Electric Field Design for Multijet Electropsinning with Uniform Electric

Article pubs.acs.org/IECR

Electric Field Design for Multijet Electropsinning with Uniform Electric Field Yuansheng Zheng,†,‡ Changming Zhuang,† R. Hugh Gong,‡ and Yongchun Zeng*,†,§ †

Textile Engneering, College of Textiles, Donghua University, Shanghai, 201620, People’s Republic of China Textiles, School of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom § Key Laboratory of Textile Science & Technology, Donghua University, Ministry of Education, Shanghai, 201620, People’s Republic of China ‡

ABSTRACT: Electric field plays a key role in producing required nanofibers in electrospinning. This study aims to achieve uniform electric field distribution in multijet electrospinning by designing the appropriate spinneret and collector geometry. Based on the three-dimensional electric field simulation, three kinds of spinneret and two kinds of collectors have been designed. The simulation results show that a flat spinneret creates a more uniform electric field distribution than the multineedle spinneret with an auxiliary plate. The two-step spinneret can intensify the electric field at the central area and create a rather uniform electric field. A smaller collector creates a more uniform electric field distribution, and a curved collector creates a more uniform surface electric field. The simulation results have been verified by experiments. The experiments show that thinner and more uniform fibers are collected when using the two-step spinneret and a smaller flat collector, and the thickness of fiber mat is more uniform.

1. INTRODUCTION Electrosoinning is a process that creates nanofibers of diameter below 1 μm through an electrically charged jet of polymer solution or polymer melt. The large surface-to-volume ratio property of the electrospun fibers and interconnected pores in the nanofibrous nonwovens enables the electrospun products to have many applications in areas of filtration media, medicine, and industry.1,2 However, the widespread industrial implementation of this technique is primarily limited by low production rate, which is 0.01−1 g/h from a single jet electrospinning process. In recent years, quite a few methods for multijet electrospinning have been developed to increase the production rate of the electrospun nanofibers. These methods include multineedle electrospining,3−5 multihole electrospinning,6−8 and free surface electrospinning.9−12 Although more and more needleless electrospinning (i.e., free surface electrospinning) applicances have been invented these days, the multineedle electrospinning is still under active investigation due to its straightforward approach to enhance the productivity. Compared to the multineedle electrospinning, the multihole electrospinning has a more simple way for polymer solution feeding and ejecting. There are many parameters, including polymer parameters, processing conditions, and electric field characteristics, that play roles in electric spinning. Most of these studies on multjet electrospinng focus on the approaches to productivity enhancement and on the effects of polymer parameters13,14 and processing conditions15,16 on fiber diameter. This paper will focus on the effect of electric field distribution of the multijet electrospinning on resultant fiber morphology and collected fiber mat. The electrospinning process, in its simplest form, consists of a spinneret to form the polymer jet, a collector, and a highvoltage supply. The spinneret and the collector are usually used as two electrodes, and an electric field is created between the © 2014 American Chemical Society

two electrodes under the applied voltage. The important role of electric field in electrospinning has been widely recognized. Kong et al.17 designed four basic electric fields by changing the shape of the electrodes. They concluded that the design of the electric field is a significant parameter in the attempt to control nanoweb formation, and the effect of the spinneret design is more significant than the effect of the collector design. Yang et al.18 created three kinds of electric field distribution by changing the spinneret structure of a single-needle electrospinning system to study the effects of electric field distribution on the fiber formation. Their results showed that more uniform electric field produced thinner fibers. Yang et al.19 designed three kinds of collection targets in a single-needle system to study the effects of surface electric field of the collector on the fiber mats. Their results showed that uniform surface electric field distribution produced more uniform thickness of the fiber mats. Angammana and Jayaram’s20 investigated the variation of the strength of the electric field at the tip of the needles in multineedle arrangements and showed local field deteriorations at the needle tips. Our previous study21,22 showed that more uniform electric field can be created by a hole spinneret with a flat surface. We have argued that more uniform electric field distribution produces finer and more uniform fibers.21−23 The aim of this study is to enhance the productivity of electrospun nanofibers with better fiber morphology by manipulating the electric field. As the geometries of the spinneret and the collector determine the electric field characteristics, several spinnerets and collectors are designed to create electric fields with more uniform Received: Revised: Accepted: Published: 14876

May 4, 2014 September 3, 2014 September 8, 2014 September 8, 2014 dx.doi.org/10.1021/ie501827b | Ind. Eng. Chem. Res. 2014, 53, 14876−14884

Industrial & Engineering Chemistry Research

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distribution in this study. We focus on the effects of spinneret and collector geometries on the electrospun fiber morphology and the properties of collected mats. Numerical simulation is used to design the electric field, and experiments are carried out to validate the spinneret and collector designs. The method for designing electrospinning configuration based on the electric field simulation results is aiming at providing a reliable tool for future industrial application.

