Experimental Investigation of the Filtration Characteristics of Charged Porous Fibers

Nanofibers fabricated through electrospinning technology have been successfully applied in various industries. The filtration advantages of nanosized electrospun fibers, particularly their high filtration efficiency and low pressure drop, make them highly suitable for addressing the problems of particulate pollution. Previous research has shown that the higher surface-to-volume ratios of nanoscale porous fibers contribute to their physical properties. However, little research on the filtration characteristics of charged porous fibers has been reported. Accordingly, this study used electrospinning to fabricate microand nanoscale PMMA fibers in order to investigate the surface voltage, pressure drop, and filtration efficiency of charged porous fiber filters. The outcomes showed that porous PMMA fibers were successfully fabricated as the mass ratios of DMF:CHCl3 gradually reached 1:15. After corona discharge, the surface voltage of the fine porous fiber (~500 nm) was –0.650 kV higher than the –0.562 kV voltage of fine smooth fiber. Filtration efficiency was evaluated using particles with an average concentration of 1.16 × 10 and with sizes ranging from 21.58–660.62 nm. After negative corona discharge, the most penetrating particulate size of filters made from fine porous fiber decreased from 168.5 to 121.9 nm, and the penetration rate dropped from 34.09% to 5.84%. For smooth fiber, the most penetrating particulate size remained unchanged at 135.8 nm, but the penetration rate dropped from 32.64% to 18.19%. This study also showed that porous fiber performed better than smooth fiber in terms of surface voltage decay and single fiber efficiency.


INTRODUCTION
Particulate pollutants (a.k.a."particulate matter," or "PM"), are currently the most prominent cause of deteriorating air quality and have long been declared by the World Health Organization (WHO) as leading health hazards (WHO, 2006;Nourmoradi et al., 2015;Chong et al., 2017).Common methods for dealing with widespread PM include electrostatic precipitators, wet washings, and filtration (Zhang et al., 2017).Among these, filtration is considered one of the most simple and effective and has been used extensively to remove particulate contamination to improve air quality (Chen et al., 2016).
One option for manufacturing effective air filters is electrospinning.Introduced in 1934, electrospinning is capable of producing inexpensive, high-quality nanoscale fibers (Tan et al., 2005;Aruna et al., 2017).In contrast to traditional spinning methods, electrospinning is a unique technology that uses electrostatic force to produce fibers from polymer solutions (Bhardwaj and Kundu, 2010).The low requirements and high adaptability of electrospinning allow fibers to be created out of a wide range of polymer candidates (Goh et al., 2013;Xue et al., 2017), and the technology continues to receive a good deal of attention from both academia and industry (Deitzel et al., 2001, Cho et al., 2015;Choi et al., 2017).
Studies indicate that electrospun fibers can employed in different fields where non-woven fiber mats are employed, such as water treatment, tissue engineering, optics, biosensors, textile engineering and air conditioning filtration (Wang et al., 2002;Wnek et al., 2003;Ahn et al., 2012;Homaeigohar and Elbahri, 2012;Kim et al., 2016).Of particular interest are electret filters produced from electrospun fibers.Electret filters are characterized by their ability to remove PM from polluted air by the electrostatic forces of the fibers of which they are comprised (Sambudi et al., 2017).Such filters are known for minimal air flow resistance (aka "low pressure drop") and high efficiency in trapping PM (Tang et al., 2017).
Electret filters derive their electrostatic forces from surface charges built up on individual dielectric fibers.In theory, particle filtration efficiency is enhanced when the surface voltage of filter's individual fibers is increased (Nifuku et al., 2001;Huang, 2013).Compared to conventional-scale or microscale fibers, nanoscale fiber has a relatively higher surface-to-volume ratio, enabling it to hold relatively more charge.
Conventional filter materials capture PM from passing air streams.As more particles are trapped within the fiber mat, the pressure differential before and after the filter (a.k.a."pressure drop") increases.Once the pressure drop's upper limit is reached, the filter can no longer function (Chen and Chen, 2017).Pressurized air blasts are often applied to remove accumulated PM.After several rounds, however, the fiber mats within such filters are crushed and damaged and the resultant pressure drop renders them nonfunctional.Charged fiber has been explored as a feasible method to overcome this issue, thereby prolonging filter life, and also as a method to improve filter efficiency (Gibson et al., 2001;Greiner and Wendorff, 2007).Methods to charge fibers include corona discharge, tribo-charging and electrostatic fiber spinning.For polymer materials, such as those used in electrospinning, corona discharge is considered most suitable (Tsai et al., 2002).Studies have shown that charged nanofibers outperform conventional fibers in filtration efficiency, pressure drop, and service life (Lolla et al., 2016).
In addition, rough or porous surface features can be introduced during electrospinning that could further enhance charge retention (Yoon et al., 2009;Noh, 2011).Porous electret materials have been shown to have excellent performance with regards to surface charge decay (Xia et al., 2003).Porous fibers are electrospun from solutions with low-boiling-point solvents.During the electrospinning process, ambient water vapor condenses onto the fiber surface.When the water droplets on the fiber evaporate, they leave pores in the surface (Luo et al., 2010).
Charged porous nanofiber is anticipated to become a key technology in the near future.Since there is little research on the properties of charged fiber and their relation to the filtration efficiency of fiber mats comprised of porous fiber, a thorough investigation of relevant characteristics is critical to expand its application.
In this study, smooth and porous PMMA polymer fibers were fabricated by electrospinning.PMMA was selected for this study because it is an excellent electrospinning material and for its known dielectric properties.The fibers were carefully characterized, and then charge properties and filtration efficiency were tested on charged fibers.

