Physics of Electrospinning

Abstract

Electrospinning is the process of producing fibers ranging from sub-micron to Nano-scale in diameter consistently and reproducibly. The Electrospinning consists of three main parts High voltage power source (up to 30 kV), Spinneret (such as a syringe, with a small diameter needle) and a conducting collector. The basic principle of electrospinning technique is that, when an electrically charged solution is feed through a small opening such as syringe pump, needle or a pipette tip then due to its charge the solution is drawn as a jet towards an oppositely charged conducting collector plate. The solvent evaporates gradually during jet travel towards the collecting plate and a charged solid fiber is laid to accumulate at the collector plate. The high voltage is connected to the end of a needle containing the liquid solution. The fiber collecting screen is expected to be conductive and it can either be a stationary plate or a rotating platform or substrate. The physics of electrospinning involves several key factors, including the electrostatic forces, surface tension and viscosity of the polymer solution.

1. Introduction

Electrospinning is a versatile and widely used technique for producing nanofibers with unique morphologies and properties. It has been studied and developed for over a century, with the first reported observation of electrospinning dating back to the late 1800s. Over the years, electrospinning has undergone several advancements, including the development of new equipment and methods to control the morphology and properties of the resulting nanofibers [1]. In this introduction, we will briefly discuss the history of the electrospinning process and some notable milestones and achievements that have contributed to its development.

The earliest known observation of electrospinning was made by British scientist William Gilbert in the late 1600s. However, it was not until the late 1800s that electrospinning was first reported as a process for producing fibers. In 1884, French scientist Charles Vincent reported the production of fibers by electrostatic means. The first patent for electrospinning was granted in 1900 to American scientist Joseph Zeleny, who used the technique to produce fibers for medical applications. In the 1930s and 1940s, electrospinning gained attention as a method for producing ultrafine fibers for air filtration and other applications. In the 1960s, the use of electrospinning expanded to include the production of synthetic fibers for textile applications. In the 1990s and 2000s, electrospinning became a popular research tool for the production of nanofibers for a wide range of applications, including tissue engineering, drug delivery, and energy storage. Since then, electrospinning has undergone significant advancements and innovations. Researchers have developed new equipment and methods to control the morphology and properties of the resulting nanofibers, including the use of multi-needle spinnerets, coaxial and triaxial electrospinning, and the addition of other materials to the polymer solution [2]. These advancements have expanded the potential applications of electrospinning and opened up new avenues for research.

Fiber production techniques have been the focus of much research over the years, with electrospinning being one of the most promising due to its versatility, adaptability, and the unique properties of the fibers produced [3]. However, other traditional and contemporary fiber formation techniques, such as melt spinning, dry spinning, wet spinning, and centrifugal spinning, still hold substantial relevance in various applications [4]. This review explores the advancements and the comparative strengths and weaknesses of electrospinning versus other fiber production techniques. Electrospinning stands out as a simple and versatile technique for creating ultrafine fibers with diameters ranging from the Nano to the microscale [5]. It offers exceptional control over fiber morphology and structure, enabling the production of complex nanofiber arrangements [3]. Its versatility in handling a wide array of materials, such as polymers, ceramics, and composites, is noteworthy [6]. Furthermore, electrospun fibers exhibit unique properties, such as high surface area-to-volume ratio, flexibility in surface functionalities, and superior mechanical properties, which are attractive for applications in filtration, tissue engineering, drug delivery, and energy storage [7]. Despite these strengths, electrospinning faces a major limitation in its low production rate, rendering it less feasible for large-scale industrial applications [3]. Some techniques have attempted to overcome this limitation, such as needleless electrospinning and multi-jet electrospinning, but the scalability issue remains a challenge [8]. On the other hand, traditional fiber spinning techniques, like melt, dry, and wet spinning, have been utilized for large-scale fiber production for years [4]. These methods enable the production of continuous fibers and provide significant control over the fiber diameter, although generally at a larger scale than electrospinning. However, these techniques struggle to match the nanoscale dimensions and high surface area that electrospinning can achieve. Additionally, the processing conditions in traditional spinning methods, such as high temperature in melt spinning and solvent use in dry and wet spinning, can limit the materials that can be used [9]. Centrifugal spinning, a more recent development, emerges as an alternative to electrospinning, offering higher throughput and the ability to produce fibers in the Nano to microscale range [10]. It eliminates the use of high voltages, making it safer, and offers more flexibility in the types of materials that can be spun [11]. Yet, the fibers produced by this method still lack the uniformity achieved by electrospinning, and the method requires significant optimization to improve the fiber properties and morphology [10]. In conclusion, electrospinning continues to hold a distinctive position among fiber production techniques due to its capability of generating unique fiber properties. Despite the competition from traditional and newer techniques, the challenges of scale-up production in electrospinning highlight the importance of ongoing research in enhancing this technology’s feasibility for industrial applications.

