Polyvinyl alcohol nanofibers were prepared by a needleless electrospinning technique using a rotating spiral wire coil as spinneret. The influences of coil dimension (e.g., coil length, coil diameter, spiral distance, and wire diameter) and operating parameters (e.g., applied voltage and spinning distance) on electrospinning process, nanofiber diameter, and fiber productivity were examined. It was found that the coil dimension had a considerable influence on the nanofiber production rate, but minor effect on the fiber diameter. The fiber production rate increased with the increased coil length or coil diameter, or the reduced spiral distance or wire diameter. Higher applied voltage or shorter collecting distance also improved the fiber production rate but had little influence on the fiber diameter. Compared with the conventional needle electrospinning, the coil electrospinning produced finer fibers with a narrower diameter distribution. A finite element method was used to analyze the electric field on the coil surface and in electrospinning zone. It was revealed that the high electric field intensity was concentrated on the coil surface, and the intensity was highly dependent on the coil dimension, which can be used to explain the electrospinning performances of coils. In addition, PAN nanofibers were prepared using the same needleless electrospinning technique to verify the improvement in productivity.
Electrospinning is a simple but effective method to produce polymer nanofibers [
Efforts to improve the electrospinning productivity have been made based on different principles, such as increasing the needle number (also called multineedle setup), using air-jacket to improve the solution flow rate, and electrospinning from open solution surface (also referred to as “needleless electrospinning”). Multi-needle electrospinning is a straightforward strategy to increase the electrospinning productivity. However, the multi-needle setup usually requires a large operating space, and the relative locations of needles have to be optimized to avoid the strong charge-repulsion between the adjacent solution jets, otherwise unevenly deposited nanofiber mat may be obtained. A regular cleaning device has to be applied to each needle to prevent the blockage of the nozzles during electrospinning, which makes the whole setup inapplicable when thousands of needles are used for the nanofiber production. Dosunmu et al. [
Recently needleless electrospinning setups have been reported to increase nanofiber production rate [
Theoretically, Lukas et al. [
For a three-dimensional setup, the geometry of the spinneret greatly influences the distribution of the electric field intensity thus affecting the electrospinning process and fiber properties. However, it has been difficult to directly measure the electric field intensity of an electrospinning setup due to the high-voltage involved. Finite element method (FEM) is a numerical technique for finding approximate solutions of partial differential equations (PDE), which is used to solve a wide range of physical and engineering PDE problems. It provides an attractive method to analyze the electric field in electrospinning. Since the practical dimensions and material properties can be used for the FEM calculation, it enables one to visualize the electric field intensity profile and to understand how this profile may be influenced by the spinneret geometry as well as material characteristics.
In the previous work, we have used disc and cylinder as spinnerets to electrospin polyvinyl alcohol (PVA) nanofibers, and demonstrated the noticeable differences between disc and cylinder electrospinning processes [
In parallel, we also used a conical wire coil to electrospin PVA nanofibers [
PVA (average molecular weight 146,000–186,000, 96% hydrolyzed), polyacrylonitrile (PAN, average molecular weight 70,000), and dimethylformamide (DMF) were obtained from Aldrich-Sigma. The fiber morphology was observed under scanning electron microscope (SEM, Leica S440). The average fiber diameter was calculated from the SEM images using an image analysis software (Image
The apparatus used for needleless electrospinning of nanofibers is depicted in Figure
(a) Schematics of spiral coil electrospinning setup; (b) magnified view of the coil; photos of spiral coil spinning processes. (c) front view and (d) side view.
The electric field was calculated using a finite element method (FEM) program package COMSOL3.5. Before the calculation, the physical geometries of the electrospinning setups (e.g., spinneret, solution container, and collector), polymer solution in the container, and collector were established according to their practical dimensions, locations, and relative permittivities. A high voltage was then set to the metal wire located at the bottom of the solution bath. The metal collector and the boundaries at an infinite distance were set as zero potential. All the other boundaries were set as continuity. The meshing and solving were performed by the software to obtain the electric field intensity and profile.