2. ELECTRIC FIELD SIMULATION FOR SETUP DESIGN The high voltage involved in electrospinning process leads to difficulty in measuring the electric field. The Ansoft Maxwell Figure 3. Electric field intensity of the multineedle electrospinning with an auxiliary plate along the x-axis at 1 mm below the needle tips with different needle lengths protruding the plate.

dimensions and material properties of the electrospinning setup. To create a more uniform electric field distribution for the multijet electrospinning system, electric field simulation is adopted to design an electrospinning configuration. Several designed configurations with various spinneret and collector geometries were simulated by Ansofe Maxwell software. In all simulation works, a voltage of 15 kV was set to the spinneret, and the metal collector and the boundaries at an infinite distance were set as zero potential. The distance between the spinneret and collector was set to 15 cm in all the simulations. The meshing and solving were performed by the software to obtain the electric field intensity and profile. 2.1. Spinneret Design. Three kinds of multijet electrospinning spinneret with the same collector were designed to examine the effect of spinneret geometry on electric field distribution. The collector used was a rounded flat target with 150 mm diameter and 2 mm thickness.

Figure 1. Schematic of multineedle spinneret with an auxiliary plate electrospinning system.

(ANSYS Inc., USA) software provides a numerical technique to simulate the electric field using the finite element method (FEM). The electric field intensity and distribution can be visualized using the FEM calculation with the practical

Figure 2. Vectors of the electric field of the multineedle electrospinning with an auxiliary plate. 14877

dx.doi.org/10.1021/ie501827b | Ind. Eng. Chem. Res. 2014, 53, 14876−14884


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Figure 4. Vectors of the electric field of the flat spinneret electrospinning configuration.

needle lengths protruding the plate. It is obvious that the maximum electric field intensity is concentrated at the needle tips, and the central needle gives the smallest peak value of the electric field intensity. With decreasing needle length protruding the plate, the uniformity of the electric field distribution increases. A more uniform electric field has lower peak values at the tips and higher electric field intensity except in the central area. When the needle length protruding the plate is reduced to 0, the spinneret becomes a flat surface with holes in it. For this case, the electric field intensity turns quite uniform except at the edge of the plate. 2.1.2. Flat Spinneret with Cylinder Electrode. Based on the simulation results of the multineedle spinneret with an auxiliary plate electrospinning system, a flat spinneret was designed to create a uniform electric field. The spinneret can be constituted by a cylindrical metal electrode and a perforated plastic plate to eject the polymer jets. We simplify the analysis without the plastic plate in the simulations. The electric fields of the spinneret with various diameters were calculated in order to study the effect of the electrode diameter on electric field distribution. The calculated electric field distribution of the flat spinneret configuration is shown in Figure 4. The electric field distribution is more uniform than that of the multineedle configuration with auxiliary plate, which is shown in Figure 2. Figure 5 shows the electric field intensity along the x-axis at 1 mm below the spinneret (i.e., z = 1 mm) with different diameters of the cylinder electrode. It is clear that there is a concentration of the electric field at the edge of the electrode. With the increasing diameter of the flat electrode, the electric field distribution in the central area of the flat electrode spinneret becomes more uniform, while the electric field intensity decreases in both central area and the edge of spinneret. It is known that a uniform electric field is created by a pair of plate electrodes of the same size. Therefore, larger electrode of the flat spinneret will lead to smaller size difference between the spinneret and the collector (i.e., negative electrode) and consequently more uniform electric field.

Figure 5. Electric field intensity of the flat spinneret electrospinning configuration along x-axis at 1 mm below the spinneret with different electrode diameters.