In the field of biomedical applications, electrospun fibers have shown immense promise, particularly in tissue engineering, wound healing, and drug delivery systems. They exhibit a structural similarity to the natural extracellular matrix, which promotes cellular growth and tissue regeneration (“Electrospun nanofibers for regenerative medicine,” 2020, Advanced Drug Delivery Reviews). In the realm of energy storage, electrospun nanofibers have emerged as compelling candidates for creating high-performance electrodes in lithium-ion batteries and super capacitors. Their high surface-to-volume ratio facilitates better electron transfer and ion diffusion, contributing to improved energy storage efficiency (“Nanofibers for lithium-ion and lithium-metal batteries: A Review,” 2020, Advanced Materials). Environmental remediation represents another significant domain of electrospun fiber applications. Electrospun nanofibers, especially those functionalized with certain nanomaterials or specific chemical groups, have demonstrated excellent capabilities for absorbing heavy metals, organic pollutants, and oil spills from water (“Electrospun nanofibers for environmental applications,” 2019, Nanomaterials). The filtration industry has also harnessed the potential of electrospun fibers, thanks to their adjustable pore sizes and high porosity. They have been proven effective for filtering out particulate matter, bacteria, and viruses from air and liquids (“Filtration properties of electrospun Nano fibrous materials for aeronautic applications,” 2015, Journal of Aerosol Science). Lastly, electrospun fibers’ high sensitivity and specific surface area make them excellent for sensor technologies, where they can be functionalized to detect various chemicals, gases, and biomaterials (“Electrospun nanofibers for chemical and biological sensors,” 2020, Materials Today Chemistry) [3, 12, 13, 14, 15, 16, 17, 18, 19].

In conclusion, the electrospinning process has a long and rich history, with significant advancements and innovations made over the years. It has become a widely used technique for producing nanofibers with unique morphologies and properties, with potential applications in a wide range of fields. The continued development and improvement of the electrospinning process hold great promise for the future of nanofiber research and applications [20].

2. Experimental techniques

The electrospinning setup consists of several key components, including a high-voltage power supply, a spinneret, a collector, and a syringe pump. While the basic electrospinning setup is relatively simple, advancements in the field have led to the development of more advanced and unique setups that offer greater control over the morphology and properties of the resulting nanofibers. Following are some of the set-up’s.

2.1 Single needle setup

One of the earliest and simplest electrospinning setups involves the use of a single-needle spinneret as shown in Figure 1 below. This setup involves the attachment of a syringe to a metallic needle, which is then connected to a high-voltage power supply. The polymer solution or melt is loaded into the syringe, which is then driven by a syringe pump to control the flow rate. The polymer solution is then ejected through the needle and forms a charged jet as it travels towards a collector plate. Researchers from the School of Chemical Engineering at the University of Adelaide, Australia used a single-needle spinneret electrospinning setup to fabricate poly(lactic acid) (PLA) nanofibers for biomedical applications. In their study, they loaded a PLA solution into a syringe attached to a 22-gauge metallic needle and connected it to a high-voltage power supply. The solution was then ejected through the needle and formed a charged jet that was collected on a grounded aluminum foil collector. The researchers varied the flow rate of the PLA solution using a syringe pump to control the diameter of the resulting nanofibers. They observed that the nanofibers exhibited high surface area-to-volume ratios, which could be beneficial for drug delivery applications [22].