Figure
Figure
SEM images of PVA nanofibers from spiral wire coil electrospinning (magnification is 1k, 5k, and 10k, resp., bar = 5
Figures
SEM images of PVA nanofibers from spiral wire coil electrospinning with different PVA concentrations. (a) 6%, (b) 9%, (c) 12%, and (d) 15% (bar = 1
The fiber diameter increased with the increase of PVA concentration in the range of 8–11 wt%. The increased polymer concentration also resulted in a reduced number of polymer jets (Figure
The electric field calculation results indicated that the electric field distributed unevenly along the coil surface, and the coil dimension affected the electric field greatly. As illustrated in Figure
Electric field intensity profiles of (a) spiral wire coil electrospinning, (b) needle electrospinning, and (c) schematic illustration of a setup used to verify the influence of protrusion length on electric field intensity. (d) Electric field intensities along the coil axis direction and (e) from fiber generators to the collector.
By combining the electric field analysis with the experimental results, the influences of coil dimension on the fiber diameter and productivity can be understood. The fiber diameter was hardly affected by the coil length
Effects of coil dimensions on the fiber diameter, productivity, and electric field intensity, (a) and (a’) coil length
When the spiral distance
Both coil diameter
In physics, the charge density on the surface of an irregularly shaped conductor is high in convex regions with a small radius of curvature. In our case, the wire with smaller diameter should have larger electric field intensity because of the smaller radius of curvature. In the case of forming greater intensity of electric field on a coil with larger coil diameter, this was attributed to the long protrusion of large coil and the influence of solution bath. In our design, the high voltage was charged on the solution bath underneath the coil. Because the liquid bath formed second electric field, it influenced the intensity of electric field on the coil surface above. This effect was verified through a simple calculation using a structure shown in Figure
The increase in electric field strength could lead to the formation of more “Taylor cones” from the coil surface and jets travelled under a higher electric field were also faster. Both effects favored to the increase in the fibre productivity.
The influences of applied voltage and electrospinning distance on the electrospinning process can be well interpreted with help of electric field analysis. For coil electrospinning, the minimum applied voltage for inducing the fiber generation was around 40 kV. Higher applied voltage led to more stable electrospinning until “corona discharge” occurred when the applied voltage was higher than a critical value. As shown in Figure
Effects of applied voltage and collecting distance on the fiber diameter and productivity. (a) and (a’) applied voltage; (b) and (b’) collecting distance.
The WAXD patterns of PVA powder and PVA nanofibers from both needle and needleless electrospinning are shown in Figure
WXRD patterns of PVA powder and PVA nanofibers.
The coil spinneret showed a better electrospinning performance compared with needle electrospinning. In coil electrospinning, the fiber diameter increased from
Comparison between needle electrospinning and spiral wire coil electrospinning, (a) fiber diameter, and (b) productivity.
The above results were based on electrospinning of PVA solutions, other materials, such as PAN, were also electrospun using the same setup and similar trends were obtained. Figure
(a) SEM images of coil electrospun PAN fibers (PAN concentrations = 12 wt%,
We have demonstrated a novel needleless electrospinning setup by using a spiral wire coil as the fiber generator. This device has been used to produce PVA and PAN nanofibers successfully. The PVA nanofiber production rate was much higher than that of single-needle electrospinning. Coil electrospinning also produced finer nanofibers than needle electrospinning. The productivity of coil electrospinning increased with the increased applied voltage and decreased collecting distance. Electric field analysis showed that high electric field intensity was generated around each coil spiral and the intensity was much higher than that of needle electrospinning nozzle. The electric field distribution was an important factor affecting the electrospinning process, fiber production rate, and resultant nanofiber property. Increasing the electric field intensity led to stronger electrostatic force resulting in thinner fibers and higher fiber productivity. This setup shows great potential in the large-scale production of nanofibers which will contribute to not only the laboratory research but also the industrialization of electrospinning.
The first two authors have equal contribution to the paper. This work was conducted in Deakin university, Austrialia, while the first author was doing his PhD study.