2.1.1. Multineedle Spinneret with an Auxiliary Plate. We began with the simulation of a multineedle spinneret with an auxiliary plate, as shown in Figure 1. Six metal needles were distributed in an equilateral hexagon configration, with an additional needle located in the center. The spacing between neighboring needles was 10 mm. The needles were inserted through the metal auxiliary plate with 60 mm diameter and 1 mm thickness. The needle length protruding the plate was adjustable to create various electric field distributions. Figure 2 shows the vectors of the electric field of this multineedle elecrospinning in the x-z plane calculated by the Ansoft Maxwell software. The arrows indicate the direction of the electric field, and their length is proportionate to the strength at that position. The color map shows the voltage distribution of the electrospinning configuration. It can be seen that an auxiliary plate could create a more uniform local electric field than those multineedle spinneret without an auxiliary plate. Figure 3 shows the electric field intensity along the x-axis at 1 mm below the needle tips (i.e., z = 1 mm) with different 14878



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Figure 6. Vectors of the electric field of the two-step spinneret configuration.

highest. Therefore, this “edge effect” of the cylinder electrode can be utilized in our spinneret design. A spinneret with twostep electrode was developed to create a strong and uniform electric field. The new design involves a step protruding from the central part of the cylinder electrode used in the flat spinneret. The step is used to enhance the electric field intensity in the central area of the flat spinneret. As shown in Figure 6, the two-step electrode also creates a more uniform electric field compared to the multineedle configuration with auxiliary plate (Figure 2). Figure 7 shows the electric field intensity along the x-axis at 1 mm below the spinneret (i.e., z = 1 mm) with different step heights. With increasing step height, the electric field intensity at the outer edge of the spinneret decreases, with a more enhanced electric field in the central area. When the step height reaches 3 mm, the electric field intensity at the step edge exceeds that at the outer edge. The 2 mm-step electrode creates the optimum electric field with nearly the same electric field intensitie at the step edge and the outer edge, presenting a quite uniform electric field.

Figure 7. Electric field intensity of the two-step spinneret configuration along x-axis at 1 mm below the spinneret with different step heights.

2.1.3. Spinneret with Two-Step Electrode. The electric field simulation results of the flat spinneret configuration show that the electric field intensity at the edge of the electrode is the

Figure 8. Electric field intensity of flat collectors with different diameter: (a) electric field intensity along z-axis and (b) electric field intensity along x-axis on the collector surface. 14879



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Figure 9. (a) Illustration of the curved collector; (b) electric field intensity along x-axis on the curved collector surface.

Figure 10. (a) Schematics of flat spinneret with round flat collector electrospinning configuration; (b) schematics of flat spinneret with curved collector electrospinning configuration; (c) sectional view of the flat spinneret.

2.2. Collector Design. As the negative electrode, the collector also plays a role in the electric field distribution in the electrospinning system. To find out how the collector geometry affects the electric field distribution, the electric fields of the round flat collector with three different diameters were simulated. A flat spinneret with 25 mm diameter was used in the collector design. Figure 8 shows the electric field intensities obtained from the collectors calculated by the Ansoft Maxwell software. The electric field intensity along the z-axis at the central line shown in Figure 8a indicates that the smaller flat collector forms a more uniform electric field. A more uniform electric field is characterized by gradual decrease of the electric field intensity in the area near the spinneret and higher electric field intensity beyond a certain distance from the spinneret (say, around 50 mm). As explained above, using a smaller collector will lead to smaller size difference between the spinneret (positive electrode) and the collector and consequently more uniform electric field.

To study the effect of the collector on electrospinning electric field, we focused on the electric field distribution close to the collector. Figure 8b shows the electric field along the xaxis on the collector surface (called surface electric field). It is clear that the surface electric field distribution of the flat collector is uneven, with the strongest electric field at the center part. The smaller collector forms a more uniform and stronger surface electric field. It has been found that the collected fibers tend to be deposited in the center area for a flat collector during the experiment. In addition to the gravity effect, the charged fibers are more readily deposited in the region of stronger electric field. To obtain a more uniform fiber collection, a collector shaped as a spherical cap (called a curved collector) was designed and the electric field was simulated. Figure 9a shows the design of the curved collector. The spherical radius is set as 250 mm. Figure 9b shows the surface electric field distributions of the curved collector with various spherical-cap heights. Compared to the flat collector, the curved collector produces a more uniform 14880



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Figure 11. (a) Schematics of multihole spinneret with flat collector electrospinning configuration; (b) arrangement of the holes.

