical Co., Ltd., Tianjin, P. R. China) were used as the photocurable phase and noncurable phase, respectively. The photoinitiator 2hydroxy-2-methylopropiophenone (Sigma-Aldrich) and the oilsoluble dye Oil Blue N (Sigma-Aldrich) were dissolved in the curable phase. Sodium dodecyl sulfate (SDS; VAS Chemical Co., Ltd., Tianjin, P. R. China) aqueous solution and polyethylene glycol (PEG; average Mn = 20 000, VAS Chemical Co., Ltd., Tianjin, P. R. China) were used as the continuous phase. Hydrophobic SiO2 microspheres (1.0 μm; Baseline Chromtech Research Center, Tianjin, P. R. China) and hydrophilic SiO2 microspheres (700 nm; Baseline Chromtech Research Center, Tianjin, P. R. China) were dispersed in different phases for selective modification of the microfibers. 2.2. Experimental Setup. The microfluidic device used is shown in Figure 1. The device was fabricated on a 50 × 20 × 3

Table 1. Viscosities of the Liquids

viscosity (mPa·s)

curable phase

noncurable phase

2% SDS aqueous solution

19.8

67.1

1.17

2% SDS + 20% PEG 20 000 aqueous solution 35.9

3. RESULTS AND DISCUSSION 3.1. Fabrication of Microfibers. The flow pattern in the microfluidic device determines the morphology of the resulting material. When 2 wt % SDS aqueous solution is used as the outer phase, three different flow patterns are obtained by changing the flow rates of the silicone oil (QSO) and HDDA (QHDDA), as shown in Figure 2. Before the two inner phases enter the main channel, both segmented and laminar flow can form in the inner phase channel. The increase in the total flow rate of the two phases leads to a transition from segmented to laminar flow. When the flow ratio of the two inner phases deviates from 1, the flow pattern tends to transition from laminar to segmented flow. When segmented flow forms in the inner phase channel, both dripping and jetting flow can form in the main channel. For the dripping flow, HDDA droplets and silicone oil droplets are generated alternatively (flow pattern A). The amount of HDDA or silicone oil droplets depends on the length of the segmented plug. The increase in the flow rate of the inner phases leads to a transition from dripping to jetting. For the jetting flow, HDDA and silicone oil threads form alternatively. Their time duration also depends on the length of the segmented plug (flow pattern B). Finally, when laminar flow forms in the inner phase channel, dripping flow will not readily form in the main channel owing to the high flow rate of the inner phases. In this situation, jetting flow always forms in the main channel (flow pattern C). Microfibers can be fabricated only when jetting flow forms in the main channel. Therefore, the following solidification experiments were carried out only in the range of flow patterns B and C (Figure 2).

Figure 1. Coaxial microdevice.

mm poly(methyl methacrylate) (PMMA) plate using micromachining technology. A glass tube (ID 500 μm) was inserted as the main channel for the multiphase flow. Another glass tube (ID 900 μm) with a shrink tip was inserted into the main channel as the inner phase channel. The inner diameter of the shrink tip was 100 μm. Two stainless steel microneedles were inserted into the dispersed phase channel as the inlets of curable phase and noncurable phase. The outer and inner diameters of the microneedle were 310 and 160 μm, respectively. The microfluidic device was sealed using another 50 × 20 × 1 mm PMMA chip, which was bonded to the first chip using the ultrasonic-assisted sealing technique. Three microsyringe pumps and four gastight microsyringes were used to pump fluids into the microfluidic device. Part of the glass tube was exposed to ultraviolet (UV) light to control the polymerization of the curable phase. The diameter of the light spot is 3 mm. 2.3. Fabrication of Microfibers. In each experiment, a solution containing 5 wt % photoinitiator and 95 wt % HDDA was used as the curable phase. Silicone oil was used as the noncurable phase. Oil-soluble dyes were dissolved in the curable phase to distinguish the two phases. The two inner phases were pumped into the microdevice through two microneedles. Two different aqueous solutions containing (1) 2 wt % SDS and (2) 2 wt % SDS and 20 wt % PEG were used as the outer phases. As the inner phases flowed through the shrink tip, a long thread formed downstream of the tip, and B

DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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shown in Figure 3, the fabricated microfibers have a simple round cross-section. The length of the microfibers depends on the flow rate ratio of the two inner phases. The fiber length increases with the increase of QHDDA/QSO. Under certain flow rates, the fiber length is highly uniform with an average deviation of less than 3%. In flow pattern C, stable laminar flow first forms in the inner phase channel. Jetting flow then forms in the main channel. Before droplets pinch off from the extended thread downstream of the shrink tip, the thread experiences several states. Figure 4 shows the evolution of the jetting flow from the tip to breakup. First, the curable and noncurable phases form stable parallel flow near the shrink tip. Then, the two inner phases begin to oscillate owing to capillary instability. The oscillation speed increases and finally causes the noncurable phase to break up into droplets. If QHDDA/QSO is high enough, the curable phase retains a continuous topology for a long time after the breakup of the noncurable phase, as shown in the dashed box. If QHDDA/ QSO is relatively low, the breakup of the two phases occurs almost simultaneously. The evolution of the flow makes it possible to fabricate microfibers with various morphologies. By adjusting the horizontal distance between the UV light and shrink tip (denoted as x), different states of the flow can be solidified. Figure 4 shows that when the UV light is near the shrink tip (x = 5 mm), continuous microfibers with a moon-like cross-section are obtained. The microfibers have uniform dimensions and cross-section morphology. As the UV light moves away from the shrink tip (x = 15 mm), for low QHDDA/ QSO, gourd-like microfibers are obtained. The cross-section of the microfibers varies along the axial direction. For high QHDDA/QSO, the noncurable phase first breaks up into droplets. Porous microfibers are then obtained. During the breakup of the noncurable phase, two types of droplets form to become pores on microfibers: main droplets and satellite droplets. As x keeps increasing (x = 20 mm), both the two inner phases break up into droplets. Microparticles are then obtained. If the UV light is placed at a fixed position, similar results can be obtained by changing the fluid flow rates. Figure 5 shows that as the flow rate of the noncurable phase increases, the microfiber morphology transitions from porous to gourd-like to moon-like. This is because the fiber morphology is determined by the relative position between UV light and the thread. The distance between the shrink tip and the position that the inner phase begin to oscillate (denoted as x1) as well as the distance between the shrink tip and the position that the noncurable phase breaks up (denoted as x2) will change with the noncurable phase flow rate. Given a fixed x, when the noncurable phase flow rate is small, the thread is short, and

Figure 2. Flow patterns with different flow rates of silicone oil and HDDA. Outer phase flow rate (Qw) = 800 μL/min.

In flow pattern B, segmented flow first forms in the inner phase channel. Then, the curable and noncurable phases alternatively enter the main channel, and segmented jetting flow forms in the main channel. After exposing to UV light, the curable phase solidifies to form segmented microfibers. As

Figure 3. (a) Photograph of segmented microfibers (Qw = 800 μL/min, QHDDA = 180 μL/min, QSO = 20 μL/min). (b) SEM image of the crosssection of segmented microfibers in panel a. (c) Variation in fiber length with QHDDA/QSO. C

DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Evolution of the jetting flow and control of the fiber morphology. The top panels are micrographs of the jetting flow. (For the image in the dashed box, QHDDA/QSO = 10. For the remaining three images, QHDDA/QSO = 5.) The middle panels are micrographs of products generated by exposing a different part of the main channel to UV light. The bottom panels are SEM images of the products.

Figure 6. Topology of the cross-section of the fluids in the main channel.

Figure 5. Variation in fiber morphology with increasing silicone oil flow rate (Qw = 800 μL/min, QHDDA = 500 μL/min, QSO = 10, 50, 100, 150, 300 μL/min, x = 5 mm).

assumed that the final equilibrium state depends only on the three interfacial tensions γij (i ≠ j ≠ k = 1, 2, 3), which are considered as the three spreading parameters Si = γjk − (γij + γki) as follows:

x2 < x. Then, the photopolymerization takes place after the breakup of noncurable phase and lead to the generation of porous fiber. As the noncurable phase flow rate increases, the thread becomes longer. We have x1 < x < x2. Then, gourd-like fiber will be obtained. When the noncurable phase flow rate is high enough, the thread will keep stable in a quite long distance, x ≪ x1. In this case, the smooth moon-like fiber will be obtained. 3.2. Control of Cross-Section Morphology. Before oscillation takes place, the cross-section morphology of the thread in the main channel is significantly influenced by the three-phase interfacial tensions. The topology of the crosssection of the fluids in the main channel is shown in Figure 6. The yellow, white, and blue areas represent the noncurable phase (phase 1), continuous phase (phase 2), and curable phase (phase 3), respectively. Torza and Mason25 studied the equilibrium configurations of the three-phase system. They

