A review on nanofiber fabrication with the effect of high-speed centrifugal force field

Abstract

Among the traditional methods for nanofiber fabrication, their inherent defects limit their application in industry. This work presents a simple and novel spinning technology to fabricate nanofiber, which uses a high-speed rotary spinneret called high-speed centrifugal spinning. Unlike electrospinning, the electric field is not required, and it could fabricate nanofiber in bulk from melt or solution materials. This work introduces the mechanism principle and development of high-speed centrifugal spinning. Besides, the high-speed centrifugal spinning is compared with the traditional spinning methods. The jet movement and nanofiber formation process under the action of centrifugal force are explained in detail. The effects of equipment parameters and spinning solution parameters on final nanofiber morphology are presented. These parameters are controllable, they include rotational speed of spinneret, length and diameter of nozzle, spinning solution concentration, spinning solution viscosity and surface tension, and collection distance.

Introduction

Nanofibers are fibers with diameters of tens to hundreds nanometers, and they have more unique advantages than ordinary fibers.^1,2 As a result of their high porosity, excellent mechanical properties, high surface ratio, and so on,^3,4 nanofibers are widely applied in tissue-engineered scaffold, drug delivery, high-performance filter media, artificial blood vessel, biochip, nanosensor, composite material, and other emerging fields.^5–7 At present, the demand for nanofibers is increasing and the quality requirement of nanofibers is more and more high, especially as innovation-driven and green development become hot topic in industrial world. Researchers pay more attention to study an environment-friendly method to fabricate nanofibers, which not only improves the productivity but also collects ordered nanofibers. ⁸

The demand for fabrication of nanofibers on a large scale is so urgent with the rapid development of nanotechnology. ⁹ Nowadays, a lot of application research and methods of fabricating nanofibers have been proposed, such as melt spinning, self-assembly, phase separation, and electrospinning. ¹⁰ Among these methods, the electrospinning method is of great concern. ¹¹ However, electrospinning has some deficiencies which need development and improvement. In the past few years, researchers paid attention to centrifugal spinning technology and hoped to fabricate ultrafine fibers and even nanoscale fibers by centrifugal force. Some scholars have proposed an idea of fabricating nanofibers by the rotation method. This is the origin of centrifugal spinning. Moreover, they have made preliminary studies on the nanofiber fabrication. ¹² These studies show that the centrifugal spinning technology has an attractive prospect and the aforementioned deficiencies of electrospinning can be overcome. First of all, fabricating nanofibers by centrifugal force does not need high electric field. Besides, this method could not cause environmental pollution and the production efficiency of nanofiber is greatly increased (average 60 g/h using a nozzle). ¹³ At present, the research on the centrifugal spinning technology is still in its infancy. There is not much progress in theoretical and technical research, such as the movement of jets and nanofibers, nanofiber collection and yarn formation, optimization design of spinning mechanism, and so on. At the same time, the existing studies about centrifugal spinning only stop at the stage of fabricating disordered nanofibers.

In this article, the principle of centrifugal spinning is introduced and the centrifugal spinning method is compared with traditional methods. In addition, this study summarizes the effects of equipment parameters and polymer solution properties on the morphology of nanofibers in the process of centrifugal spinning.

The mechanism principle of traditional methods for fabricating nanofibers

At present, the traditional methods of fabricating nanofibers mainly include electrospinning method, melt-spinning method, self-assembly method, phase separation method, and so on.^14,15 Because of its simple principle, simple operation and low cost, electrospinning spinning is the most commonly used method to fabricate nanofibers.^16,17

The principle of electrospinning

The method of fabricating nanofibers can be traced back to the rayon patent which was applied by Formhals in 1934. The patent proposed to fabricate artificial fibers by using a high-voltage electric field known as electrospinning. Many scholars have studied the principle of this method to fabricate nanofibers and created a precedent for fabricating nanofibers by electrospinning in the past 10 years. ¹⁸ Rutledge and Fridrikh ¹⁹ studied the dependence of jet on operating variables and summarized the continuum level models of jet instability. In order to collect ordered nanofibers, Beachley et al. ²⁰ designed a roller-type collection equipment which was driven by a motor. They found that it has strong relationship between the orderly degree of nanofibers and spinning time. Besides, they studied the relationship between the mechanical properties of fibers and the roller rotational speed. Lu et al. ²¹ fabricated porous carbon nanofibers with conductivities and various surface areas via centrifugal spinning. The porous carbon nanofibers could be used in electric double-layer capacitors. A wheel-like collection device was designed by Theron et al. ²² The continuous nanofibers could be winded at the tip-like edge of the collection device.