Materials
Precursor solution for smooth fiber was prepared by first dissolving PMMA (Polymethyl Methacrylate, M w = 350,000 g mol -1 , ALDRICH®) in DMF (N,N-Dimethyl formamide, Sigma-Aldrich), and then solvent levels were adjusted to accommodate various concentrations.For porous fibers, the end solvent was changed to a 1:15 mix of DMF and trichloromethane (CHCl 3 ) were mixed in different proportions, as shown in Table 1.Submicron and nanoscale porous fibers were then fabricated using precursor solutions with different PMMA concentrations.

Electrospinning
Parameters involved in fiber fabrication by electrospinning include: temperature, humidity, PMMA solution concentration, and propulsion speed (Barham et al., 1984;Han et al., 2004;Tan et al., 2005;Zhang et al., 2009;Huang et al., 2011;Cheng et al., 2017).In this study, all parameters other than solution concentration were fixed.Two types of filter materials, smooth fiber and porous fiber, were fabricated using different concentrations, characterized, and then evaluated for surface charge properties and filter efficiency.
Electrospinning devices used in this study included a syringe-and-needle set (22G, ID = 0.84 mm), a ground electrode with countertop (with collection carriers, 5.5 cm × 5.5 cm iron mesh and 2 cm square iron foil), and a high voltage power supply (SL30, Spellman).The needle was connected to the high voltage supply, which could generate positive DC voltages up to 30 kV.The fiber filters collected by the carrier were kept in a culture dish.A schematic diagram of the fabrication device is shown in Fig. 1.
For electrospinning, two concentrations each of PMMA/DMF and PMMA/ (DMF & CHCl 3 ) solution were prepared: 100 mg mL -1 and 200 mg mL -1 .The syringe pump (KDS 100, KD Scientific) was used to transport the precursor solution with different volume flows.The distance between the needle tip and the ground electrode was 15 cm, and the voltage applied to the polymer solutions was 10 kV.All experimental procedures were carried out at room temperature.

Measurement and Charging
After the fibers were fabricated, their shapes and diameters were characterized by Scanning Electron Microscope (SEM,  Model 5136MM, Tescan).Fiber diameter was determined by averaging readings of one hundred sets of fibers selected from SEM fiber morphology.The weight and thickness of fiber filter were also measured, using an electronic analytical and precision balance (SI-234, DENVER) and a thickness gauge (H type, PEACOCK).The porosity (P m ) of fiber was calculated by Eqs. ( 1), ( 2) and ( 3). (2) In Eq. ( 1), P m is porosity of fiber, V m is volume of fiber, and V m /V indicates the filling density of the fiber.In Eqs. ( 2) and (3), ρ PMMA is the density of PMMA (1.19 g cm -3 ), and L, W, and t are the length, width, and thickness of the overall filter being measured.
A corona discharge device was designed and built for this study (Fig. 2).The needle tip was connected to a DC power supply (SL50PN300, Spellman), and the grounding electrode was connected to the iron net used for collecting fiber.Given that the properties of every fiber filter material were different, the output value of the constant current source was set at 0.2 mA.To avoid spark discharge and subsequent fiber burn caused by a constant source of current, the interelectrode distance was fixed at 10 mm.After charging, the surface charge potential of each test filter was measured with an electrostatic meter (FMX-003, SIMCO).