Figure 1.

Basic electrospinning set-up with a single needle [21].

2.2 Multi needles setup

Another commonly used setup is the multi-needle electrospinning setup as shown in Figure 2. This setup involves the use of multiple needles arranged in a specific pattern to electrospun several fibers simultaneously. Multi-needle electrospinning is useful for the production of large quantities of nanofibers in a short amount of time and for the production of more complex fiber morphologies. Custom-built electrospinning setup that consisted of six needles arranged in a circular pattern were used. The needles were connected to a high-voltage power supply and a syringe pump was used to control the flow rate of the polycaprolactone (PCL) solution. The solution was then electrospun into nanofibers, which were collected on a rotating cylindrical drum. The researchers varied the distance between the needles and the collector to control the alignment of the nanofibers. They observed that the aligned PCL nanofibers had enhanced mechanical properties and could promote cell adhesion and proliferation [24].

Figure 2.

Multi-needle electrospinning equipment [23].

2.3 Needle-less electrospinning setup

Needleless electrospinning is a technique used to produce nanofibers from a polymer solution or melt without using a needle as shown in Figure 3. Instead, the polymer solution is extruded from a capillary orifice, which is usually a small nozzle or spinneret. The process is similar to traditional electrospinning, but the use of a capillary orifice instead of a needle has some advantages [26, 27].

Figure 3.

Needleless electrospinning [25].

One advantage of needleless electrospinning is that it reduces the risk of contamination. In traditional electrospinning, the needle can become contaminated with the polymer solution, which can affect the quality of the nanofibers produced. With needleless electrospinning, the risk of contamination is reduced because there is no needle to become contaminated.

Another advantage of needleless electrospinning is that it can produce fibers with a more uniform size distribution as shown in Figure 4. This is because the capillary orifice can be designed to produce a more uniform flow of polymer solution, which can result in more uniform nanofibers. In contrast, traditional electrospinning can produce nanofibers with a wider size distribution due to variations in the flow rate of the polymer solution from the needle [29].

Figure 4.

Needle-less electro spraying [28].

Needleless electrospinning can be achieved using various techniques, such as centrifugal electrospinning, electro spraying, and electro hydrodynamic atomization as shown in Figure 5. Each technique has its advantages and disadvantages, and the selection of the appropriate technique depends on the specific application and requirements of the nanofiber product [5, 31].

Figure 5.

Needleless electro hydrodynamic atomization [30].

2.4 Coaxial electrospinning setup

This is another advanced electrospinning setup used to produce core-shell nanofibers. This setup involves the use of a coaxial spinneret, which consists of two concentric tubes. The polymer solution or melt is loaded into the inner tube, and a different polymer solution or melt is loaded into the outer tube. The polymer solution from the inner tube forms the core of the nanofiber, while the polymer solution from the outer tube forms the shell as shown in Figure 6. For example, researchers have used coaxial electrospinning to produce nanofibers with a drug-loaded core and a polymer shell for controlled drug delivery applications [33]. In the study conducted by [34] they loaded a poly lactic-co-glycolic acid (PLGA) solution containing a model drug into the inner tube of the coaxial spinneret and a PLGA solution without the drug into the outer tube. The two solutions were electrospun simultaneously to form core-shell nanofibers, which were collected on a grounded aluminum foil. The researchers varied the flow rate and composition of the two solutions to control the drug loading and release profile of the resulting nanofibers. They observed that the nanofibers exhibited sustained drug release over a period of several weeks, demonstrating the potential of this technique for controlled drug delivery applications.

Figure 6.

Co-axial electrospinning equipment [32].

