Electrospraying and electrospinning are both electrohydrodynamic mechanisms which are used for the fabrication of nano/microparticles and nano/microfibers.
Fundamentals of Both the Processes
The electrostatic force is applied to produce electrically charged jets out of viscoelastic polymer solutions. Solvent is evaporated in time and nano-micro structures are obtained once the process is complete.
Concentration of the polymer solution dictates the structures of the resulting materials. Spherical particles are observed at lower solution concentrations. As the concentration approaches a critical value, bead-on-string morphologies are typically generated i.e. electrospun fibers with particulates. Above the critical solution concentration uniform fibers can be produced.
Fig. SEM images of electrosprayed or electrospun PAN with beads, bead-on-string and fibers structure obtained from solutions with different polymer concentration (w/v): (a) 2%, (b) 4%, (c) 6%, (d) 8%, (e) 10%, (f) 12% DMF/EA solutions.
As well as their independent usages, combining these two methods makes it possible to obtain hybrid fiber-particle composite material.
Nanofibers are one of the pioneering nanostructures. This is due to their novel properties i.e. low density, high pore volume, controllable mechanical properties and extreme high surface area. This is compared to many other conventional bulk materials. Nanofibers also represent a new generation of reinforcement for the fabrication of well-tailored and unique nanocomposites. Nanofibers have extensive and variable uses in critical application areas including environmental technology, bioengineering, healthcare, catalysis, energy and sensors.
Benefits of Electrospinning
Electrospinning offers a straightforward, simple and inexpensive process which enables the production of homogenous, continuous and uniform nanofibers from submicron diameters down to nanometer sizes as well as many different processing techniques (i.e. drawing, phase separation, template synthesis and self-assembly). Electrospinning also allows the production of nanofibers from various materials i.e. inorganics and organics in different assemblies and configurations.
The process is initiated by application of a high voltage which creates an electric field between the droplet of the polymer solution and the collector. The spherical solution droplet elongates due to the accumulating charge on the surface of the drop and forms a conical shape as the electrostatic force is increased.
Once the force reaches a certain limit and overcomes the surface tension of the drop, the polymer solution forms a jet moving towards the collector. During this movement the solvent evaporates and dried nanofibers deposit on the collector to form an electrospun mesh.
Fig. Schematic diagram of the polymeric jet path from the needle to the collector (Angammana, C. J. (2011).
Schematic illustration of the Taylor cone and jet formation: (A) the applied electric field creates surface charges in the polymeric solution; (B) increasing the voltage the drop is elongated; (C) when the voltage is higher than VC, the polymeric jet is formed and travels towards the collector due to the charge repulsion (Casasola, R. (2016)).
There are three main parameters that should be optimized before and during the process of electrospinning in order to maintain nanofibers with the desired morphologies. These are: environmental conditions; solution properties; and operation parameters. The solution properties are given as molecular weight of the polymetric material, viscosity, surface tension and the conductivity of the polymetric solution.
Furthermore, the electric current also has a detectable effect on the fiber morphology. The operating parameters are applied to flow rate, high voltage and the distance between the tip of the spinneret and the collector. The solidification process during fiber formation, concerning the evaporation rate of the solvent, depends on the environmental conditions which means that changing conditions such as relative humidity and temperature also change the characteristics of fibers.
Dielectric effect of solvent
Molecular weight and solution viscosity
|Tip to collector distance
This information has been sourced, reviewed and adapted from materials provided by Inovenso.
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