Electrospinning with curved nozzle spinneret for producing poly (vinyl Alcohol) nanomembrane for filtration

To produce a consistent and fine nanofiber with a higher yield, electrospinning with a redesigned spinneret has been used. The principle that a curved or sharp edge activates a greater electric field intensity that stretches the jet into a thinner and denser electro spinning nanoweb is the subject of this research. In the electrospinning process, a curved nozzle spinneret outperforms a circular nozzle spinneret and a standard needle type in terms of electric field intensity and dispersion. It has been discovered that a high electric field intensity causes a 30% reduction in fiber diameter, the development of a denser fiber web, and an increase in production rate up to 280 mg h−1.For testing as an air filtration material, the electro-spun nano web of Poly (vinyl alcohol) (PVA) is accumulated as a membrane and sandwiched with polypropylene nonwoven fabric. The nonwoven membrane can filter particles down to 2–3 microns, whereas the sandwiched PVA nano-fiber can filter particles into nanometers. This research could lead to the low-cost manufacture of Nano-membranes using a simple electrospinning setup.


Introduction
The manufacture of nanofibers relies heavily on electrospinning. Although, it is noted that needle electrospinning might offer only low production with varying diameter of fibers [1]. In this study, the performance of a curved nozzle spinneret as a feeder electrode in producing Poly Vinyl Alcohol (PVA) nanofiber is compared to that of a circular nozzle spinneret and conventional needle electrospinning in terms of jet path, fiber diameter, and web deposition. The preliminary stabilization of the Taylor cone is highly crucial in gaining a stable nanofiber throughout this process as the initial spun jet can quickly become unstable due to processing and environmental factors [2]. A curved nozzle spinneret is employed as an electrode in this electrospinning setup to provide stable initial spin jets, provide steady spraying of the electro spun fiber on the substrate, and expedite solvent evaporation. Insufficient area for solvent evaporation prior to fiber solidification using circular and needle spinneret is one of the major limitations highlighted in the cited research papers. The curved nozzle spinneret, on the other hand, provides enough spaces to control solvent evaporation. The fiber diameter and deposition area are measured using this experimental setup. The rate of production using a curved nozzle spinneret was also tested and found to be more effective [3].
In terms of applicability, electro spun nanofiber-based filters outperform microfiber filters in terms of filtration efficiency and service life [4]. However, these filters still have significant limitations, such as inadequate filtration performance due to thick fibers (>100 nm) and a relatively large pore size. The improved electrode reduces the fiber thickness to less than 100 nm, making it more suitable [5]. Electrospun fibers with a diameter in the micro to nanometer range, a high porosity, and small pores with high pore interconnectivity are employed in a wide variety of applications including filtration, tissue engineering, energy conservation, reinforcement, and environmental protection [6]. The needle electrospinning process produces nanofibers at a rate of between 10 and 100 mg h −1 [7]. Within electrospinning process, minimum rate of production owing to single jet forming as well as the materials blocking up at the tip of needles are the limitations of single needle spinneret [8]. Electrospinning with several needles demands a big operating area and needle space. Additionally, due to frequent blockage and bead formation, production is intermittent [9]. Due to the faster solution evaporation prior to fiber creation, free surface electrospinning also produces more non-uniform nanofibers. Electrospinning is often characterized as revolving or stationary based on the movement of the spinneret [10]. Controlling solvent evaporation is impractical in the revolving type, however in the stationary type, it is easy to design and resize the spinneret to control solvent evaporation and generate homogeneous electrospun fiber. The solution is reserved and the flow is controlled in a curve nozzle spinneret. Thus, electrospinning with a curved nozzle spinneret results in reduced polymer blockage, a smaller fiber diameter, and increased output with a denser web due to the increased electric field generation.
The next sections discuss the efficiency of the modified nozzle curve spinneret on compact jet path, electric field intensity, nanofiber diameter, and deposition area. According to Nurfaizey et al [11], the magnitude movement of the deposition area of electro spun nanofiber was investigated using an auxiliary electric field. According to Wu et al [12], when an auxiliary electrode was used, the fiber diameter changed due to the auxiliary field, improved convergence, and deposition of nanofibers. Arras et al [13] demonstrated that by applying an auxiliary electric field to a turnable auxiliary electrode, they were able to generate an aligned fiber with an intensive deposition region without using the target electrode. Bellan et al [14] suppressed chaotic whipping using an electrode and concentrated on the fiber deposition area. Kim et al [15] used a cylindrical auxiliary electrode to create a programmable three-dimensional scaffold structure in relation to fiber deposition and alignment. According to Yang et al [16], the von Koch curve can be used to manufacture a fine and uniform nanofiber. It enabled a high level of output at a low cost. According to Liang Wei et al [17], the applied voltage, collector distance, and solution flow rate all have an effect on the morphology, diameter, and productivity of annular spinnerets. Soliman et al [18] conducted an analysis on the control of porosity by varying the fiber diameter and packing density of the fibers. A. Mukhopadhyay [19] discussed how sandwiching nanofiber between non-woven fabric strengthen applications. Song et al [20] investigated the porous structure, porosity, air permeability, and quality factor of a nanofiber membrane used for aerosol filtering. Yan et al [21] studied the effect of solution temperature on fiber diameter, packing density, and fiber production in nanofiber mats. Kim et al [22] examined the usage of auxiliary electrodes of varying dimensions in order to determine the electric field concentration and the effect of the electric field at the nozzle tip. According to Kim et al [23], a slightly modified electrospinning technique included an extra electrode, an air blowing system, and a guiding electrode.
Numerous research studies have demonstrated that nanofibrous filters have a good capacity for removing particles between 0.1 and 0.5 μm. additionally; they discovered that the solitary Poly Vinyl Alcohol PVA electrospun web had higher filtration efficiency than standard polypropylene nonwoven filter fabric, but a lower strength. The same was sandwiched between the polypropylene nonwoven textiles to add strength. It has the potential to be employed as a filter fabric for microorganisms, microparticles, and pollutants, which is a significant advancement in light of the COVID pandemic and environmental pollution that have surfaced internationally.

Experimental details 2.1. Polymer material and solution preparation
Poly(vinyl Alcohol) (PVA) of molecular weight (M w = 80,000-89,000) fully hydrolysed, degree of saponification = 99%) and distilled water are obtained from Southern India Scientific Corporation. A solution of PVA is formulated through dissolving PVA powder in obtained water, heated at 60°C in a hot-water bath for about 30 min and stirred actively using a magnetic stirrer up to 6 h to obtain a clear and uniform solution.

Set up of nozzle spinneretelectrospinning and experiments
Various nozzle spinnerets are investigated in this electrospinning method under the following processing conditions: temperature (25°C), humidity (65%), voltage (30 kV), flow rate (0.5 ml hr −1 ), distance between spinneret and collector (12 cm), collector speed (1100 rpm), and 3-h time period with PVA concentration of 7.5%. The nozzle spinneret has a diameter of two centimeters at the bottom, a height of five centimeters, a diameter of four centimeters at the top, and an orifice radius of four centimeters. The electrospinning process using nozzle spinnerets is illustrated schematically in figure 1.
Various process conditions, such as applied voltage, flow rate, and distance, are explored prior to selecting the ideal process settings. The experiment utilizes an applied voltage range of 1 to 30 kV. It has been shown that at voltages less than 10 kV, no fiber production occurs. Between 10 and 20 kilovolts, polymer droplets flow outside the collector, but between 20 and 25 kilovolts, fiber production with additional beads on the collecting plate is observed. At 30 Kv, a fiber with a homogeneous structure and nearly no beads forms. The experiment regulates the flow rate between 0.1 and 10 ml hr −1 . Between 0.1 and 0.4 ml hr −1 , a small amount of fiber producti on with no uniform structure is detected. The fibred is generated in a homogeneous structure without a bead at 0.5 ml hr −1 . At flow rates greater than 0.5 ml hr −1 , droplet production and spillage are observed all around the collector. The distance between the spinneret and the collector varied between 1 and 12 cm in this experiment. Between 1 and 5 cm, solidification of fiber takes longer and solvent evaporation is slow. Between 5 and 10 cm, solidification of fiber occurs rapidly and solvent evaporation is gradual, in comparison to the results obtained between 1 and 5 cm. At 12 cm, it is observed that fiber solidification and solvent evaporation occur quite rapidly. As a result, the ideal process settings are determined as 30 kV voltage, 0.5 ml hr −1 flow rate, and 12 cm distance between spinneret and collector. The nozzle of spinnerets was used as a feeder electrode in figure 2. When a high voltage of 30 kV is supplied, the solution flows through the nozzle spinnerets, increasing the electric field intensity between the electrodes (spinneret to collector). When it was extended, it produced a cone structure known as a Taylor cone. Between distances of 12 cm from the spinneret tip to the collector, the polymer solution showed simultaneous instability and extension. Meanwhile, the solvent evaporated and the nanofiber formed. Finally, the nanofibers formed a web on the collector, which rotated at 1100 revolutions per minute. To determine the PPE (Particulate Penetration Efficiency) in air filtration, the nano web was collected on a substrate layer made of 18 grams per square meter polypropylene non-woven fabric (GSM).

2.3.
Characterization of the pva polymer and analytical measurement 2.3.1. Scanning electron microscopy (sem) analysis The diameter and shape of the PVA nanofibers are determined using SEM. A sample of nanofiber web (1 cm×1 cm) is collected and bonded to an aluminium stub using double-sided carbon adhesive tape. The stub is placed in a sputter coater device for ten minutes to coat the sample with a thin layer of gold, which eliminates image  artefacts caused by excessive surface charge. SEM microscopy is used to examine the nano fiber materials (SEM). The diameter is determined using image analysis software (Image Pro+4.5) by randomly selecting pictures from the SEM (figures 3, 4 and 5). The diameter of each spinneret is measured 10 times, and the mean, median, standard deviation, maximum value, and minimum fiber diameter are calculated as given in table 1.

Spectroscopy (Ftir) analysis for fiber web
The PVA nano web was analyzed using FTIR spectroscopy to determine the distinctive peak of the produced membrane's functional group. The existence of hydroxyl (OH) and CH2 is shown by the 3300 cm −1 and 2942 cm −1 peaks, correspondingly, while the terminal polyvinyl groups and carbonyl (C=O) stretching bond are indicated by the 1088 cm −1 and 1735 cm −1 peaks, respectively [25]. The composition of the PVA nanofiber membrane is validated by FTIR analysis, which reveals the presence of a variety of functional groups. Graphs 6 (A) shows the infrared spectrum of a PVA electrospun nano web is Fourier transformed.  The composition of the PVA nanofiber membrane is validated by Raman spectroscopy, which demonstrates the existence of many functional groups in PVA. Figure 6(B). The figure illustrates the FTIR and Raman spectroscopy results for a PVA electrospun nano web.

Experimental resultsand discussions
3.1. Theroy behind curved nozzle spinneret Any curved surface of the electrode concentrates the electric charges, resulting in an increased electric field. The rise in degree of curvature from a flat surface is depicted in figure 7. The increased electric field causes an increase in the mobility of charged materials.
The electric field applied on the external part of the spinneret with different shapes is given by the formula (equation (1)) At curved surface, the charge density is higher as more charges are present on the surface. It is known that equation (2) is Where; E is electric field, V is voltage and d is distance between equipotential lines. In curved surface, distance is small compared to circular surface, which, in turn, increases the electric field. A huge number of charges allocate themselves in such way that the repulsive force effect is reduced with the fact that it is directly perpendicular to the surface. Figure 8 shows the repulsive force effect of the charges on curved surface. The Koch curve [16] is further created by initiating with an equilateral Triangle, and then recursively altering each line segment as shown in figure 9.
Due to the circular surface's lack of curvature, it possesses a weak electric field. The lines of force and the field are equally spaced in a flat conductor. In the case of a curved surface, the lines of force are closely spaced closer to  the conductor than they are away from it. As a result, the curved surface generates a stronger electric field near the conductor, which aids in initiating the electrospinning process.

Effect of curvednozzlespinneret on fiber diameter
According to Aya Hamed et al, using a conical spinneret as a feeder in an electrospinning setup resulted in a slight reduction in nanofiber diameter when compared to the needle type [26]. However, this article conducted a series of testing with various types of nozzle spinnerets, including star nozzle, bi-lobal nozzle, and tri-lobal nozzle, to see which one could create fiber with a smaller diameter. According to the average fiber diameter attained, curved nozzle spinnerets are expected to be more effective at manufacturing fine nanofibers as small as 70 nm.

Effect of curvednozzle spinneret on production
Chien-Teng Hsieh et al shown that needle electrospinning provides a modest yield of 10-100 mg h −1 [7], which is sufficient for project purposes only. However, this work is focused on producing a high yield for industrial or commercial use. At every 60 min of spinning time, the curved nozzle spinneret produces more than the needle and round nozzle spinnerets. This is because of the structural geometry of the curved nozzle spinneret, which   resulted in high electric field intensity, an uninterrupted jet, and the creation of fewer beads. Additionally, it could be explained by the strong synergy between the electric field and viscous force in the first droplet of PVA solution. Table 2 provides the output of nozzle spinnerets and needle.

Effectof curvednozzle on deposition area of fibers
Aya Hamed et al [26] demonstrated that the electric field decreases as the spinneret dimension increases, which may result in nanofiber creation straying from the collector. The spinneret with a top diameter of 3 cm, a base diameter of 6 cm, a height of 7 cm, and an opening of 0.4 cm was employed in their work. The purpose of this work is to present a curved nozzle spinneret with more pointed edges and a reduced dimension in order to obtain a larger electric field. When the electric field is increased, the fiber deposition is more concentrated on the spot than when using a circular nozzle spinneret or a needle spinneret. With a curved nozzle spinneret, the fiber deposition area is reduced to less than 5 cm. However, the deposition area of a needle spinneret and a circular nozzle spinneret is 13.5 and 6 cm, respectively (table 3). On a collector plate and beyond, the fiber deposition region created by the needle spinneret is collected. In a circular nozzle spinneret, the fiber web is formed randomly around the collecting area, but in a curved nozzle spinneret, the fiber web is formed in the collection plate's center. Figure 10 illustrates the deposition area for the (A) curved nozzle spinneret, (B) circular nozzle spinneret, and (C) needle spinneret.

Effect of packing density with curvednozzle spinneret in nanoweb
The thickness of the resulting web (figure 11) has been determined using SEM analysis of the fiber's crosssectional area, and it ranges between 50 m and 110 m. The effective area of fiber packed density is calculated by subtracting the porosity area from the total area, taking into account the web thickness. The packing density of fibers in a curved nozzle spinneret, a circular nozzle spinneret, and a needle spinneret is calculated to be 0.6079, 0.56591, and 0.3110, respectively. This demonstrates that the curve nozzle spinneret achieved a better fiber packing density and productivity than the needle and round nozzle spinnerets. The formula below formula is applied to calculate the web's fiber packing density (equation (3)). Table 4 shows the different types of spinnerets and their outcomes. where, G is the grammage, Z is the thickness of the media and r Fi is the density of fibers.

Effect of nanofibrous membrane in air filtration
Filtration is one of the fastest growing segments for the nanomembrane due to its ability to be engineered very precisely to meet exacting specifications and stringent regulatory requirements for air, liquid, bacteria, and dust filtration in gas respirators, vacuum cleaners, kitchen hood filters, and dust removal, among other applications. It is demonstrated that the fiber shape (beaded or fine nanofibers) has a significant effect on the membranes' filtration performance. The inclusion of beads increases the distance between the fibers, resulting in a reduction in pressure drops. Additionally, the most critical parameters determining the quality of nanofiber membrane filters are the fiber diameters (or distribution of fiber diameters), the fabric porosity, and the fabric's homogeneity. While nanofiber membranes include a huge number of microscopic pores smaller than the size of the targeted particles, they also capture particles smaller than the pore diameters due to the single fiber filtration mechanism.  Additionally, nanofibers exhibit a unique nano-size aerodynamic impact when their fiber diameters are less than 100 nm. This slip flow effect enables particulates to deposit and remain on the surface of the fine nanofibers with an increased pressure drop and particle retention capacity, while the dust remains easily cleaned by shaking loose particles from the surface or by using an automated clean air back-pulse system. Numerous nanofiber filters are so frequently constructed in composite structures, which typically consist of a thin layer of nanofibers covering a spun-bond nonwoven foundation substrate. Additionally, as illustrated in figure 12, the PVA nano web is sandwiched between the polypropylene nonwoven textiles to increase filtration efficiency and robustness. We utilize a nonwoven fabric with an area density of 18 gm −2 and a thickness of 0.2 mm. A solitary PVA nano-web is more efficient at filtering than conventional non-woven and woven filters but has a lower strength [24]. The web's air permeability is determined using an air permeability tester (ASTM D737-18). A sample of a 5 square cm nano-web is tested and measured under a 120-pascal front-to-back air flow pressure. Among other benefits, the web created by the curved nozzle spinneret results in a reduction in porosity size and an increase in fiber packing density, resulting in improved filtration performance. Table 5 air permeability and porosity of the Nano web filter is experimented. It shows that when the deposition of fiber per unit area is more, there is a decrease in the porosity, resulting in higher filtration efficiency. Measurement of air permeability and distribution of fiber deposition in a focused way suggest that the Nano fiber produced from a curved spinneret has a great potential in air-filtration. Normally, the value of the porosity of the nanoweb is 0 to1.
The porosity has been calculated based on the formula given below in equation (4).
Where m is the weight (mg) of the membrane samples measured by an electronic weighing machine, r is the density of PVA raw material, z is the thickness (mm) of the membrane samples and S is the samples size (mm 2 ) of the relevant samples. Knudsen number (Kn) derived for the sandwiched web made out of Circular, curved, needle nozzle spinneret with the below stated formula (equation (5)) is 1.8694 where λ is the mean free path of air molecules (66 nm at ambient temperature and pressure) and d f is fiber diameter. It implies that higher the Knudsen number, higher is the air filtration efficiency. The Knudsen number varies from 0.25-10. The character of hydrophilic PVA nano web and the hydrophobic nonwoven fabric opens up a new avenue in the field of filtration. Absorbency of the PVA nano web is expected to be greatly helpful in filtering out micro particulates.

Conclusion
A curved nozzle spinneret is utilized in the modified electrospinning process, and its performance is compared to that of a circular nozzle spinneret and a needle spinneret. This novel spinneret generates a high-intensity and uniform electric field, which results in fiber diameter reduction, increased productivity, concentrated deposition area, compacted jet path, high fiber packed density, and, most importantly, more uniform fiber due to the reduction of abrupt solvent evaporation. Additionally, as the electric field grows, the jet's focus toward the collector increases, flight time lowers, the number of bending cycles drops, the fiber diameter increases, and the fiber becomes more compact. This approach has been shown to be more effective in achieving fiber diameters as small as 70.619 nm. Nanofiber output reached around 280 mg h −1 , indicating a better productivity than conventional needle electrospinning. When fiber size is reduced, the most penetrating particle size (MPPS) decreases, resulting in increased total filtering efficiency. The increased filtration effectiveness is due to the greater surface area per volume available for particle capture, particularly for small submicron meter particles. The sandwiched filter's porosity and air permeability demonstrate its potential for use as a filter. This spinneret would be advantageous in electrospinning, allowing for the production of consistent nanofiber webs at a far  cheaper cost. It may be utilized efficiently as a filter membrane in sandwiched or raw form to catch a large number of pollutants and bacteria. In future work, we will examine how the efficacy of antimicrobial, antiviral, electret-enhanced nanomembranes can be used to improve the filtering of extremely tiny micro and nanoscale materials that can be employed as face masks and other personal protective equipment (PPE) (Personnel Protective Equipment).

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.