⎛ S1 ⎞ ⎛ ⎞⎛ γ12 ⎞ ⎜ ⎟ ⎜−1 1 −1⎟⎜ ⎟ ⎜ S2 ⎟ = ⎜−1 −1 1 ⎟⎜ γ23 ⎟ ⎜ ⎟ ⎝ 1 −1 −1⎠⎜ γ ⎟ ⎝ 31 ⎠ ⎝ S3 ⎠

(1)

Three possible equilibrium configurations can be obtained: (a) complete engulfing (or core−shell) when S1 < 0, S2 < 0, S3 > 0; (b) partial engulfing (or Janus) when S1 < 0, S2 < 0, S3 < 0; (c) nonengulfing when S1 < 0, S2 > 0, S3 < 0. To adjust the cross-section morphology of the microfiber, aqueous solutions containing either (1) 2 wt % SDS or (2) 2 wt % SDS and 20% wt. PEG is used as the outer phase. The measured interfacial tensions of the different systems and the predicted configurations of each system are shown in Table 2. Figure 7 shows D

DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Predicted and Actual States of the Systems system

SDS concentration in outer phase (% m/m)

PEG concentration in outer phase (% m/m)

1 2

2 2

0 20

γ12 (mN/m) γ13 (mN/m) 2.92 6.48

0.54 0.54

predicted cross-section configuration

actual cross-section morphology

2.42 8.72

Janus core−shell

Janus core−shell

not only affected by the interfacial tensions but also by the flow conditions. These results indicate that the equilibrium model does not perfectly describe the dynamic flowing system. In addition to the cross-section morphology, the dimensions of the microfiber are difficult to predict with a theoretical model. Therefore, computational fluid dynamics (CFD) simulations were carried out to provide a better prediction of the flow. 3.3. CFD Simulation of Janus Laminar Flow. We discussed the equilibrium state of Janus laminar flow in Section 3.2. However, the equilibrium state cannot always be reached in a dynamic flowing system. This is especially so when materials are prepared through in situ solidification methods where solidification usually occurs before the equilibrium state is reached. Moreover, the dimensions of the fiber are difficult to predict by theoretical models, owing to the complexity of the Janus flow. Another method is needed to predict the Janus laminar flow process. CFD simulation combined with the level set method (LSM) is a powerful technique for studying the evolution of liquid−liquid interfaces in microchannels. We previously developed a modified level set method27,28 and used it to simulate the liquid−liquid two-phase flow in microchannels. The same method is used in the current study to simulate the Janus laminar flow. To describe the three-phase flow, two level set functions (LSF), ϕ1 and ϕ2, are introduced, as shown in Figure 9, with ϕ1 = 0 and ϕ2 = 0 representing the interfaces.

Figure 7. SEM images of microfibers fabricated using systems (a and c) 1 and (b and d) 2.

SEM images of the cross sections of the obtained fibers when using the two different systems. The predicted equilibrium configurations are in good agreement with the experimental results. A Janus fiber with a moon-like cross-section is obtained when using system 1. A core−shell fiber is obtained when using system 2, indicating that the two inner phases move toward each other and experience a transition from partial engulfing to complete engulfing when flowing in the main channel. This is due to the effect of the three-phase interfacial tensions. The experimental results show that the morphology of the cross-section is also affected by the flow rates of the three phases. The section area largely depends on the flow ratio between the curable and noncurable phases. As shown in Figure 8, the gap within the cross-section decreases with the increase of the QHDDA/QSO. The topology of the cross-section is also determined by the angle α marked in Figure 6. Hasinovic et al.26 studied the topology of a Janus drop. α can be calculated by the following equation: α = 180 − acos[0.5(γ232 + γ132 − γ122)/(γ23γ13)]

γ23 (mN/m)

Figure 9. Two intersecting LSFs describing the three interfaces of Janus laminar flow.

