Introductory Chapter: The Electrospinning

*Khaled H. Mahmoud and Khalid S. Essa*

## **1. Introduction**

The production of nanofibers from a polymer solution or melt may be accomplished using an electric field called electrospinning. Due to the peculiar characteristics of the nanofibers produced by this procedure, there has been a significant interest in various areas, including medicine, biotechnology, and materials science [1]. Originally conceived as a theoretical concept, electrospinning has evolved into a versatile and widely used process with diverse applications across several sectors. Due to its ability to produce nanofibers with exact control over their structure and composition, this method benefits researchers and industries [2].

## **2. History of electrospinning**

L.J. Cohn and J.A. Sprague first proposed the process of electrospinning in 1934, and they also secured a patent for it. At the same time, Formhals also obtained a separate patent for a method that closely resembled electrospinning [3, 4]. However, the first applications and understanding of electrospinning were limited, and the technique did not get much attention from the research community at that time. Taylor's work in the 1960s had a significant role in the progress of electrospinning by developing a model for the hopper configuration, where an electric field produces liquid droplets [5, 6]. His partnership with JR Melcher resulted in the creation of the "Leaky Dielectric Model" for conductive liquids [7].

Electrospinning gained practical importance in the 1990s as a consequence of researchers creating more controlled techniques and beginning to investigate applications in a range of sectors [8, 9]. In particular, Darrel Reneker and Gregory were able to effectively decrease a variety of organic polymers to the nanoscale by the use of electrospinning [10, 11]. In order to manufacture ultrafine fibers with dimensions that are smaller than 5 micrometers, Darrel Reneker was the pioneer who introduced the method of submitting a polymer dispersion to high voltage levels. This was the beginning of the technology's rise to prominence. In addition, during this time period, advancements in polymer science and materials engineering led to the creation of electrospinning as a method that had the potential to be regarded as practical [12].

Electrospinning is an intriguing technique used to produce fibers with advantageous structural characteristics, as seen in **Figure 1**.

**Figure 1.** *Different types of fibers synthesized* via *electrospinning method.*

## **3. Fibers manufactured by electrospinning**

## **3.1 Porous fibers**

These fibers are known as porous fibers because they have holes or empty areas within their structure. The size and form of the pores further define their practical applicability, but in general, porous fibers are used in filtering because of their capacity to trap particles [13].

## **3.2 Core-shell fibers**

The core of these fibers is made of a distinct substance, while the shell is made of a different material. Due to the fact that each component may contribute its own unique traits, this structure makes it possible to create one-of-a-kind combinations of attributes [14].

## **3.3 Helical fibers**

A twisted or coiled structure is characteristic of fibers that are spiral or helical in form. It is possible for the fibers to acquire intriguing mechanical and optical characteristics as a result of this winding [15].

## **3.4 Hollow fibers**

The structure of hollow fibers is characterized by the presence of voids or empty areas. The empty area may be filled with a particular material, making this design ideal for applications, such as medicine administration, where it can be filled with specific substances [16].

## **3.5 Multichannel fibers**

Multi-channel fibers, often known as fibers having several routes or channels, provide unique capabilities that are not found in other fiber kinds. Due to their versatility, multichannel fibers provide a promising and adaptable foundation for many technological applications. This is especially accurate in fields where meticulous regulation of fluid flow or sensing is crucial. Researchers persist in exploring and developing innovative uses for these fibers in many applications and areas of research [17].

## **4. Basic electrospinning methods**

There are a few distinct methods that are used in the manufacturing of fibers from polymers. These methods include wet spinning, dry spinning, liquefying, and gel spinning. Every approach has its own set of qualities, benefits, and applications that are unique to him. Following is a summary of the distinctions that exist between these various spinning processes:

## **4.1 Wet-spinning**

Extruding a polymer solution through a spinneret and into a coagulation bath is the process involved in wet spinning. It is the presence of a non-solvent for the polymer in the coagulation solution that causes the polymer to solidify and form fibers. The use of wet spinning is advantageous because it may be used for polymers that are difficult to dissolve in conventional solvents. The manufacture of fibers with high strength is made possible as a result. Wet spinning is often used in the production of rayon and acrylic fibers, among other applications [18].