2.5 Microfluidic electrospinning setup

This setup is used for the production of nanofibers with precise control over their morphology and properties. This setup involves the use of microfluidic channels to control the flow rate and concentration of the polymer solution before it is electrospun as shown in Figure 7. Microfluidic electrospinning is useful for the production of nanofibers with more complex morphologies and structures, such as patterned and multi-layered fibers. Researchers have used microfluidic electrospinning to produce nanofibers with a gradient of pore size for tissue engineering applications [36]. In the study conducted by [37] they used a microfluidic chip consisting of two parallel channels, one for the PCL solution and the other for a pore-forming agent. The two solutions were then electrospun through a single needle to produce nanofibers with a gradient of pore size. The nanofibers were collected on a grounded aluminum foil and then treated with ethanol to remove the pore-forming agent. The researchers observed that the resulting nanofibers exhibited a gradient of pore size, which could be beneficial for tissue engineering applications.

Figure 7.

Microfluidic pump electrospinning setup [35].

3. Electrospinning process

The physics of electrospinning is complex and involves several mechanisms, including electrostatic repulsion, surface tension, and solvent evaporation. The process begins when a high voltage is applied to a droplet of polymer solution, creating an electric field that causes charges to accumulate on the surface of the droplet. When the electric field reaches a critical value, electrostatic repulsion overcomes surface tension and a thin jet of material is ejected from the droplet. As the jet travels through the air, solvent evaporation causes it to stretch and thin, resulting in the formation of a fiber.

Several factors can influence the electrospinning process, including the properties of the polymer solution (such as viscosity, conductivity, and surface tension), the applied voltage, the distance between the spinneret and the collector, and the properties of the collector (such as its shape and surface energy). Understanding the physics behind electrospinning is important for optimizing the process and producing fibers with desired properties. The process can be divided into three main stages: (1) initiation, (2) elongation, and (3) solidification as shown in Figure 8.

Figure 8.

Basic Schematic of three main phases in electrospinning technique [38].

3.1 Initiation phase

In the initiation stage, a high voltage is applied to the polymer solution or melt. The voltage creates an electric field that induces charges on the surface of the solution, resulting in the formation of a Taylor cone. The Taylor cone is a conical shape formed at the tip of the spinneret, where the electric field strength is the highest. The formation of the Taylor cone is a critical step in the electrospinning process, as it determines the initial shape of the jet. The formation of the jet involves a complex interplay of electrical, fluidic, and surface tension forces, which are influenced by several factors such as the properties of the polymer solution, the electric field strength, and the distance between the spinneret and the collector.

The electrostatic force is the driving force behind the initiation of the jet. When a high voltage is applied to a conductive polymer solution or melt, charges accumulate on the surface of the solution. This accumulation of charges creates an electrostatic field that induces a polarization of the solution. As the electric field strength increases, the polarization becomes more pronounced, and the surface tension of the solution decreases [39]. Eventually, the surface tension becomes so low that it can no longer support the weight of the solution, and a droplet forms at the tip of the spinneret.

Once the droplet forms, the electrical repulsion between the charges on the surface of the droplet overcomes the surface tension, and a charged jet is ejected from the droplet. The charged jet then experiences a series of instabilities, such as the Rayleigh instability, which cause the jet to break up into droplets. The droplets solidify into nanofibers as they travel towards the collector [40].

Polymer solution properties: The viscosity of the polymer solution affects the formation of the droplet at the tip of the spinneret, while the conductivity and surface tension influence the formation and stability of the charged jet. A high conductivity can result in a more stable jet, while a low surface tension can promote the formation of a more uniform jet.

Electric field strength: A higher electric field strength can lead to the formation of a more stable and uniform jet, but can also result in the formation of multiple jets or the formation of beads along the fiber. The electric field strength can be controlled by adjusting the voltage applied to the spinneret and the collector [2].