Figure 12. SEM images of PEO fibers collected from four different collectors: (a) D = 150 mm flat collector; (b) D = 300 mm flat collector; (c) D = 450 mm flat collector; and (d) curved collector.

electric field on the collector surface. A higher spherical-cap produces more uniform surface electric field.

The spinneret consists of an electrode and a plastic plate made of polytetrafluoroethylene (PTFE) with 2 mm thickness. The holes with 0.5 mm diameter were drilled in the plastic plate. The electrode was made of aluminum with 50 mm diameter. A solution chamber was formed between the electrode and the plastic plate. A 2 mm-outer-diameter metallic tube connected with a 2 mm-outer-diameter insulated tube inserted in the center of the cylindrical electrode was used to provide solution feeding. For the multihole spinneret, 19 holes with concentrical distribution shown in Figure 11b were adopted. The polymer solution was forced from a syringe via a syringe pump (KDS 220, KD Scientific, Inc. USA) to the spinneret. A high voltage power supply (ES-60P 10 W/DDPM, Gamma High Voltage Research, USA) was applied to the spinneret and the aluminum foil-grounded collector.

3. EXPERIMENTAL SETUP A group of experiments was carried out with a single-hole flat spinneret to investigate the effect of the collector geometry on the fiber diameter and fiber mat thickness. The collectors studied in the experiments are the flat collectors of 150 mm, 300 mm and 450 mm in diameter, and the curved collector with 45 mm spherical-cap height. The schematic illustration of the electrospinning setups of this group of experiments is shown in Figure 10. Another group of experiments using a 19hole flat spinneret and a 19-hole two-step spinneret with a flat collector were carried out to study the influence of the spinneret geometry on elctrospinning process and the resultant fibers and fiber mats. The apparatus used in this group of experiments is depicted in Figure 11. 14881



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A 5.0 wt % poly(ethylene oxide) (PEO, Mv = 600 000 g/mol, Sigma-Aldrich, Inc., USA) aqueous solution was used in this study. The solution viscosity was 970 mPa.s, and the solution conductivity was 95.28 μs/cm. The distance between the spinneret and collector was 25 cm. The solution was prepared and all experiments were performed at about 25 oc in air at 40− 60% RH. The electrospun fibers were observed with a scanning electron microscope (SEM) (JSM-5600LV, Japan). Five SEM images taken from different sample locations were used to measure the fiber diameter. The mean diameter of the fiber was calculated using 300 measurements randomly selected. The ImageJ image analysis software (NIH, USA) was used to determine the fiber diameters. The areas of fiber mats were searched by scanning the aluminum foil in an EPSON scanner. The thicknesses of the fiber mats were measured with a micrometer.

Figure 15. Fiber mats thickness at central line collected by the four collectors.

straight section followed by a whipping process. Our previous study22,23 and other researchers18,24 have shown that the straight section is of the order of several centimeters. Therefore, the area of z > 50 mm may be situated in the whipping area. For the smallest flat collector (i.e., 150 mm in diameter), the higher electric field intensity in the whipping area results in larger electric field force to stretch the fiber and consequently produce finer fibers. The curved collector produces the finest and the most uniform fibers. Figures 14 and 15 respectively show the deposited patterns and the thickness of the fiber mat collected from the four collectors after 60 min. It can be seen that the fiber mat from the 150 mm-diameter flat collector is smaller than those from the two larger flat collectors. Compared to the flat collectors, the curved collector collects a rather round mat. The thickness of the mat was measured every 1 cm along the central line of fiber mat (i.e., the x-direction). Although the 150 mm-diameter flat collector produces more uniform thickness of the mat than the two larger flat collectors, the small size of the collector would be a limit in mass production. It is evident that the curved collector produces a uniform thickness in a large area. 4.2. Effect of Spinneret Geometry. A 19-hole flat spinneret and a 19-hole two-step spinneret shown in Figure 11 were used to investigate the influence of geometry of spinneret on the fiber diameter and fiber mat. In this group of experiments, the spinning voltage was 38 kV, the distance between the spinneret and collector was 25 cm, and the flow rate of the micropump was 9.5 mL/h (i.e., 0.5 mL/h per hole). Figures 16 and 17 show the SEM image and the fiber diameter distribution of the resultant nanofibers produced by the flat spinneret and the two-step spinneret with two steps of hole arrangement. It can be seen that the fibers obtained from the two-step spinneret are finer and more uniform than those from the flat spinneret. For the flat spinneret, the fibers electrospun from the outer layer of the holes are smaller than

