# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 2 Electrospraying Technology
### Mechanism of Electrospraying
\(\alpha\), \(\beta\) and \(\omega\) are special constants determined by self-solvent properties [66; 77; 78; 79]. The scaling laws of emitted current (Equations (6) and (8)) and diameter of the jet of organic solvents with viscosity (Equations (7) and (9)) were further proposed by [PERSON] et al. in 1996 [77]. As for high viscosity solution (\(\delta_{\mu}=(\frac{\mu^{3}K}{\rho\gamma^{2}\varepsilon_{0}})^{\frac{1}{3}}\geq 1\)):  
\[I=2.4(QK\gamma)^{\frac{1}{2}} \tag{6}\]  
\[D_{j}\approx 0.7(\frac{Q\varepsilon_{0}}{K})^{\frac{1}{3}} \tag{7}\]  
As for low viscosity solution (\(\delta_{\mu}=(\frac{\mu^{3}K}{\rho\gamma^{2}\varepsilon_{0}})^{\frac{1}{3}}\leq 1\)):  
\[I=8.6(\frac{QK\varepsilon_{0}\gamma^{3}}{\rho})^{\frac{1}{4}} \tag{8}\]  
\[D_{j}\approx 0.6Q^{\frac{1}{2}}(\frac{\rho\varepsilon_{0}}{\gamma K})^{ \frac{1}{6}} \tag{9}\]  
where \(\gamma\), \(\varepsilon_{0}\), \(\rho\), \(Q\), \(\mu\) and \(K\) are the liquid-gas surface tension, vacuum electrical permittivity, liquid density, liquid flow rate, liquid viscosity, and liquid electrical conductivity, respectively. The changes in trends of emitter current, as well as the jet diameter (or droplet size) with the changing of flow rate (or conductivity, surface tension), are similar not only between high conductive and high viscous solvents but also between low conductive and low viscous ones. [PERSON] et al. concluded more precise scaling laws about emitted current and droplets size for two different solutions (low viscosity as well as low conductivity solvent and high viscosity as well as high conductivity solvent) [80]. Finally, a general and unified description about scaling laws related to various parameters for the emitted current and droplet size under different boundaries was given [73]. The scaling laws allow obtaining a deeper and complete understanding of the electrospray mechanism based on the influences of the solution properties and operating parameters. Thus, according to the experimental situation, the selection of the appropriated theoretical approach predicts the formation of the cone-jet mode and provides a theoretical guide for the design of the structure and the morphology of electrospray droplets by modifying the properties of the solution [81; 82]. [PERSON] et al. found that the spray current (\(I\)) follows the scaling laws of the flow rate (\(Q\)) of the polymeric solution are found to be \(\,I\,\sim\,Q^{\frac{1}{4}}\) as well as the droplet size (\(D_{d}\)) scaling laws \(\,D_{d}\,\sim\,Q^{\frac{1}{2}}\) one [58]. Besides, [PERSON] et al. also concluded that an appropriate scale law predicts the size of polymer particles produced from the polymer volume fraction and the operating parameters of the electrospray process [43]. From these laws of scale, the efficiency of the control and design of the structure, and the morphology of the polymer particles during the electrospray process can be optimized by modifying the solution properties, such as the solvent system and operating parameters. Also, when scaling laws are used to analyze the process and outcomes of encapsulating active substance via coaxial electrospraying of the polymeric solution, the physical properties of inner and outer liquids need be considered at first. The emitted current mainly depends on the flow rate of the driving liquid, which has higher electrical conductivity. [PERSON] et al. found that its dependence on the flow rate of driving liquid fits well the \(\,I\sim Q^{\frac{1}{2}}\) law in the process of coaxial electrospraying [72]. Meanwhile, [PERSON] et al. concluded that the relationship between the diameter of the outer jet and the flow rate of the outer liquid follows the \(\,D_{j-outer}\,\sim Q^{\frac{1}{2}}_{outer}\) scaling law [83]. The relation between the droplet size and the liquid flow rate (\(D_{d}\,\sim Q\)), depends on the viscosities and electrical conductivities of inner and outer liquid solutions.