# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 4 The Effect of Operating Parameters and Solution Properties in PCL Electrospraying  
As a multi-physical process, the process and results of electrospraying, Taylor cone formation, and particle morphology are determined not only by the properties of the starting solution, but also by the setting of the machine parameters, i.e., (i) the applied voltage, (ii) the working distance, (iii) the liquid flow, and (iv) the nozzle type. The adjustment of the various working parameters, to obtain a conical jet during the electrospray process, is mainly governed by the physico-chemical properties of the solutions. The study of the relationships between the conical jet working range and the related properties of the solutions provides a better understanding of the importance of choosing appropriate solvent systems and theoretical knowledge for future system optimization. The value of the voltage applied between the nozzle and the collector is one of the parameters to be controlled for process optimization since it influences the stretching of the jet and then the formation of a Taylor cone at the end of the nozzle. Its increase leads to a gradual increase in electrostatic forces acting on the surface of the charged droplets. The spray mode gradually changes from drip to multi-jet mode, including micro-drop, pulsed cone-jet, and stable cone-jet modes as the applied voltage increases [62; 158]. Drop-to-drop mode is obtained for low voltage values when the electrostatic forces acting on the surface of the spray droplets are not sufficiently strong to exceed that of the surface and viscoelastic tension, and it leads to the production of films. In the dripping regime, drops of electrified liquid form at the end of the capillary until the combined effect of gravitational and electrical forces exceeds their surface tension. The droplet break-up occurs at a relatively low frequency, and their spherical morphology is maintained as long as gravitational and surface forces play primary roles. As the voltage increases, the shape of the ejected liquid is affected by its wetting properties. The use of this regime allows obtaining particles of high average diameter with broad size distribution. By further increasing the tension, a pulsating cone jet appears. For higher voltages, the spray mode switches to stable conical jet mode, since the electrostatic force is strong enough to overcome surface tension and viscoelastic force and achieve a balance between different forces. In this regime, particles of small mean diameter and with a narrow size distribution are obtained. Beyond this tension, the multi-jet is observed, and then if the applied potential increases further, the jet disintegrates. Thus, depending on the inherent characteristics of the solutions used (electrical conductivity, surface tension, and viscosity), there is an appropriate voltage range for obtaining a stable cone-jet regime (Table 5). In general, the conical jet window of the voltage applied in the electrospray process is between several kilovolts and a dozen kilovolts. These differences are mainly related to variations in the molecular weight of the PCL used, the concentration, and the solvent selected, which modify the electrical conductivity, viscosity and surface tension of the prepared solutions. As the concentration of the polymer solution increases, surface tension and viscosity increase, while electrical conductivity decreases. Thus, a stronger electrostatic force is required to form the stable jet cone for a solution with a higher polymer concentration. Besides, the use of toluene requires a much higher applied voltage to form a stable cone due to the very low electrical conductivity and dielectric constant of this solvent. In most cases, an increase in concentration leads to a widening of the voltage range and/or an increase in it. This increase is also correlated with a variation in the electrical conductivity of the solutions. The applied electric field required forming a cone on the tip of the nozzle decreases with increasing conductivity.