Distance between the spinneret and the collector: The distance can influence the size and shape of the droplet and the jet, as well as the alignment and morphology of the resulting nanofibers. A shorter distance can result in a more uniform jet and more aligned fibers, while a longer distance can result in a more random orientation of the fibers [41, 42].

Ambient conditions: High humidity can increase the surface tension of the polymer solution and lead to the formation of larger droplets and less stable jets.

3.2 Elongation phase

In the elongation stage, the electric field causes the surface of the Taylor cone to become unstable, resulting in the formation of a jet. The jet is propelled towards the collector due to the repulsive electrostatic forces between the charges on the surface of the jet and the charges on the collector. As the jet travels through the air, it undergoes elongation and thinning due to a combination of electrostatic repulsion, surface tension, and air resistance. The elongation stage of electrospinning is a critical step that determines the final morphology and properties of the resulting nanofibers.

The viscosity of the polymer solution can be controlled by adjusting the concentration of the polymer or by adding a viscosity modifier, such as a polymer solvent or a surfactant. The conductivity of the polymer solution can be controlled by adding conductive additives, such as carbon nanotubes or graphene. The surface tension of the polymer solution can be controlled by adding a surfactant or by adjusting the pH of the solution. A study by Huang et al. [39] investigated the effect of polymer concentration on the morphology and properties of electrospun nanofibers. The study found that increasing the polymer concentration resulted in thicker fibers with larger diameters, while decreasing the concentration led to thinner fibers with smaller diameters. This highlights the importance of controlling the viscosity of the polymer solution during the elongation stage of electrospinning.

Another approach to controlling the elongation stage of electrospinning is to modify the electric field strength. This can be achieved by adjusting the voltage applied to the spinneret and the collector or by using a multi-needle spinneret to generate multiple jets with different electric field strengths. A higher electric field strength can result in a faster stretching rate and thinner fibers, while a lower electric field strength can result in thicker fibers. A study by Zeng et al. [43] explored the use of multi-needle electrospinning to control the morphology and properties of electrospun nanofibers. The study found that using a multi-needle spinneret resulted in the formation of multiple jets with different electric field strengths, which allowed for greater control over the fiber diameter and morphology. This highlights the potential of using advanced electrospinning techniques to improve the precision and reproducibility of electrospun nanofibers.

A shorter distance can result in a faster stretching rate and thinner fibers, while a longer distance can result in thicker fibers. However, a shorter distance can also increase the likelihood of secondary droplet formation along the jet, which can result in the formation of beads along the fiber.

High humidity can increase the surface tension of the polymer solution, leading to the formation of thicker fibers. High temperature can decrease the viscosity of the polymer solution, resulting in faster stretching rates and thinner fibers. However, high temperature can also lead to faster solvent evaporation, affecting the final morphology and properties of the resulting nanofibers. In a study by Khajavi et al. [44] the effect of humidity on the morphology and properties of electrospun nanofibers was investigated. The study found that increasing humidity led to an increase in fiber diameter due to an increase in surface tension. This demonstrates the importance of controlling the environmental conditions during the elongation stage of electrospinning to achieve the desired fiber morphology and properties.

For example, the use of co-electrospinning, where multiple polymers are electrospun simultaneously, can be used to control the morphology and properties of the resulting nanofibers. Electrospinning with the aid of external fields, such as magnetic or acoustic fields, can also be used to control the elongation stage and orientation of the resulting nanofibers. In a study by Lee et al. [45] the effect of surfactant concentration on the morphology and properties of electrospun nanofibers was investigated. The study found that increasing the surfactant concentration led to a decrease in fiber diameter due to a decrease in surface tension. This demonstrates the potential of surfactants as a means of controlling the elongation stage of electrospinning.

