Electrospinning in brief:
Electrospinning is one of the methods to prepare nanofibers of polymer or polymer composite materials from corresponding solutions or melt. Electrospinning process is almost similar to extrusion spinning used in textile industries. As compared to extrusion spinning, electrospinning uses electrical force instead of mechanical force. Electrospun fibrous membranes find place in diverse applications like sensors, filters, fuel cell membranes, scaffolds for tissue engineering, organic electronics etc. Different parameters like flow rate, viscosity, surface tension, electric field, rotation speed of collecting drum, etc. affect electrospinning process.
Electrospinning in detail:
Electrospinning is a simple and versatile method to produce polymer nanofibers by accelerating a charged polymer jet in a very high electric field. The diameter of the fibers are in the range of 10µm to 10 nm, which is typically 1-3 orders less than that obtained by the conventional spinning process. A schematic diagram of a typical electrospinning setup is shown in Fig. 1. The main components of the electrospinning process can be classified as (i) syringe (or pipette), (ii) high voltage power supply and (iii) counter electrode or substrate. Polymer in solution (or melt) form is loaded in the syringe and is connected to the positive terminal (it also can be negative terminal) of the high voltage power supply. For continuous production of nanofibers, the solution should be pushed at a constant flow rate. Generally, a syringe pump serves this purpose. In the electrospinning process a high voltage is used to create an electrically charged jet of polymer solution or melt. One electrode is connected to the spinning solution/melt and the other attached to the collector. In most cases, the collector is simply grounded, as indicated in Fig. 1. The electric field is concentrated at the tip of the needle that contains a pendant droplet of the solution held by its surface tension. Accordingly, charges are induced on the surface of the drop. Mutual charge repulsion and the tendency of the surface charges to move towards the counter electrode, result in an electrostatic force against surface tension. As the intensity of the electric field is increased, the hemispherical surface of the fluid at the tip of the capillary tube elongates to form an inverted cone known as the Taylor cone. On increasing the electric field further, a critical value is reached when the repulsive electrostatic forces overcome the surface tension forces and a fine jet of charged polymer solution is ejected from the tip of the cone. This jet is further subjected to elongation process and instabilities, which results in the jet becoming very long and thin as they move towards the counter electrode. The polymer strands now start moving away from each other due to mutual repulsion and ultimately collect on the counter electrode as a random coil of nanofibers. Once the jet comes into the atmosphere, the low boiling point solvent evaporates, leaving behind only the charged polymer strands.