4. EXPERIMENTAL VERIFICATION AND DISCUSSION 4.1. Effect of Collector Geometry. Three different sizes of flat collectors and a curved collector used in the simulation

Figure 13. Fiber diameter produced by the four collectors.

work were adopted in the first group of experiments. The processing parameters in the experiments were 25 kV applied voltage, 15 cm working distance, and 0.5 mL/h flow rate. Scanning electron microscopy (SEM) images shown in Figure 12 compare the fibers collected with the four collectors. It seems that the curved collector produces straighter and more uniform fibers. The measurements of the fiber diameter obtained from the four collectors are shown in Figure 13, indicating that for the three flat collectors a smaller collector produces finer and more uniform fibers. It has been shown (Figure 9a) that the electric field is stronger in the area of z > 50 mm for the 150 mm-diameter flat collector than for the other two flat collectors. The electrospun jet typically has a

Figure 14. Fiber mats collected by the four collectors: (a) D = 150 mm flat collector; (b) D = 300 mm flat collector; (c) D = 450 mm flat collector; and (d) curved collector. 14882



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Figure 16. SEM images of electrospun fibers from different hole positions in the two multihole electrospinning configurations.

worth mentioning that, all the holes can spin stably under the voltage of 30 kV in the two-step spinneret electrospinning system. For the flat spinneret, only the outer holes can spin stably under this voltage. The polymer drop at the inner holes and the center cannot form a jet due to the lower electric field intensity in the central area. When the voltage increases to 38 kV, all the holes of the flat spinneret can spin stably.

5. CONCLUSIONS In this work, we designed several electrospinning configurations with various geometries of the spinneret and collector to create uniform electric field. The electric field distributions of these configurations were simulated by Ansoft Maxwell software using finite element method, and the simulation results were verified by the experimental work. The following conclusions are drawn: (1) The simulation results of different spinnerets electrospinning configurations indicate that a flat spinneret creates a more uniform electric field distribution compared with the multineeedle with an auxilary plate electrospinning configuration, and a two-step spinneret can reinforce the electric field in the central area of the flat spinneret. It is predicted that finer and more uniform fibers will be produced by the multihole flat spinneret and two-step spinneret. The experimental results confirm the prediction. (2) The simulation results of different collectors indicate that smaller flat collector creates more uniform electric field and higher electric field strength on the collector surface. The experiment results show that finer fibers and more uniform fiber mats were collected by the smaller flat collector. The curved collector produces a more even fiber mat in a larger area due to the more uniform surface electric field. The electric field of the electrospinning configuration (the shape, size, and material of the spinneret and collector) can be designed using electric field simulations before experimentation. It provides a promising industrial method for designing electrospinning configuration.

Figure 17. Fiber diameter from different hole position in the two configurations under their critical voltages.

those from the inner layer. While in the two-step spinneret, the fibers obtained from out layer has a slightly larger average fiber diameter, but fibers from different layers are much more uniform than the that from flat spinneret. This is likely to be due to the more uniform distribution of the electric field created by the two-step spinneret. Figure 18 compares the fiber mats collected from the two spinneret configurations under the same spinning voltage. It is noted that, due to the short collecting time of 5 min, the collected mat has not yet been a continuous web. We can see that the mats produced from the two-step spinneret (Figure 18b) are in close proximity, while the flat spinneret produces the mats with a scattered distribution. The mats obtained from the outer holes in the flat spinneret are spaced far apart from those obtained from the holes situated in the central part. It is 14883



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Figure 18. Fiber mats collected from (a) the flat spinneret configuration and (b) the two-step spinneret configuration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 21 67792627. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (11272088), the Fundamental Research Funds for the Central Universities, the Chinese Universities Scientific Fund, and the China Scholarship Council.

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Electric Field Design for Multijet Electropsinning with Uniform Electric

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