# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 2 Electrospraying Technology
### Mechanism of Electrospraying
The spray mode changes gradually from dripping mode to cone-jet mode with increasing the applied voltage. Then, the spray mode is further transmitted from stable cone-jet to multi-jet mode (unstable cone-jet mode) when the applied voltage beyond a certain range [61]. Therefore, a suitable applied voltage to reach a balance among electrostatic, viscoelastic forces and surface tension in a droplet at the tip of the nozzle is required for achieving the cone-jet of the electrospray solution. The value of working distance from nozzle to collector also affects the formation of Taylor cone and the breakup of the charged droplets via changing the strength of electric field. According to [PERSON] et al. [62], the achievement of a stable cone-jet rises with the use of a lager tip-to-collector distance and smaller flow rate. Apart from operating parameters, the success of stable cone-jet and the breakup of droplets are also closely related with the physical properties of electrospray solution [63; 64; 65]. The value of electrical conductivity of electrospray solution affects the charge quantities on the surface of droplets and further influences the strength of electrostatic force. Several authors have concluded that cone-jet mode was obtained in air only if the electrical conductivity of the liquid was set in a certain range [66; 67]. As for the upper limit, [PERSON] et al. [68] have found that jet changes from a permanent to an intermittent jet, when the conductivity of the solution beyond than \(10^{-5}\) S/m. For lower limit, estimates generally vary between \(10^{-8}\) and \(10^{-10}\) S/m. Besides, the range from the upper and lower limits of conductivity broadens when the solution has low surface tension. On the one hand, if the surface tension is too high, the formation of the Taylor cone at the tip of the nozzle is maintained due to the change in spherical shape of the liquid. On the other hand, corona discharge results from excessively high surface tension at the tip prevent the electrostatic atomization of charged droplets [69]. According to [PERSON] et al., the atomization of charged droplets in cone-jet mode cannot be achieved if the surface tension of electrospray liquid exceeds 50 mN/m [70]. The upper limit of the surface tension of the solutions varies, according to the properties of solute and solvent, and the operating parameters of the process. Furthermore, the viscosity of the solution needs also to be considered, since it influences the formation of cone-jet. Some researchers concluded that the solution with higher viscosity is efficient to maintain the cone-jet mode during the electrospray process [71; 72]. These relationships among operating parameters/solution properties and the characteristics of electrospraying (including jet's diameter, droplet size, and emitted current) are also calculated quantitatively by scaling laws [73; 74]. In 1994, [PERSON] et al. firstly proposed emitted current (\(I\)) scaling law (Equation (2)) and jet's diameter (\(D_{j}\)) scaling law (Equation (3)) based on the electrospraying of some pure organic solvents with high conductivity and low viscosity [75]. Meanwhile, the scaling laws of droplet size (\(D_{d}\)) (Equation (4)) as well as emitted current (Equation (5)) of non-polar organic solvents were also calculated [76]. \[I=f(\varepsilon)(\frac{\gamma KQ}{\varepsilon})^{\frac{1}{2}} \tag{2}\]  
\[D_{j}\sim D_{d}=\alpha(\frac{\varepsilon}{\varepsilon_{0}})^{\frac{1}{2}}( \frac{Q\varepsilon_{0}}{K})^{\frac{1}{3}} \tag{3}\]\[I=\beta(\frac{QK\varepsilon_{0}\gamma^{3}}{\rho})^{\frac{1}{4}} \tag{4}\]  
\[D_{j}\sim D_{d}=\omega Q^{\frac{1}{2}}(\frac{\rho\varepsilon_{0}}{\gamma K})^{ \frac{1}{6}} \tag{5}\]  
where \(\varepsilon_{0}\), \(\gamma\), \(\rho\), \(K\), \(\varepsilon\) and \(Q\) are electrical permittivity of a vacuum, surface tension of solution, density of solution, electrical conductivity of solution, dielectric constant of solution and flow rate of electrospray liquid, respectively.