## **4.2 Dry spinning**

The process of dry spinning involves the extrusion of a polymer solution into a heated air chamber by use of a spinneret. The fibers become hardened when the solvent evaporates, leaving behind the fibers. In the case of polymers that are capable of being dissolved in volatile solvents, dry spinning is an advantageous technique. It enables the manufacture of fibers that have certain qualities, such as porosity, among other characteristics. In the context of applications, dry spinning is often used for the production of acetate fibers and some kinds of polyesters [19].

## **4.3 Melt spinning**

Melting a polymer and then extruding it *via* a spinneret is the process that is involved in the liquefying process, which is also referred to as melt spinning. When the polymer is allowed to cool, it becomes solid and forms fibers. A number of advantages include the fact that liquefying is appropriate for thermoplastic polymers that may be melted without suffering any chemical transformations. This is a basic procedure that has relatively minimal costs associated with manufacturing.

Molten spinning is often used in the production of nylon, polyester, and polypropylene fibers, among other applications [20].

### **4.4 Gel spinning**

Gel spinning is a process that includes the creation of a gel from a polymer solution or melt. The gel generated by this process is then pulled or stretched in order to align the polymer chains. High-strength fibers are formed by the gel that has been aligned and then hardened. Gel spinning results in fibers that have good strength and modulus, which is a significant advantage. It is possible to improve the mechanical characteristics of the material by aligning the polymer chains during the drawing process. The production of high-performance fibers such as aramid fibers (e.g., Kevlar) is often accomplished by the use of gel spinning [21].

## **5. Variation in electrospinning**

Electrospinning may be categorized into various variations and types, each designed for specific purposes or desired outcomes. The selection of the electrospinning technology is based on the desired characteristics of the nanofibers, including their composition, structure, shape, and specific parameters relevant to the intended application.

#### **5.1 Conventional electrospinning**

This is the most basic kind of electrospinning, in which a polymer solution or melt is delivered to a spinneret using a syringe pump. A high voltage is delivered between the spinneret and a grounded collector, causing nanofibers to develop [22].

#### **5.2 Coaxial electrospinning**

The process of coaxial electrospinning includes the simultaneous spinning of several fluids with the use of needles that are either concentric or coaxial. Through the use of this technique, it is possible to create core-shell structures, in which one component acts as the core and another substance encloses it as a shell [23].

#### **5.3 Needleless electrospinning**

Needleless electrospinning employs a spinneret-free setup, such as a rotating disk or drum, instead of the conventional spinneret utilized in the process. This approach is highly suitable for large-scale production and has the capacity to streamline the electrospinning process [24].

#### **5.4 Emulsion electrospinning**

When one liquid is suspended in another in the form of tiny droplets, the result is an emulsion. One method for creating droplets within a continuous phase is emulsion electrospinning. This technique makes use of emulsions, which are combinations of two liquids that do not readily blend together. As the liquid evaporates, the electric field induces the formation of fibers [25].

## **5.5 Rotary jet-spinning**

The production of nanofibers is accomplished by the use of a spinning spinneret in this method. A centrifugal force is generated as a result of the spinneret's spinning, which makes it possible to deposit fibers on a collector in a regulated manner [26].

## **5.6 Bubble electrospinning**

The electrospinning process is altered by the incorporation of a gas using the bubble electrospinning technology. Depending on the gas and polymer solution mixture, this process might potentially lead to the formation of porous structures or hollow fibers [27].

## **5.7 Melt electrospinning**

The use of molten polymer rather than a polymer solution is what is involved in the process of melt electrospinning. As the molten polymer reaches the collector, it undergoes the process of electrospinning, which results in the formation of fibers [28].

## **5.8 Side-by-side electrospinning**

Side-by-side electrospinning is a technique that involves electrospinning two or more polymer solutions concurrently *via* separate channels, yet the resulting solutions are collected together. This enables the development of composite fibers that include components that are unique from one another [29].

## **5.9 Multi-jet electrospinning**

A multi-jet electrospinning setup uses a number of nozzles or jets to electrospin several polymer solutions simultaneously. Complex fiber structures may be created using this technique [30].

## **6. Electrospinning process configuration**

An electrospinning setup consists of three primary components: (i) a voltage source, (ii) a metal fiber, and (iii) a semiconductor collector [31]. Both of these components are essential to the process. It is possible to further subdivide the semiconductor collector into three primary groups, which are as follows: (a) fixed flat plate collectors, (b) rotating drum collectors, and (c) rotating disk collectors [32].

