1) Introduction

Electrospinning is a technique which is developed recently from last decade (around 2012) for the fabrication of continuous fibres from the liquid molten melts of polymers, metals and ceramics. These ultrathin fibres can have the diameter ranging from submicron to nanometre scale range.[1]^[1]This method involves the application of an electric field to a polymer solution or melt, which causes the formation of a charged jet that is subsequently stretched and solidified into a fibre. The resulting fibres have a high surface area-to-volume ratio and can exhibit unique physical and chemical properties that make them useful in a wide range of applications, including tissue engineering, drug delivery, energy storage, and environmental remediation. Electrospinning has gained considerable attention in both academia and industry.^[2]However, several factors such as polymer concentration, solution concentration, flowrate and electric field intensity can affect the process of electrospinning.^[1]

2) Principe of electrospinning

Electrospinning is a method that involves the use of electrohydrodynamic  to create ultra-fine fibres from a liquid droplet. This process requires a simple setup consisting of a high-voltage power supply, a syringe pump, a spinneret, and a conductive collector as shown in Figure 1. The power supply can be either DC or AC. The spinneret, which is usually a hypodermic needle, extrudes the liquid to form a droplet due to surface tension. When the droplet is electrified, the surface charges of the same sign repel each other and cause the droplet to deform into a Taylor cone, which then generates a charged jet. Initially, the jet extends in a straight line before whipping motions occur due to bending instabilities. As the jet stretches and thins, it quickly solidifies to form solid fibre(s) on the collector. The electrospinning process can be divided into four steps, which include the charging of the droplet and the formation of the Taylor cone or cone-shaped jet, the extension of the charged jet, the thinning of the jet in the presence of an electric field, and the solidification and collection of the jet as solid fibre(s) on the grounded collector. More details on these steps can be found in the following subsections.^[3]

Figure1: A setup for Electrospinning. ^[4]Springer Authors Choice

2.1) Formation of Taylor Cone upon Charging a Liquid Droplet

During electrospinning, a syringe pump is typically used to feed liquid through the spinneret at a constant and adjustable rate. As a potential difference is created between the spinneret and collector, charges of opposite polarity within the liquid will separate, and charges of the same sign as the spinneret's polarity will accumulate on the surface of the droplet, resulting in excess charges. As the voltage increases, more charges will accumulate, causing the density of surface charges on the droplet to increase. Although surface tension flavors a spherical droplet to minimize the total surface free energy, electrostatic repulsion tends to deform the droplet's shape and increase its surface area to alleviate the repulsion. The droplet is expected to take a shape that balances the sum of the electrostatic energy and the surface free energy. ^[3]

Fiuure 2: Formation of length of fibres as a function of applied voltage. ^[3]ACS Authors Choice

2.2) Stretching of the Charged Jet

Electrically charged fluid is ejected from the top of a cone and then accelerated by the electric field towards a collector. As depicted in Figure 2, the jet extends in the direction of the electric field and initially follows a straight path, which is known as the near-field region.[3] To prevent the jet from breaking into droplets, the fluid must have viscoelastic properties. The current passing through the jet is generated by the movement of surface charges. The velocity, length, and diameter of the straight segment of the jet can be easily measured.^[3]

Figure 2. Diagram showing the path of an Electrospun jet.^[3]ACS Authors Choice.

2.3) Thinning of jet

To create ultrathin nanofibers through electrospinning, it is important to induce rapid growth of the whipping instability in the fluid jet, which causes it to bend and stretch. The bending generates a lateral force called FR, which further promotes the bending. This force causes the jet to rapidly bend by 90 degrees, resulting in a looping, spiraling, and gradually thinning coil. The elongation mostly occurs in the loops due to bending motion. As the jet elongates, it becomes thinner in diameter to maintain its path. With further elongation, the jet forms a smaller coil triggering another stage of bending instability, the second bending instability. However, if the jet solidifies before the second bending instability occurs, the diameter of the loops no longer increases, resulting in an "envelope cylinder." Factors such as solvent evaporation can also affect the shape of the envelope cone, making it more complicated. The loops become smaller in diameter as the solvent evaporates, making it difficult to further stretch the jet. ^[3]

2.4) Solidification of Jet

As the charged jet elongates, it solidifies to form fibres due to either solvent evaporation or melt cooling. When the solidification process is slow, the elongation can continue for a longer period, resulting in fibres with a smaller diameter. According to one study, the cross-sectional radius of a dry fibre was only 1.3 × 10^^-3 times that of the initial jet due to stretching and solvent evaporation. Once the fibres solidify, the surface charges remain, but all instabilities cease.^[3]

3) Applications

Nonwoven mats of thin nanofibers are formed after accumulation on a collector, which possess a high surface-to-volume ratio and porosity. These unique characteristics make them useful for various applications, including air and water filtration for removing particulate matter, toxic ions, and molecules, while demonstrating high selectivity and durability. ^[4]In wastewater treatment, electrospun nanofiber mats have shown promise in filtering particulates ranging from 3 to 10 μm in size, as well as removing heavy metals, ions, and organic pollutants such as dyes, pesticides, and plasticizers through physisorption, chemisorption, and electrostatic attraction mechanisms. Overall, the use of electrospun nanofiber mats in filtration and wastewater treatment presents an effective and sustainable solution for environmental remediation.^[5]

References