(2)

Equation 2 indicates that α only depends on the interfacial tensions. However, in the current experiments, α varies with the flow rates of the fluids, which is different from the equilibrium configuration of Janus droplets in a stationary frame. As shown in Figure 8, it is obvious that angle α increases with the increase of QHDDA/QSO. In other words, in a Janus colaminar flow, α is

The governing equations for the two LSFs are written as Dϕi Dt

u ·∇ϕi − (⇀ u ·∇ϕi)|ϕi = 0 (i = 1, 2) =⇀

(3)

Figure 8. SEM images of fiber cross sections (Qw = 800 μL/min). (a) QHDDA = 150 μL/min, QSO = 450 μL/min; (b) QHDDA = 300 μL/min, QSO = 300 μL/min; (c) QHDDA = 450 μL/min, QSO = 150 μL/min; (d) QHDDA = 500 μL/min, QSO = 100 μL/min. E

DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research The governing equations of mass and momentum can be found in our previous studies,27,28 so they are not repeated here. The geometry of the simulation domain is shown in Figure 10. The inner diameters of the dispersed channel and outer

Figure 10. Geometry of the simulation domain.

channel are 100 and 500 μm, respectively. The lengths of the dispersed channel and outer channel are 500 μm and 5.0 mm, respectively. The simulation domain consists of 3.6 M unstructured, tetrahedral meshes. The mesh size is 12 μm for the main channel and is refined to 2.5 μm for the dispersed channel. The commercial software package CFX 5.6 was used to perform the steady-state simulation. The convergence criterion was set to be when the normalized root-mean-square of the equation residuals reaches 0.00001. The simulation results are shown in Figure 11. Figure 11a shows the effect of QHDDA/QSO. The outer diameter of the fiber changes little when fixing the flow rate of the outer phase and the total flow rate of the two inner phases. Increasing the flow ratio QHDDA/QSO causes the wane of the moon-like fiber to decrease, while the predefined angle α increases. The simulation results are in good agreement with the experimental results shown in Figure 8. Figure 11b shows that the outer phase flow rate has little effect on the morphology of the crosssection. Additionally, we also measured the diameters of the curable phase (d0) and noncurable phase (di). Figure 12 shows that the simulation results agree well with the experimental results. The above results show that the CFD simulation satisfactorily predicts the fiber size and morphology. 3.4. Fabrication of Patchy Microfibers. Materials with varying surface patches (patchy materials) have attracted much attention because they can exhibit properties that cannot be obtained with symmetric materials. Patchy materials are usually prepared in time-consuming multistep procedures, which are subject to variable reproducibility. In the current experiment, the stable Janus laminar flow provides a unique environment for the curable phase. That is because the curable phase can simultaneously contact with two different phases. Reagents for surface modification could contact the curable phase from either the outer phase or the noncurable phase to realize asymmetrical modification. The solidification and asymmetric modification of the microfiber occur simultaneously, so patchy microfibers could be synthesized in a single step. In the current study, microfibers asymmetrically coated with microparticles are prepared in situ. When SiO2 microparticles are dispersed in the outer phase (Figure 13a), particles collide with the interface between phases 2 and 3. After collision, some of the microparticles tend to be adsorbed at the interface. Microparticles adsorbed on the interface are then fixed in place upon solidification of the curable phase. Patchy microfibers with microparticles coated only on the convex surface are

Figure 11. (a) Simulation results at Qw = 800 μL/min. (a1) QHDDA = 150 μL/min, QSO = 450 μL/min; (a2) QHDDA = 300 μL/min, QSO = 300 μL/min; (a3) QHDDA = 450 μL/min, QSO = 150 μL/min; (a4) QHDDA = 500 μL/min, QSO = 100 μL/min. (b) Simulation results at QHDDA = 300 μL/min and QSO = 300 μL/min. (b1) Qw = 400 μL/min, (b2) Qw = 800 μL/min, (b3) Qw = 1600 μL/min.

obtained. When SiO2 microparticles are dispersed in the noncurable phase (Figure 13b), particles collide with the interface between phase 1 and 3. Microparticles are then selectively coated on the concave surface. Figure 14 shows SEM images of the fabricated patchy microfibers. In Figure 14a, particles are distributed only on the convex surface. In Figure 14b, large amounts of particles are uniformly distributed on the concave surface with few particles on the convex surface. EDS spectra in Figure 14d show that the white particles in the SEM images are SiO2 microparticles, confirming the selective coating on the microfibers.