The electrospinning device is mainly composed of a 1–60 kV high voltage power supply, a spinning container (mainly a syringe), a nozzle (needle), a collection plate, and a number of wires. ²³ The sketch of electrospinning is shown in Figure 1. There are many materials used for electrospinning, such as natural polymers, synthetic polymers, and their mixtures. ²⁴ In the process of spinning, the polymer solution is stored in the spinning container. The nozzle is connected to the positive polarity of high-voltage electric field, and the collection device is connected to the negative polarity. A droplet named “Taylor cone” will be formed at the tip of nozzle because there is an electrical potential difference between the polymer solution and collection plate. ²⁵ When the electrostatic field force overcomes the surface tension and viscous force of polymer solution, the jet is formed. ²⁶ Subsequently, the solvent volatilizes, and the nanofibers are collected at the collection plate.^27,28

Figure 1.

Sketch of electrospinning.

The fabrication of nanofibers by a high-voltage electric field is simple and reliable, but there are some insurmountable inherent defects which limit the large-scale application of electrospinning. ¹³ The main reasons are as follows. First, the high electric field is required in the process of nanofiber fabrication, especially in multi-needle electrostatic spinning. At the same time, the droplet at the tip of nozzle is unstable. It is easy to cause the nozzle clogging or the droplet dripping. Second, the production efficiency is low because the nanofiber production of each nozzle is only 0.1 g/h in the laboratory scale units. ¹³ The low production efficiency is the major bottleneck, which restricts the large-scale application of electrospinning with single nozzle. Third, electrospinning is sensitive to material conductivity, and only the polymer solution with certain conductivity could be used to electrospinning. The electrical potential difference needs to be increased when the polymer solution is less conductive. ¹³ Therefore, the type of spinning solution used for electrospinning is limited due to the polarity requirements of polymer solutions. At the same time, spinning jets could not be produced if the viscosity of melt materials is high in the process of melt electrospinning.

Researchers have not stopped the theoretical and practical exploration for overcoming the limitations of electrospinning in the last few years. ²⁹ In order to overcome the limitation of feeding rates and improve the production efficiency, many new electrospinning devices have been developed on the basis of conventional single nozzle electrospinning. Bubble electrospinning and multiple nozzle electrospinning are considered to have the potential for mass production. Liu et al. ³⁰ demonstrated the ability to fabricate uniform and continuous nanofibers by bubble electrospinning. They pointed out that the bubbles could be easily electrospun into nanofibers when the surface tension of spinning solution was less. Elmarco company has developed needle-free nanospider technology, which is designed for fabricating the highest quality nanofibers and enhancing productivity. Chen et al. ³¹ used electrospinning with multiple needles to increase the production efficiency of linear nanofibers. However, the jets are mutually exclusive because they carry the same kind of charges when they are ejected from the Taylor cone, which will lead to the morphology and diameter uniformity of nanofibers being poor. Dosunmu et al. ³² designed a cylindrical porous ceramic tube nozzle and a circular collection device to increase the nanofiber production efficiency. The multiple nozzles could fabricate nanofibers simultaneously, as shown in Figure 2. These are effective ways to increase the nanofiber production. However, when multiple needles or nozzles are arranged densely, the spinning jets undergo mutual repulsion and deviate from the original trajectory. Thus, the nanofibers cannot be received by the collection device. Li et al. ³³ combined centrifugal force with electrospinning to fabricate nanofibers for reducing the high-voltage electric field in electrospinning. This is centrifugal electrospinning, which is also called double spinning. The working principle is shown in Figure 3. The design idea is to fabricate nanofibers by rotating the needles of electrospinning. It can produce nanofibers with same quality, improve the production efficiency, and reduce the voltage from 30 to about 3 kV. However, it still needs a high-voltage electric field and the positive polarity of the high-voltage electric field is rotating with the needles. It is dangerous and cannot collect ordered nanofibers. These studies have some instructional significance to fabricate nanofibers by centrifugal force. But they are still transformed from electrospinning, and the preparation process is complicated.

Figure 2.

Electrospinning of polymer nanofibers from multiple jets on a porous tubular surface.

Figure 3.

Schematic diagram of centrifugal electrospinning.

The principle of melt-blown method

The process of fabricating nanofibers by the melt-down method basically follows the traditional melt spun technology. There are many advantages of melt-blown spinning such as high efficiency, low cost, and easy to large-scale production. ³⁴ At the same time, the solvent does not need to be processed specially. The schematic diagram of melt spinning principle is shown in Figure 4. The fiber webs that are obtained by the melt-blown method are micron or submicron scale webs with diameters of 40–2000 nm. ³⁵ When nanofibers are fabricated by thermoplastic polymer, the technology will show relative economic advantage and have the potential of fabricating nanofibers on a large scale. ³⁴ Hence, melt-blown spinning becomes an important method for fabricating nanofibers. For example, Rieter company (Switzerland) has invented a single-mode head-melted device for fabricating nanofibers and the single fiber with a diameter of only 500 nm. ¹ Hills company (USA) has made great development of the melt-blown method, too. The average diameter of single fiber made by this company is less than 250 nm, and the fibers with diameter of 50–400 nm are more than 90%. At present, Hills has completed parallel and multi-component composite nanofiber test. The production capacity of single nozzle production line is 1.6 kg/h, and it is estimated to be increased to 12 kg/h after commercialization. However, the melt extrusion rate of each spinning component hole will decrease when ultrafine fibers are fabricated. Because the spinning materials must be heated and be in the molten state for a long time, the spinning materials are prone to oxidative decomposition. Besides, the melt extrusion rate is low and the melting pressure of spinning components needs to remain stable, the design requirements for the spinneret hole must be very strict. ⁴

Figure 4.