## **7. Working principle**

At the location of the liquid droplets created at the needle's tip, an adequate voltage is applied. The liquid gets charged. Electrostatic repulsion counteracts the surface tension. Thus, the droplet is stretched, and at a critical point, a stream of liquid bursts from the surface and spirals into a cone-like structure termed a "Taylor cone." Once the Taylor cone is constructed, the fluid jet is directed to the metal collector. The formation of solid fibers may be attributed to the liquid's thickness, cooling, or

evaporation. The solvent's swirling action causes the Taylor cone to evaporate off the collector during flight, coating the collector with a substance other than fiber [33–35].

## **8. Parameter optimizations for the electrospinning process**

The quality of electrospun fibers generated from polymer solutions may be influenced by critical parameters, which can be classified into three subclasses as given in **Figure 2** [36]:


## **8.1 Solution parameters**

A lot of things about the solution affect the properties of electrospun fibers made from polymer solutions. These include how fast the solvent evaporates, how thick the polymer solution is, how concentrated it is, and how high the surface tension is [37]. The concentration and viscosity of the produced fiber have a direct proportional relationship and have comparable effects. The creation of a bead is attributed to a polymer solution with low viscosity or low concentration, while a polymer solution with greater viscosity and concentration results in fibers with a larger diameter. Typically, raising the polymer solution concentration leads to a larger fiber diameter. The polymer solution exhibits bead formation due to its elevated surface tension, whereas a decrease in surface tension promotes the creation of smooth fibers. The polymer's concentration, surface tension, and solvent viscosity influence the electrons' spin rate and the fibers' shape [38].

## **8.2 Process parameters**

The surface tension of the polymer solution has a significant impact on the critical voltage, which is the voltage at which the charged jet begins the electrospinning process. The diameter of the electrospun fibers exhibited an inverse relationship with the applied voltage, such that lowering the voltage resulted in an increase in diameter, and

#### **Figure 2.**

*Optimizing parameters for electrospinning of fibers.*

vice versa. The primary factor influencing fiber solidification is the distance between the tip and the collector since it determines the amount of time the threads have to dry before reaching the collector. An insufficient gap encourages the development of beads, while a greater length yields bead-free fibers [39].

## **8.3 Environmental parameters**

Humidity and temperature are regarded as factors that are distinctive to the environment. The characteristics of electrospun fibers are influenced indirectly by these factors. By raising the temperature, the rate of evaporation rises, leading to an elevation in viscosity and concentration. Consequently, this promotes the creation of fibers with a greater diameter. Furthermore, the viscosity of the solution often reduces as the temperature rises. With an increase in humidity, there is a corresponding rise in the average diameter of the fibers [40].

## **9. Applications of electrospun nanofibers**

Electrospun nanofibers have been used in a broad variety of sectors as a result of their exceptional characteristics, which include a large surface area, a tiny diameter, and a shape that can be adjusted. The many domains in which electrospun nanofibers are used are outlined in **Figure 3**.

Electrospun nanofibers have a number of significant uses some notable applications of electrospun nanofibers include:

## **9.1 Biomedical**

Electrospun nanofibers have the potential to serve as drug delivery systems, offering precise and regulated release of medications while enhancing their bioavailability. Nanofibrous scaffolds are made to look like the extracellular matrix. This makes it easier for cells to attach and for tissues to grow back in areas like bone tissue engineering and wound healing. Electrospun nanofibers may be used to generate wound

**Figure 3.** *Applications of electrospun nanofibers.*

dressings that are both permeable and very porous. These dressings help encourage healing by maintaining a moist environment and avoiding bacterial infection.

### **9.2 Textile**

Nanofibers may be introduced into textiles to offer new capabilities, such as resistance to water, antibacterial characteristics, and better breathability. These functionalities can further enhance the textiles' overall performance. Electrospun nanofibers have the potential to improve the protective characteristics of clothing, making it more resistant to the effects of environmental conditions, diseases, and toxins.

