Electrospinning | Electrospinning Process | Conventional Process of Electrospinning

Introduction to Conventional Electrospinning
Electrospinning is a relatively simple process to produce nanofibre from polymer solutions or melts. Its roots go back to the early 1930s when the process was patented by Formhals. The advantage of the electrospinning process is its technical simplicity and its easy adaptability. It is based mainly on applying an electrical field, by using high voltage source, between the tip of a nozzle and a collector in order to generate sufficient electrostatic force to overcome the surface tension in a droplet of polymer solution at the nozzle tip. 
Conventional electrospinning setup.
When the surface tension is overcome, the hemispherical surface of the fluid at the tip of the nozzle stretches to form a conical shape known as the Taylor cone . Further increase of the electric field’s strength will deform the Taylor cone until a jet stream is extruded from the cone’s apex. During the process, and depending on the solution properties and operating conditions, the solvent evaporates as the jet moves toward the collector which decreases the jet radius and increases the polymer concentration and viscosity. When the solvent is fully evaporated, the jet stretching stops and results in producing fibre of highly reduced diameter which deposits on the grounded collector in the form of a random nonwoven structure. The process of the electrospinning is well described in many papers . Nanofibres in the range of 10 to 1000 nm diameter can be achieved by choosing the appropriate parameters such as viscosity, concentration, applied voltage, distance between the two electrodes, and nozzle tip (needle) diameters. However, the instability, the whipping of the fibre, and the beads formation remain important problems in the electrospinning process. This paper aims to validate experimentally the functionality of the new upward electrospinning approach introduced by Abdel Hady. In this new approach, as the fibre formation is produced and a jet is directed upwards, the gravitational force and the surface tension will work against the electrostatic force, which introduces more stretching to the fibre.
 
The New Upward Electrospinning Approach
Electrospinning requires a very simple and economical setup; Figure 1 shows a schematic view of the conventional electrospinning (downward) setup. It consists of a nozzle, a collector, and of a high voltage electric source with positive or negative polarity. The nozzle may be a syringe needle or any other capillary tube; a syringe pump will carry the solution from the syringe to the nozzle.
The collector can be made of any shape according to the requirements, like a flat plate or rotating drum, and separated from the nozzle by a defined distance. One electrode from the power supply is connected to the nozzle (needle) in order to charge the polymer solution and the other is attached to the opposite polarity collector (usually a grounded conductor).

Electrospinning Process
A schematic diagram of electrospinning is as shown in Figure 1. The process makes use of electrostatic and mechanical force to spin fibers from the tip of a fine orifice or spinneret. The spinneret is maintained at positive or negative charge by a DC power supply. When the electrostatic repelling force overcomes the surface tension force of the polymer solution, the liquid spills out of the spinneret and forms an extremely fine continuous filament. It has the misleading appearance of forming multiple filaments from one spinneret nozzle, but current theory is that the filaments do not split.

These filaments are collected onto a rotating or stationary collector with an electrode beneath of the opposite charge to that of the spinneret where they accumulate and bond together to form nanofiber fabric.
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Figure 1.   Schematic representation of electrospinning process .
The distance between the spinneret nozzle and the collector generally varies from 15 –30 cm. The process can be carried out at room temperature unless heat is required to keep the polymer in liquid state. The final fiber properties depend on polymer type and operating conditions. Fiber fineness can be generally regulated from ten to a thousand nanometers in diameter .
Polymer-Solvents Used in Electrospinning.
The polymer is usually dissolved in suitable solvent and spun from solution. Nanofibers in the range of 10-to 2000 nm diameter can be achieved by choosing the appropriate polymer solvent system [5]. Table 1 gives list of some of polymer solvent systems used in electrospinning.
           Fiber Type
Fiber size, in Micrometer
Fiber surface area per mass of fiber material m2/g
Nanofibers
0.05
80
Spunbond fiber
20
0.2
Melt blown fiber
2.0
2

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