**2.2. Applied voltage**

The electrospinning process generates fiber when the applied voltage surpassed a given value required to balance the surface tension of the solution or melt. The electrical field intensity is estimated as the applied voltage divided by the distance between the tip and collector in most research. A higher electric field intensity value is obtained either through decreasing the distance between the tip and collector or by applying higher voltages. In 1969, Taylor deduced the threshold voltage of electrospinning [24], which defined the relationship between thresh‐ old voltage and the processed material. That is to say when the applied voltage exceeds this value, it breaks the balance between the electric force and the surface tension of the droplet, so a jet is ejected.

For solution electrospinning, it is commonly found that fiber diameter decreases with an increase in the applied voltage. However, for some polymer solutions like polyvinyl alcohol (PVA) and polyethylene oxide (PEO), they do not follow this rule [25]. The applied voltage has an important effect on non-woven morphology as well. Too weak or too strong an electrical field intensity, may cause beads of fiber in solution electrospun, non-woven fabrics and result in a rough surface [25]. For different materials, there is a suitable applied voltage range when other parameters are fixed.

For melt electrospinning, because of the dielectric properties and high viscosity of the polymer melt, the applied voltage is more than 2 times that used in solution electrospinning. Therefore, 20–100 kV is needed to polarize the polymer melt, and induce the generation of polymer jets [26], while usually 5–20 kV is loaded on the end of the syringe needle in solution electrospin‐ ning. Increasing the voltage is a common measure used to obtain finer fiber in melt electro‐ spinning, but it can cause corona or breakdown if the voltage is too high. Ratthapol [27] proposed a vacuum melt electrospinning method, improving the threshold voltage, in which the loading voltage can reach 1–30 kV/cm without breakdown, however, this method may prove costly if used in large-scale production.