#### **9.3 Defense**

Electrospun nanofibers' unique qualities make them useful in defense and military technology. Ballistic textiles made using nanofibers are lightweight, flexible, and projectile-resistant. Electrospun nanofibers may be functionalized to make chemicaland biological-resistant garments. Gas filtration using nanofibrous membranes protects military troops from harmful gasses and chemical warfare weapons. Electrospun nanofibers enhance air quality in confined military areas by filtering particulates. Nanofibers can make lightweight, flexible communication antennas. They may be used as sensors to provide real-time environmental data. Electronic warfare methods like signal jamming and interference may use electrospun nanofibers. Nanofibers having radar-absorbing or deflecting qualities may be used in military stealth equipment. Electrospun nanofibers may be utilized to make antibacterial wound dressings for speedier field injury recovery.

### **9.4 Filtration**

Nanofibrous membranes are well-suited for air and water filtration applications due to their expansive surface area and minuscule pore size. These membranes have the ability to extract particles and impurities from both the air and water. Electrospun nanofibers may be used for the specific separation of oil and water, making them potentially valuable in environmental cleanup and industrial applications.

#### **9.5 Sensor**

Nanofibers may be functionalized for sensing applications, which can detect changes in temperature, humidity, or certain chemical compounds. This technology applies to the field of sensor technology. Flexible Electronics: Electrospun nanofibers have the potential to be used as components in electronic devices that are both flexible and lightweight. These devices include capacitors and sensors.

#### **9.6 Energy storage**

Nanofibers have the potential to be exploited in energy storage devices, such as fuel cells and batteries, in order to enhance the performance of electrodes and the efficiency with which they conduct energy. Nanofibrous materials synthesized via the electrospinning approach have the potential to increase the efficiency of solar cells by offering a large surface area for the absorption of light and by enhancing the movement of electrons.

## *Introductory Chapter: The Electrospinning DOI: http://dx.doi.org/10.5772/intechopen.114224*

Nanofibers that have been electrospun are becoming more versatile, and current research efforts are helping to broaden their uses across a variety of sectors. Because they may be modified in terms of their qualities and usefulness, they are very useful in tackling unique difficulties that are present in a variety of sectors.

## **10. Future prospects**

Electrospinning is a method for creating a wide range of one-dimensional materials with adjustable structural properties. A lot of ground has been covered, but we still have a ways to go. Attaining consistent size and shape in nanofibers, while simultaneously satisfying the demands of end-users continues to pose a significant challenge. There are fewer diverse uses for natural polymers because they lack the mechanical and chemical characteristics of synthetic polymers. Researchers are working on hybrid polymer systems that include synthetic and natural polymers to address this. These systems are electrospun and provide improved functionality, particularly in biotechnology. In tissue engineering, adding nanoparticles after electrospinning therapy will increase its effectiveness. Studying the interactions between nanofiber designs and various drug-related release patterns is crucial. There are many unanswered questions and untapped potential applications for electrospun nanofibers in the energy sector, in filtering applications, and as sensors. To be useful, nanofiber membranes need upgrades to their surface area, pore diameters, and other properties.

## **Author details**

Khaled H. Mahmoud1 and Khalid S. Essa<sup>2</sup> \*

1 Department of Physics, College of Khurma University College, Taif University, Taif, Saudi Arabia

2 Faculty of Science, Geophysics Department, Cairo University, Giza, Egypt

\*Address all correspondence to: khalid\_sa\_essa@cu.edu.eg

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 2** Physics of Electrospinning

*Sharvare Palwai*

## **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.

**Keywords:** nanofibers, membranes, spinneret, sensor, nanoscale polymers

## **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

### *Physics of Electrospinning DOI: http://dx.doi.org/10.5772/intechopen.113010*

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, 12–19].

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 highvoltage 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

**Figure 1.** *Basic electrospinning set-up with a single needle [21].*

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].

## **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

**Figure 2.** *Multi-needle electrospinning equipment [23].*

## *Physics of Electrospinning DOI: http://dx.doi.org/10.5772/intechopen.113010*

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].

## **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].

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].

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 3.** *Needleless electrospinning [25].*

**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 drugloaded 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

**Figure 6.** *Co-axial electrospinning equipment [32].*

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.

## **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**.

## **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

#### **Figure 8.**

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

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

#### *Physics of Electrospinning DOI: http://dx.doi.org/10.5772/intechopen.113010*

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.

## **Author details**

Sharvare Palwai Physics, Tuskegee University, United States

\*Address all correspondence to: psharvare@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 3**