4. CONCLUSIONS We present a simple way to fabricate microfibers with anisotropic shapes and surface properties. Janus colaminar flow is formed in the microchannel. The noncurable phase serves as a template for the curable phase to form various shapes. Asymmetric microfibers with various morphologies can be produced by photopolymerization in different evolutionary states of the flow. The morphology of the cross-section is also affected by the interfacial tensions and flow rates of the fluids. Theoretical analysis and numerical simulation predicting the effect of the interfacial tensions and flow rates of the fluids on the morphology of the cross-section agree well with the experimental results. Microfibers with SiO2 particles selectively coated on the convex or concave surface can be fabricated by dispersing particles in the outer or noncurable phase, respectively. Replacing SiO2 particles with other modification F

DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 12. Comparison of simulation and experimental results for the diameters of the curable and noncurable phases. (a) Qw = 800 μL/min, QHDDA + QSO = 600 μL/min; (b) QHDDA = 300 μL/min, QSO = 300 μL/min.

reagents could potentially realize the selective modification for a wide variety of applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wenjie Lan: 0000-0002-7833-8824 Xuqiang Guo: 0000-0002-0781-1477 Shaowei Li: 0000-0002-7761-9210 Notes

Figure 13. Synthesis of microparticle-coated patchy microfibers. Microparticles coated on the (a) convex and (b) concave surfaces.

The authors declare no competing financial interest.

Figure 14. (a) SEM image of patchy microfibers with SiO2 microparticles coated on the convex surface. (b) SEM and (c) magnified SEM image of patchy microfibers with SiO2 microparticles coated on the convex surface. (d) EDS spectra of the patchy microfibers at surface points 1−4. Gold is present because the samples are coated with gold to prevent charging during SEM observation. G

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hollow fiber membrane with a facile double co-axial microfluidic device. J. Membr. Sci. 2017, 526, 9−17. (20) Lan, W. J.; Li, S. W.; Xu, J. H.; Luo, G. S. Controllable synthesis of microscale titania fibers and tubes using co-laminar micro-flows. Chem. Eng. J. 2012, 181, 828−833. (21) Meng, Z. J.; Wang, W.; Xie, R.; Ju, X. J.; Liu, Z.; Chu, L. Y. Microfluidic generation of hollow Ca-alginate microfibers. Lab Chip 2016, 16, 2673−2681. (22) Chaurasia, A. S.; Jahanzad, F.; Sajjadi, S. Flexible microfluidic fabrication of oil-encapsulated alginate microfibers. Chem. Eng. J. 2017, 308, 1090−1097. (23) Choi, C. H.; Yi, H.; Hwang, S.; Weitz, D. A.; Lee, C. S. Microfluidic fabrication of complex-shaped microfibers by liquid template-aided multiphase microflow. Lab Chip 2011, 11, 1477−1483. (24) Jung, J. H.; Choi, C. H.; Chung, S.; Chung, Y. M.; Lee, C. S. Microfluidic synthesis of a cell adhesive Janus polyurethane microfiber. Lab Chip 2009, 9, 2596−2602. (25) Torza, S.; Mason, S. G. Coalescence of two immiscible liquid drops. Science 1969, 163, 813−814. (26) Hasinovic, H.; Friberg, S. E.; Kovach, I.; Koetz, J. Janus emulsion drops: equilibrium calculations. J. Dispersion Sci. Technol. 2013, 34, 1683−1689. (27) Lan, W. J.; Li, S. W.; Wang, Y. J.; Luo, G. S. CFD simulation of droplet formation in microchannels by a modified level set method. Ind. Eng. Chem. Res. 2014, 53, 4913−4921. (28) Lan, W. J.; Li, S. W.; Luo, G. S. Numerical and experimental investigation of dripping and jetting flow in a coaxial micro-channel. Chem. Eng. Sci. 2015, 134, 76−85.

ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (Grants 21406266 and 21576147) and the Science Foundation of China University of Petroleum, Beijing (Grants 2462013YJRC025 and C201606).

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DOI: 10.1021/acs.iecr.7b03745 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX