High‑Throughput Production of High‑Quality Nanofibers via Modified Free‑Surface Electrospinning

Background

Electrospinning is widely regarded as a straightforward and powerful technique for producing polymer nanofibers. Due to their high surface area, surface energy, and surface activity, electrospun nanofibers have found applications across diverse fields—including nonwoven fabrics[1], reinforced fibers[2], drug delivery systems[3], tissue engineering[4], fuel cells[5], composites[6], filtration[7], photonics[8], sensorics[9], supercapacitors[10], wound dressing[11], and more[12,13,14,15].

Traditional single‑needle electrospinning, however, suffers from low throughput, typically delivering only 0.01–0.1 g h⁻¹[16]. Consequently, there has been a concerted push to enhance production rates. Notable advances include multi‑needle systems[17], porous‑tube designs[18], free‑surface electrospinning (FSE) driven by magnetic and electric fields[19], rotating roller generators[20], needleless conical metal wire‑coil spinnerets[21], large rotating‑cone spinnerets[22], pyramid‑shaped copper spinnerets[23], gas‑pump‑induced bubble generators[24], and needle‑disk spinnerets[25]. Numerical simulations of charged jets and studies of electric[27] and magnetic field[28] effects have further elucidated jet dynamics[26].

In this work, we present a modified free‑surface electrospinning (MFSE) platform that couples a cone‑shaped air nozzle with a copper‑tube reservoir to achieve high‑throughput fabrication of quality nanofibers, building upon bubble electrospinning (BE)[24]. The copper reservoir and nozzle generate multiple jets, initiating the electrospinning process. We experimentally quantify diameter distribution and throughput, showing that increased applied voltage improves both quality and yield. Compared to BE, MFSE tolerates higher voltages, yielding finer fibers with a narrower diameter distribution and substantially higher throughput.

Surface‑active agents are routinely used to lower the surface tension of polymer solutions, thereby facilitating bubble formation and stabilization[29]. Our prior work demonstrated that even a modest addition of sodium dodecyl benzene sulfonate (SDBS) markedly reduces surface tension, promotes bubble generation, and enhances the mechanical performance of electrospun polyvinyl alcohol (PVA) nanofibers[30]. Accordingly, we incorporated SDBS into the electrospinning solution to generate bubbles at the liquid surface. We investigated the impact of bubbles on fiber morphology and production both experimentally and theoretically, confirming that bubble presence tends to decrease quality and throughput.

Methods

Materials

PVA (M_w ≈ 1750 ± 50 g mol⁻¹) and SDBS were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). A 7 wt % aqueous PVA solution was prepared by dissolving PVA powder in deionized water, followed by the addition of 0.3 wt % SDBS. The mixture was stirred at 90 °C for 2 h until homogeneous. All reagents were analytical grade and used without further purification.

MFSE Apparatus

The MFSE setup is illustrated in Fig. 1. It comprises a variable high‑voltage generator (0–150 kV, TRC2020, Dalian Teslaman Technology Co., LTD), a gas pump (TEION4500co, Eiko, Japan), a right‑circular cone‑shaped air nozzle with a gas tube, a vertical copper‑tube reservoir (inner diameter = 40 mm, height = 30 mm), and a grounded collector positioned above the reservoir. The nozzle has a 20 mm height, a 40 mm base diameter, and a 1.5 mm top diameter, fabricated from polyethylene (PE). Its tip aligns flush with the reservoir top. The positive terminal of the generator connects directly to the reservoir, defining the spinning voltage.

Schematic of the MFSE apparatus

In contrast, the BE apparatus employs a polymer tube reservoir with a thin polymer nozzle and a slim metal needle serving as the positive electrode. This configuration limits the achievable applied voltage. Consequently, BE typically produces nanofibers at only ~3 g h⁻¹ [24].

MFSE Process

Following established protocols[23,29] and our prior work[30], we operated the MFSE system with the following parameters: PVA 7 wt %, SDBS 0.3 wt %, applied voltage ranging from 30 to 70 kV, and a working distance of 13 cm between nozzle and collector. Experiments were conducted at 20 °C and 60 % relative humidity.

The PVA solution was poured into the reservoir, raising the liquid surface above the nozzle. Upon gradual opening of the gas valve, a bow forms around the nozzle due to surface tension. When the applied voltage exceeds the threshold, multiple jets initiate from the convex liquid surface, as depicted in Fig. 2.

Photograph of the MFSE without SDBS. a Liquid surface; b spinning process

Adding SDBS generates bubbles (10–30 mm diameter) on the free surface. These bubbles break into smaller ones, and when the reduced surface tension reaches a critical value, multiple jets are ejected from the bubble surfaces, as shown in Fig. 3.

Photograph of the MFSE with SDBS. a Liquid surface; b spinning process

Measurements and Characterizations

Jet dynamics were recorded with a high‑definition camera (25 000 fps, KEYENCE VW‑9000, Japan). Fiber diameter and morphology were assessed via scanning electron microscopy (SEM; Hitachi S‑4800, Japan). Samples were dried at room temperature and sputter‑coated with gold (10 min, IB‑3, Eiko, Japan). Diameter distributions were quantified using ImageJ (National Institute of Mental Health, USA). Electric‑field simulations employed Maxwell 2D (ANSOFT Corporation, USA).

Results and Discussion

Effect of Applied Voltage on the PVA Nanofibers

SEM images and diameter distributions for MFSE and BE fibers at varying voltages are shown in Fig. 4. At 30 kV, MFSE fibers exhibit an average diameter of 148 ± 8.53 nm, whereas BE fibers average 190 ± 8.26 nm—demonstrating finer, more homogeneous MFSE output. The diameter distribution narrows with increasing voltage in the MFSE process.

SEM images of PVA nanofibers. a MFSE at different applied voltages (a‑1 30 kV, a‑2 40 kV, a‑3 50 kV, a‑4 60 kV, a‑5 70 kV); b BE (30 kV). Inset: BE process photograph. Right figures are the corresponding diameter distributions.

Figures 5 and 6 illustrate how applied voltage affects average diameter and production in MFSE. Below 30 kV, jets are scarce due to insufficient electric force. At 70 kV, the electric force accelerates jet rise, yet the rapid ascent reduces further stretching, resulting in a non‑monotonic diameter trend: initial decrease followed by a slight increase. Production, however, rises steadily with voltage. These findings confirm the pivotal role of voltage in MFSE performance.

Effect of applied voltage on the average diameter

Production versus applied voltage using MFSE

Because the BE system’s positive electrode is a polymer needle, it operates at a lower voltage, limiting production to ~3 g h⁻¹ [24].

Effect of SDBS on the PVA Nanofibers

Table 1 reports the enhanced electrical conductivity and reduced surface tension of PVA solutions with 0.3 wt % SDBS. Figures 7 and 8 present SEM images and diameter distributions for fibers spun from SDBS‑containing solution at 60 kV over varying spin times. While the average diameter is larger than that of pure PVA, the diameter distribution remains stable across time, and the throughput reaches 12.5 g h⁻¹. These results indicate that bubble generation, while increasing diameter, slightly lowers production—likely due to energy lost in bubble deformation and breakup, which reduces jet acceleration and thus throughput.

SEM images of PVA nanofibers prepared by MFSE at different spinning times (a‑1 5 min, a‑2 10 min, a‑3 15 min, a‑4 20 min, a‑5 25 min). Right figures are the corresponding diameter distributions.

Effect of spinning time on the average diameter of PVA nanofibers prepared by MFSE

Mechanical testing of membranes fabricated from PVA with and without SDBS is summarized in Table 2. Both tensile strength and elongation‑at‑break improve upon SDBS addition, underscoring the surfactant’s role in enhancing mechanical performance.

Theoretical Analysis

Since the electric field is the primary driver of jet formation, we modeled the field distribution around the free surface and bubbles using Maxwell 2D. Figure 9 displays the simulated fields for a 13 cm working distance and 60 kV applied voltage, considering a copper reservoir (40 mm × 30 mm), bubble diameters of 20 mm and 25 mm, and solution properties (surface tension 45 mN m⁻¹ for pure PVA, 33 mN m⁻¹ with SDBS; conductivity 8.8 µS cm⁻¹ and 43 µS cm⁻¹, respectively).

Simulation of electric field distributions at 60 kV (working distance 13 cm). a Around the free surface; b Around the bubbles.

Figure 9a reveals a highly heterogeneous, intense field at the curving free surface, particularly near the reservoir edge—a region where jets first emerge. In contrast, bubbles exhibit lower field intensity, explaining why bubble‑induced jets stretch less and contribute to larger fiber diameters and reduced throughput. The theoretical predictions align well with experimental observations.

Conclusions

We introduced a modified free‑surface electrospinning system that couples a cone‑shaped air nozzle with a copper‑tube reservoir, enabling high‑throughput fabrication of high‑quality nanofibers for extended spinning periods. Systematic studies confirm that increasing applied voltage enhances both fiber quality and production. Compared to bubble electrospinning, MFSE tolerates higher voltages, yielding finer fibers, tighter diameter distributions, and markedly higher throughput.

Incorporating SDBS into the PVA solution generates surface bubbles, which, while slightly increasing fiber diameter, modestly reduce production—likely due to energy loss during bubble formation and breakup. Mechanical testing shows that SDBS improves tensile strength and elongation‑at‑break of the resulting membranes.

Electric‑field simulations using Maxwell 2D corroborate the experimental findings, demonstrating that higher field intensity at the free surface drives jet formation more effectively than at bubble sites.

Abbreviations

BE:

Bubble electrospinning

Co., Ltd.:

Limited company

FSE:

Free surface electrospinning

MFSE:

Modified free surface electrospinning

PE:

Polyethylene

PVA:

Polyvinyl alcohol

SDBS:

Sodium dodecyl benzene sulfonate

SEM:

Scanning electron microscopy

wt%:

Weight fraction