Filtration Performance Test
Fig. 3 is a diagram of a lab-scale fiber measurement system, which included a particle generator, a test device, and measurement equipment.
Submicron NaCl test particles were produced by a constant output atomizer.The face velocity of the filter was set at 10 cm s -1 and air flow was monitored and regulated by a mass flow controller (MFC, PC-540, PROTEC).In order to eliminate particle charge imbalance in the test aerosol and improve the stability of the system, a radioactive Am-241 alpha-source neutralizer was used to create positive and negative ions within the air stream.The intermingling of these ions and any charged particles thus created a Boltzmann charge balance equilibrium within the test aerosol stream.
Test particles in constant concentration were then supplied to the mix tube.The mix duct was made of acrylic, with an outer diameter of 35 cm and a length of 110 cm.Clean air was delivered to the tube and mixed with the aerosol stream containing particles.After the mixing process, the particulate air flow was introduced into the test duct.The test duct was also made of acrylic, with an outer diameter of 5 cm and a length of 40 cm.Two sampling ports (1 & 2) were located at the upstream and downstream of the filter, both at a distance of 9 cm from the filter to measure the aerosol concentration.
Since the sizes of resulting particles ranged from 20 nm to 600 nm, a Particle Mobility Particle Sizer (SMPS, Model 3080 & 3025A, TSI) was used to measure particle concentration and distribution up-and downstream of the filtration system.The difference between the number of particles upstream and the number downstream can be used to determine the penetration efficiency of the fiber filter with different particle size intervals.The penetration efficiency equation is shown in Eq. ( 4): where P is the penetration defined as the ratio (%), C in and C out represent the aerosol concentrations (counts cm -3 ) measured in up-and downstream of the filter, respectively.In addition, before feeding the precursor solutions through particles in the filter, a differential pressure gauge (PEL 1131110, PRODUAL) was connected to the test duct at a distance of 2 cm away from fibers under test to measure pressure drop.

Penetration Efficiency
Single-fiber efficiency theory was used to study the  microfluidic transmission rate of the charged and noncharged fiber filters.Single fiber efficiency is a calculated theoretical value and is compared against experimental data in order to verify that other factors did not affect the results.
The single fiber filtration efficiency E Σ was calculated using Eq. ( 5): where d f is fiber diameter, P is particle penetration efficiency, α is packing density of the fiber, and t is the measured thickness of the filter.

Electrospinning of Smooth Fiber and Porous Fibers
There are several factors affecting the diameter and shape of electrospun fibers, including the concentration, conductivity, molecular weight, viscosity and surface tension of the polymer precursor solutions, as well as the collection distance and operating voltage of the electrospinning apparatus (Bhardwaj and Kundu, 2010).Of these, the concentration of the polymer solution is primary.Therefore, this study used different concentrations of PMMA solution to produce fibers of different diameters.Fig. 4 shows SEM micrographs of fiber morphologies from different concentrations of PMMA solution (DMF), which produced fibers that are smooth and free of beads.The diameters of fibers fabricated from 100 mg ml -1 PMMA was 585.82 ± 67.83 nm, and 2477.47 ± 107.11 nm for those fabricated from 200 mg mL -1 PMMA.The results showed that fiber diameter was positively correlated with increased PMMA concentration.These findings were consistent with those reported by other investigators (Tan et al., 2005;Sihn et al., 2008).In addition, there was less variation in diameter among fibers fabricated from the low-concentration polymer solution.A possible explanation is that "Taylor cones" formed by low-concentration solutions at the dispensing needle's tip deform and disperse more evenly under the influence of the applied electric field.
In this study, porous fibers were generated by the evaporative condensation method, where electrospinning was carried out using low-boiling-point solutions.In this model, the temperature immediately adjacent to the fibers being formed dropped due to the high volatility of the trichloromethane (a solvent with a low boiling point) in the solvent mix.As a result, water droplets condensed on the fibers being formed, occupying significant space on the surface.After the water droplets evaporated, pores remained on the fiber surface (Bognitzki et al., 2001;Casper et al., 2004;Luo et al., 2010;Li et al., 2015).Tip clogging due to rapid solidification (because of the volatility of trichloromethane) was prevented by the presence of DMF (a less volatile solvent) in the solvent mix, in a 1:15 ratio of DMF to trichloromethane.Fig. 5 shows SEM micrographs illustrating the morphology of porous fibers and diameter distribution diagrams for different PMMA concentrations.Fig. 5(A1) shows the fibers resulting from a PMMA concentration of 100 mg mL -1 .Small pores can be found on these "fine" fibers, as well as some creases.Fig. 5(A2) shows that the average fiber diameter for PMMA concentration 100 mg mL -1 was 562.69 ± 96.18 nm.Fig. 5(B1) is the SEM image for porous fibers fabricated from a PMMA concentration of 200 mg mL -1 .Pores of various sizes can be easily observed on these fibers.Fig. 5(B2) shows that the average diameter of the fibers was 2639.80 ± 268.71 nm.The diameter of the porous fibers also positively correlated with PMMA concentration.

Fiber Properties
The filtration efficiency of a filter is affected by many internal factors, including fiber diameter, packing density, thickness of bulk filter material and fiber charging.The smaller the fiber diameter, the shorter the distance between fibers, which leads to higher packing densities.The higher the packing density, the smaller the gap between fibers, and so it becomes harder for particles to penetrate and escape, which leads to better filtration efficiency.
In addition, the thickness of the filter determines the length of time it takes for the particles to pass through.The thicker the filter, the longer the retention time, and therefore, the higher the chance for particles to become trapped.
In this paper, the fibers produced by electrospinning are divided into two categories: (1) "fine" fibers with average diameters of about 500 nm and (2) "crude" fibers with average diameters of about 2500 nm.The properties of these fibers are summarized in Table 2.
On the other hand, there are many external factors influencing fiber filtration, among which face velocity is  the most important.A low face velocity not only increases contact time between particles and fibers but also enhances the effect of particle diffusion, which increases the filtration efficiency of the filter.In this study, a differential pressure meter was connected to the upstream and downstream of the fiber filter to measure the pressure drop under different face velocities.From the results shown in Fig. 6, we can see that: (1) Pressure drop is related not to the type but to the size of the fiber; (2) the smaller the fiber diameter, the greater the pressure drop; and (3) the pressure drop increases linearly with the increase of face velocity.These results were consistent with those of other studies (Ji et al., 2003;Chen and Hsiau, 2009).For all further evaluations, the following parameters were kept fixed: the thickness of fiber filter between 0.03-0.06mm; packing density between 7-13%; and face velocity at 10 cm s -1 .Further discussion on fiber charging and filtration characterization will be based on these values.

Residual Voltage and Penetration Measurement
Table 3 shows the surface voltages of porous and smooth fiber filters immediately after corona discharge.Both positively-and negatively-charged fiber filters were evaluated.When given a positive charge, the 100 mg mL -1 porous fiber was 0.220 kV higher than that of smooth fiber, and the 200 mg mL -1 porous fiber was 0.093 kV higher than that of smooth fiber.When given a negative charge, the 100 mg mL -1 porous fiber was higher than that of smooth fiber by -0.088 kV, and the 200 mg mL -1 fiber was higher than that of smooth fiber by -0.124 kV.This may be because porous fibers have a higher surface-to-volume ratio and are therefore able to carry more surface voltage.
The results also showed that the surface voltage of the fiber is positively correlated to the concentration of PMMA solutions.The thickness and filling density of 100 mg mL -1 PMMA smooth fibers are lower than those of the 200 mg mL -1 PMMA smooth fiber.A possible explanation might be that "fine" fibers do not have enough space to accommodate higher charge densities, resulting in lower surface voltages.
Fiber surface voltage stabilization diagrams after positive and negative charging are shown in Figs.7 and 8, respectively.Results of this experimental research indicate that the surface voltage capacity of porous fiber is higher than that of smooth fiber.After completion of positive charging and standing for 240 hours, the surface voltages of 100 mg mL -1 PMMA and 200 mg mL -1 PMMA smooth fiber attenuated from 0.323 kV to 0.152 kV and from 0.368 kV to 0.062 kV, respectively.For porous fibers, positive surface voltages decreased from 0.543 kV to 0.261 kV and from 0.461 kV to 0.226 kV, respectively.On the other hand, after completion of negative charging and standing for 240 hours, the surface voltages of PMMA smooth fibers at 100 mg mL -1 and 200 mg mL -1 attenuated from -0.562 kV to -0.194 kV and from -0.350 kV to -0.019 kV, respectively.The surface voltages of negatively-charged porous fiber decreased from -0.650 kV to -0.224 kV and from -0.474 kV to -0.211 kV, respectively.For negatively charged fiber filters, surface voltage attenuation was more obvious than that of the positively-charged fiber.This is possibly because electrons may be more easily dissipated in excess than stolen in deficit when subjected to friction with air.In addition, regardless of whether the fibers were positively or negatively charged, maximum voltage decline occurred during the first 24 periods, particularly for the 100 mg mL -1 fibers.We speculate that voltage decline began immediately after the charging was complete and after the external electric field was removed.
Plausible explanations for the maximum surface voltage loss in the first 24 hours are: (1) With higher surface-tovolume ratios, there are proportionately larger areas making contact and subsequently experiencing higher friction with the surrounding air; and (2) higher electric potential represents larger charge density, which makes the charge carriers on the fibers' surface more repulsive to one another.It is possible that either or both of the above situations led to the rapid decline of surface voltage during the initial process of discharge and to the fact that the phenomenon occurred faster in positive charging.However, this is only a preliminary inference, and further study is necessary to further prove this argument.

Penetration Efficiency of Charged Fiber
After the fiber filters were charged, significant changes in penetration efficiency and fiber characteristics were observed.Single fiber efficiency is one of the important indicators of filter quality.Fig. 9 shows the distribution of sodium chloride particles concentrations up-and downstream of the filtration system, as measured by SMPS.The average number of particles was 1.16 × 106 ± 5.47 × 103 # cm -3 ; the median particle size (CMD) was 66.98 ± 0.49 nm, and the measured particle size range was 14.6-661.2nm.Fig. 10 is the comparison of penetration rate between     charged smooth and porous fibers, both fabricated from 100 mg mL -1 PMMA.Fig. 10(a) shows that the average penetration rate of smooth fibers was reduced from 22.60% to 11.85% after the fibers were positively charged; for porous fibers, the reduction was from 23.06% to 7.10%.The average single fiber efficiency of the smooth fibers increased from 0.200 to 0.288; for porous fibers, from 0.225 to 0.411.
The above results show that the penetration rate of both smooth and porous fibers decreased after charging, the latter to a larger degree.Similar results were found in single fiber efficiency.A possible explanation, previously put forth by other researchers, is that at the 500 nm scale, porous fibers may carry more charge and have higher collection efficiency due to larger specific-surface area (Xia et al., 2003).
The penetration rates of 100 mg mL -1 PMMA smooth fibers and porous fibers after negative charging are shown in diagram Fig. 10(b).The average penetration rate of the smooth fiber filter was reduced from 25.60% to 12.48%, after the fibers were negatively charged.For porous fiber filters, the rate was reduced from 24.34% to 3.95%.The average single fiber efficiency of smooth fiber filter increased from 0.193 to 0.298; for porous fiber, it increased from 0.185 to 0.432.From above results, we found both positive and negative charging exerted similar effects on penetration rate and single fiber efficiency.Negative charging had slightly better outcome, and we speculate that this was due to larger variations of surface voltage in discharge period.Fig. 11 is the comparison between the penetration rates of 200 mg mL -1 PMMA smooth and porous fibrous filters after corona discharge.Fig. 11(a) shows that the average penetration rate of smooth fiber filters decreased from 69.04% to 39.26% after positive corona discharge.For porous fibrous filters, the rate decreased from 55.55% to 32.91%.The average single fiber efficiency of the smooth fiber filter increased from 0.100 to 0.264, and for porous fiber filter, from 0.267 to 0.514.At the same time, we found  that, before positive charging, the penetration rate of porous fiber was lower than that of smooth fiber.A possible reason is that the surface features of the porous fiber affected the collision properties of particles.Particle capture became easier, and the filtration efficiency was enhanced.
The penetration rates of 200 mg mL -1 PMMA smooth and porous fibers after negative charging are shown in Fig. 11(b).The average penetration rate of smooth fiber decreased from 64.63% to 42.25%, while porous fiber decreased from 57.69% to 36.66%.The average single fiber efficiency of smooth fiber increased from 0.160 to 0.326; while porous fiber increased from 0.221 to 0.418.Both positive and negative charging exerted similar effects on penetration rate and single fiber efficiency.
The data show that, for 200 mg mL -1 fibers, while charging decreased the individual penetration rates of the filters, there was no significant difference in the penetration rate differentials between porous fiber and smooth fiber before and after charging.In other words, porosity alone did increase filter efficiency but did not amplify the effects of charging.We speculate that, at the 2,500 nm scale, although the specific-surface areas of the porous fibers were larger, it was still not enough to carry sufficient charge to amplify penetration rate.However, in terms of single fiber efficiency and penetration rate, porous fibers were shown to be superior to smooth fibers in most cases.
Xia's research indicated that, after charging via corona discharge, the charge stability of porous PTFE membranes was more stable compared to nonporous PTFE filter, regardless of the polarity of the charge (Xia et al., 2003).Similarly, this study found that after either positive or negative corona charging, the filtration efficiency of filters made of PMMA micro-and nanoscale fibers improved significantly.In addition, 100 mg mL -1 PMMA porous fibers showed markedly higher efficiency after charging over PMMA smooth fiber of the same 500 nm scale.Conversely, there is no significant difference in efficiency between the two types of 200 mg mL -1 fibers (2,500 nm scale).We speculate that, at the 500 nm scale, the proportionately larger increase in surface due to porosity may be the key factor in performance efficiency.This is only a preliminary inference, however, and more study is required in this area.

CONCLUSIONS
Solutions to air pollution, especially for airborne particles matter, have become a necessity in our daily lives and in industrial applications.The main mechanisms of conventional fiber filtration include interception, inertial impaction, diffusion, and gravitational settling.Charged fibers bring the additional mechanism of electrostatic force into the arena.According to the results of our experiments, static electricity can improve the filtration efficiency of fiber filters.In addition, the filtration efficiency of porous fibers was shown to be better than that of smooth fibers.Fine fiber (on a scale of 500 nm) demonstrated better filtration efficiency than crude fiber (on a scale of 2500 nm).
From our experimental results, the following conclusions were drawn: 1.Higher surface voltages can be detected on PMMA fibers that are smaller in diameter, whose surface-tovolume ratio is larger and capable of accommodating proportionately more charge carriers.2. The penetration rate of 100 mg mL -1 porous PMMA fiber filters improved much more than that of smooth fiber filters after electrostatic charging, but no significant difference was observed between the two types of 200 mg mL -1 PMMA fiber.The speculative cause is that the 100 mg mL -1 PMMA fiber gained more specificsurface area through porosity than the 200 mg mL -1 PMMA fiber.3.In most cases, porous fibers showed superior performance to smooth fibers in single fiber efficiency and filtration quality.4.This study focused on charged fibers fabricated from PMMA.However, there are many other candidates for fiber fabrication.Activated carbon materials, for example, show great promise in this field since they are already used in fiber filters and can adsorb gaseous pollutants.We suggest activated carbon porous nanofiber as a candidate for further research.5.The characteristics of charged fibers were affected by both the method of charging and the charging properties of the fibers themselves.This study found that corona discharge was an excellent process for charging fibers, but we suggest that future studies look for even more effective charging methods and for ways to delay or eliminate charge dissipation, especially since succeeding in the latter was shown to improve filtration efficiency and quality.

Fig 3 .
Fig 3. Experimental set-up of Filtration Performance test.

Fig. 6 .
Fig. 6.The relationship between pressure drop and face velocity of fiber filter.

Fig. 7 .
Fig. 7. Decreasing pattern of voltage on a positive fiber surface.

Fig. 8 .
Fig. 8. Decreasing pattern of voltage on a negative fiber surface.

Fig. 9 .
Fig. 9. Size distribution of test particles input of filter.

Fig 10 .
Fig 10.Comparison of the penetration of charged fibers at 100 mg mL -1 (a) Positive (b) Negative.

Fig 11 .
Fig 11.Comparison of the penetration of charged fibers at 200 mg mL -1 (a) Positive (b) Negative.

Table 1 .
Preparation parameters of fiber filter.

Table 3 .
Surface voltage after fiber charging.