2008 ME Graduate Student Conference

Electrospinning process is used in the manufacturing industry to generate nanofibers in an efficient way. The principle of this process is the deformation of a drop by the application of an electric field. The strength of the electric field generates force that makes the drop to elongate. Four cases are studied in order to show that the force distribution varies along the drop surface. In case (1) a sphere shape is used to simulate the water droplet shape. In case (2) and (3) an ellipsoid shape is used, and in case (4) the Taylor cone shape is used. The size of the semi-axis (a) in case (3) is double in size as the same semiaxis in case (2). FEMLAB [5] is used for the simulation under the electrostatic mode. The electric potential distribution is obtained, then the force distribution within the surface of the water droplet is calculated using a MATLAB [6] code. The results presented in this studies shows that the electric potential increases its range from 0 V to 412.142 V in case (1) up 0 V to 704.966 V in case (2), 0 V to 2295.745 V in case (3), and 0 V to 11117 V; for the same experiment configuration and electric field strength. These results indicated that as the shape elongates the electric potential values increase within the water droplet surface. Another result that is relevant to this study is the electric field distribution within the surface of the water droplet. In all of the cases, the maximum electric field values are located in the equatorial zone of the drop. As this electric field increases, it will generate larger electric forces. As these forces exceeded the surface tension of the fluid, the generation of a jet is induced. As we know, this is the principle of electrospinning which is demonstrated by the results. The application of an electric field to a substance is a method used in the oil and petroleum industries to separate water from oil [1]. The electric field creates forces that generate the coalescence of the aqueous phase present in the oil which facilitates the separation. This physical behavior has been studied by numerous researchers. J. S. Eow et al [1] experimentally showed that the drop elongated due to different electric field strength until the instability generated the formation of smaller droplets. Taylor [5] studied the formation of a conical interface between two fluids with his work; it was shown theoretically that this geometric shape only can exist in equilibrium when the cone has a semi-vertical angle of 49.3o. This is the physical principle of the electrospinning process. Some of the applications of this process are: filter media, fiber-reinforced plastics, sola and light sails, mirrors in space, biomedical applications, protective cloth, etc [4]. The typical arrangement for electrospinning process is shown in figure 1, from D. Li and Y. Xia [3]. Figure 1. Geometric Configuration for Electrospinning[3]. The arrangement consists mainly of three elements: a power supply, a metallic needle or capillary tube, and a collector. The high voltage is applied between the needle and the collector, which generates an electric field between them. The polymer solution is fed by using a syringe pump when the drop is at the needle, it experiences the electrostatic forces which induced the deformation of the drop due to instability produced when the electric potential exceed its critical value [1]. When these forces are strong enough to exceed the surface tension of the polymer solution, the formation of a liquid jet is observed. This jet formed nanofibers in the collector when the voltage was applied. The diameter, alignment, and composition of the nanofibers depend on the processes parameter. Such parameters are: properties of the polymer solution and operational condition of the process, such as feeding rate of the polymer solution, strength of the applied electric field, and separation from the needle to the collector [3]. In this study, we presented the results for the simulation of the deformation of a water droplet by the application of an electric field using four different shapes. The studied configuration is presented in figure 2. A water droplet is placed between two 15 cm diameter aluminum