# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 2 Electrospinning
This encourages greater polymer-polymer contact and thus an increased solution viscosity. Therefore, choosing a suitable solvent for the polymer is necessary to produce an appropriate viscosity for electrospinning. The initial formation of the jet also relies on electrostatic force overcoming the surface tension of the solution, thus a balance between surface tension and applied voltage is necessary to maintain stability. If the surface tension is too high, the jet will not be able to maintain stability and beads will form within the jet. Finally, the solution should be sufficiently conductive to carry charges that can assist in stretching the jet and overcoming the surface tension. The parameters of the electrospinning setup must also be optimized for the successful formation of homogenous fibres [38]. The formation of a jet relies on a voltage that is greater than the critical voltage (\(V_{C}\)). This is the voltage at which the surface tension is overcome and repulsive forces begin to stretch the jet. The \(Vc\) can be calculated according to the [PERSON] calculation [38; 40]:  
\[V_{C}^{2}=4\frac{H^{2}}{h^{2}}\left(ln\left(\frac{2h}{R}\right)-1.5\right)(1.3 \pi R\gamma)(0.09) \tag{2}\]  
where \(H\) is the distance between the spinneret and collector, \(h\) is the length of the liquid column, \(R\) is the inner radius of the needle or spinneret, and \(\gamma\) is the surface tension of the spinning solution. Below this voltage, electrospinning will not occur. Above \(V_{C}\), there is  
Figure 1: Basic electrospinning process and variables. a voltage range that will allow the formation of stable and continuous nanofibres. As the voltage increases, the jet accelerates, moving the solution from the tip of the needle to the collector faster. If the voltage is too high, the base of the jet becomes unstable and may recede into the spinneret. However, the effects of voltage on nanofibre diameter are still unclear. An increase in voltage can encourage the jet to stretch, resulting in finer fibres. However, it also accelerates the speed of the jet, which causes the fibres to move toward the collector more quickly, resulting in larger fibre diameters. As the voltage affects the electrical field, so too does the distance between the spinneret and collector. The distance between the tip of the spinneret and the collector can be adjusted to manipulate the electric field strength and the acceleration of the jet [38]. When the distance is small, the electric field becomes stronger, and the acceleration of the jet is high. This can result in fibres with larger diameters and residual solvent, as there is little time for the jet to stretch and for the solvent to evaporate. At a larger distance, there is more time for the jet to stretch, thus thinner fibres may form. However, increasing the distance too much may lead to thicker fibres due to a weakened electric field strength. Furthermore, the flow rate determines the amount of polymer solution that is exposed to the electric field and drawn into the jet at a given time. Although it is thought that the flow rate has a lower impact on the electrospinning process, it must still be optimized to avoid interruptions in electrospinning or the formation of beads. Finally, the ambient temperature and humidity should be considered to achieve the desired fibre morphology. An increase in ambient temperature can cause a decrease in the viscosity of the polymer solution, which may result in an increase in fibre diameter [41]. Low humidity can increase the rate of solvent evaporation, potentially leading to roughness in fibre diameters. High humidity may result in water droplets from the air condensing onto the jet, creating a porous fibre structure [38]. High humidity can also cause the discharge of electricity into the air, posing a potential safety hazard. Although electrospinning is a relatively simple and inexpensive process to perform, there are many factors that can impact the resultant fibre diameter.