# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 2 Electrospraying Technology  
### Mechanism of Electrospraying  
Electrospraying is a method of liquid atomization that achieves the stretching and breakup of polymeric solution to obtain micro- or nanoscale particles via means of electrical forces [51]. Six types of forces govern the electrospray process, i.e., (i) gravity of polymeric solution, (ii) electrostatic force generated from external electric field between nozzle and collector, (iii) Coulomb repulsion force among adjacent charged carriers on the surface of jet, (iv) viscoelastic force of polymeric solution, (v) surface tension in the interface between air and liquid, and (vi) frictional force between charged jet and surrounding air (Figure 1) [52; 53]. Among these forces, electrostatic force, Coulomb repulsion forces, viscoelastic force, and surface tension affect the stretching as well as atomization of polymeric droplets obviously during the electrospray process. When polymeric solution flows out of the nozzle, the charge distribution and carried charge quantity on the surface of the polymeric solution will change in varying degrees (some extent) according to its electrical conductivity and dielectric constant because of the polarization effect coming from the external electric field. At the same time, initially, the uncharged liquid becomes charged jet and is further stretched towards the direction of electrostatic attraction. However, compared to gravity and electrostatic force that accelerates the moving and stretching of polymeric solution from the nozzle to the collector, the surface tension and viscoelastic ones prevent this moving and elongation because of their opposite behavior on the electrospray solution. When these forces reach a balance at a certain range, the droplets at the tip of the nozzle are stretched from the spherical surface into conical surface [54]. In 1964, [PERSON] proposed, for a perfectly conducting liquid, a first explanation of the conical shape formation, corresponding to a hydrostatic balance between electrostatic forces and surface tension [55]. The presence of the conical surface at the tip of the nozzle during electrospraying is also called Taylor cone. According to [PERSON]'s theory, when the charge quantity distributed on the surface of droplets reaches the value range between 50% and 80% of the Rayleigh limit (Equation (1)), the breakup and fission of charged droplets will occur due to Coulomb repulsions among charged droplets beyond the binding force coming from surface tension that held the droplets together [56]. The aggregation among electrospray particle is depressed during their flight from the nozzle to collector due to Coulomb repulsions, to induce the formation of tiny solid particles after solvent evaporation. Figure 1: Schematic representation of the formation of cone jet as well as the breakup process of charged jet in electrospraying process indicating the controlling forces. \[q_{\rm lim}^{2}=8\pi^{2}\epsilon_{0}\gamma D_{d} \tag{1}\]  
where \(q_{\rm lim}\), \(\epsilon_{0}\), \(\gamma\), and \(D_{d}\) are the charge quantity of Rayleigh limit, the dielectric constant of the medium surrounding the droplets, the surface tension of electrospray liquid, and the diameter of droplets, respectively. The formation of a Taylor cone and the fission of charged droplets are crucial for preparing PCL particles with idea structure as well as morphology via electrospray. On the one hand, the size of the electrospray particles can be dispersed into micrometer or even nanometer range by the Coulomb repulsions, and the mono-dispersion of particles is obtained under cone-jet mode. On the other hand, the effective design and control of the structure, as well as the morphology of electrospray particles and the process reproducibility is achieved easily under cone-jet mode due to its stability and continuity. The effects of various parameters on the formation of the Taylor cone as well as the breakup of the charged droplets have been the topics of several research projects [57; 58; 59; 60].