3.3 Solidification phase

In the solidification stage, the solvent evaporates from the jet, resulting in the solidification of the polymer. The rate of solvent evaporation is critical in determining the final morphology of the fibers. Rapid solvent evaporation leads to the formation of smooth fibers, while slow solvent evaporation leads to the formation of beaded fibers. In a study by Scaffaro et al. [46] the effect of solvent evaporation rate on the morphology and properties of electrospun nanofibers was investigated. The study found that a rapid solvent evaporation rate led to the formation of smooth fibers, while a slow evaporation rate led to the formation of beaded fibers. This highlights the importance of controlling the rate of solvent evaporation during the solidification stage of electrospinning.

High viscosity and surface tension lead to the formation of thicker fibers, while low viscosity and surface tension lead to the formation of thinner fibers. Conductive solutions promote the formation of straight fibers due to the higher electrostatic forces acting on the charged jet. Increasing the concentration of the polymer solution can lead to the formation of thicker fibers, but can also lead to the formation of beaded fibers due to the slower solvent evaporation.

Higher voltages lead to the formation of thinner fibers due to the increased electrostatic forces acting on the charged jet. However, excessively high voltages can lead to the formation of irregular and discontinuous fibers. In a study by Reneker et al. [47] the effect of applied voltage on the morphology and properties of electrospun nanofibers was investigated. The study found that higher voltages led to the formation of thinner fibers due to the increased electrostatic forces acting on the charged jet. However, excessively high voltages can lead to the formation of irregular and discontinuous fibers. This highlights the importance of optimizing the applied voltage to achieve the desired fiber morphology and properties.

Shorter distances lead to the formation of thinner fibers due to the reduced air resistance acting on the charged jet. A study by Lee et al. [48] investigated the effect of distance between the spinneret and collector on the morphology and properties of electrospun nanofibers. The study found that shorter distances led to the formation of thinner fibers due to the reduced air resistance acting on the charged jet. This demonstrates the potential of controlling the spinneret-collector distance to achieve the desired fiber morphology during the electrospinning process.

4. Conclusion

The field of electrospinning is a captivating amalgamation of physics and material science. As a technological process, it has carved out a unique position, enabling the generation of micro and nanoscale fibers with diverse applications across industries such as biomedical, textiles, energy, and environmental science. Electrospinning relies on the principles of electro hydrodynamics, exploiting the Columbic forces to create a high-speed jet of polymer solution or melt. As the solvent evaporates, it leaves behind a thin fiber that can be collected in various forms, depending on the configuration of the collector. The high surface area to volume ratio, direct result of the physics of the process, enables electrospun materials to have enhanced properties in terms of reactivity, sensitivity, and diffusivity, thus overcoming the limitations of traditional fiber fabrication methods. One of the critical advantages of electrospinning is the versatility of fiber diameter it can produce, ranging from a few nanometers to several micrometers, by merely adjusting the solution properties and process parameters. The ability to generate ultrafine fibers consistently grants this technique an edge over conventional methods like drawing or phase separation, whose outcome can vary significantly. Moreover, electrospinning offers the flexibility to create fibers with tailored properties, including the porosity, surface chemistry, and orientation, thus opening up numerous possibilities for applications where these properties play a pivotal role. From bioengineering, where the structure of electrospun nanofibers can mimic the extracellular matrix for tissue scaffolding, to filtration and protective clothing applications, where pore size control is crucial, the ability to customize these parameters offers a distinct advantage. The simplicity and versatility of the setup not only provide a cost-effective solution but also allow for various adaptations and modifications. For instance, using multiple needles or a free surface setup for co-axial or multilayer fibers, implementing a rotating or patterned collector for aligned or patterned fiber mats, or adding post-processing steps for further fiber modifications. In conclusion, the physics of electrospinning has revolutionized the fabrication of micro and nanofibers, offering unprecedented advantages over traditional methods. Its ability to generate ultrafine fibers with a high surface area to volume ratio, the versatility in fiber properties, and a straightforward, adaptable setup make electrospinning a powerful technique with broad-spectrum applications. As we delve deeper into the realm of nanotechnology and advanced materials, electrospinning continues to hold enormous potential for novel, high-impact solutions across various fields of science and technology.