Schematic diagram of melt spinning.

The principle of self-assembly method and phase separation method

The phase separation is an effective method for fabricating microencapsulation and fiber scaffolds. The process of the phase separation method includes dissolution, gelation, extraction with different solvents, condensation, and drying. ³⁶ Polylactic acid hollow fiber membranes with various structures were prepared by phase separation. But, the disadvantage of this method is that the cost is high because it consumes a lot of coagulants. Besides, it is difficult to control the condensation reaction and the nanofibers are easily doped with other chemicals. The self-assembly method refers to a technology in which the structural units of nanomaterial aggregate into a stable structure spontaneously by the interaction of non-covalent bonds.^37,38 However, it takes a long time to prepare nanofibers or nanofiber films by self-assembly, and the stability of the product is poor. The sketch of the self-assembly method is shown in Figure 5. These two methods of fabricating nanofibers are not common in the industry.

Figure 5.

Sketch of the self-assembly method.

The research progress of centrifugal spinning

In 1924, US patent proposed that fibers could be fabricated in a centrifugal manner. ³⁹ In 1986, European patent (EP0168817) presented that spinning material could be extruded through hole to form fibers by centrifugal force. After that, the centrifugal spinning has been improved by many patents in the United States and Europe. For example, the patent US2008050304 proposed an idea to fabricate nanoscale carbon fibers by using centrifugal force. A typical centrifugal equipment was designed by Wagner et al. ⁴⁰ The centrifugal force was generated by rotating a disk with holes so that thermoplastic materials such as molten metal or glass could be stretched to form fine fibers. The patent of “Melt and solution centrifugal spinning device for preparing non-woven” was applied by Changchun Institute of Applied Chemistry. ⁴¹ It was a new centrifugal spinning technology which used spacers with perforated rings to guide multiple fibers. These studies have explored the experience of fabricating fibers but the nanofibers still could not be fabricated.

In order to fabricate high-quality nanofibers by centrifugal force, researchers studied the factors influencing nanofiber quality. Badrossamay et al. ⁴² designed a method to fabricate nanofibers by rotary jet spinning. They pointed out that if the rotation speed increased, the nanofiber diameter reduced and the uniformity of the diameter distribution was better when the concentration of polymer solution was constant. In addition, if the rotational speed was constant, the concentration of polymer solution must be higher than a critical value so that the fibers can be fabricated. Huttunen and Kellomäki ⁴³ fabricated fine fibers from heating spinning polymer via the principle of cotton candy machine, which used centrifugal force. Senthilram et al. ⁴⁴ designed a rotating nozzle to get orderly nanofibers. They qualitatively analyzed the influence of rotation speed and solution concentration on the fiber diameter. Sarkar et al. designed a new method to fabricate fine fibers by using the centrifugal force, which was called forcespinning. Besides, they proposed that the nanofiber extension process could be divided into five processes, as shown in Figure 6.^45,46

Figure 6.

Centrifugal spinning fiber extension process: (a) jet exit, (b) orbital trajectory, (c) fiber vibration from aerodynamic, (d) orbital expansion, and (e) fiber collection method.

The principle of fabricating nanofibers by centrifugal spinning is using centrifugal force instead of electric force. ⁴² The spinning solution is injected into the spinning container, then it is ejected from the nozzle. ⁴⁵ There is a droplet with conical shape formed at nozzle under the action of surface tension, viscous force, and centrifugal force. ⁴⁷ In the process of centrifugal spinning, both conductive and non-conductive materials can be used to fabricate nanofibers. In addition, the clean production of nanofiber fabrication can be achieved because the spinning polymer does not need to add conductive solvent. ⁴⁸ In recent years, Z.-M.Z. has made a series of preliminary studies on nanofiber fabrication by centrifugal spinning and proposed combining the centrifugal spinning with ordered nanofiber collection. This is the main idea of the high-speed centrifugal spinning, as shown in Figure 7. Polyoxyethylene nanofibers with nanometer diameter fabricated by high-speed centrifugal spinning have smooth surface and uniform diameter distribution, as shown in Figure 8. These theoretical analyses and experimental studies show that the method of fabricating nanofibers by centrifugal force field has very important theoretical value and application prospect.

Figure 7.

The principle of nanofiber motion and yarn formation under the high-speed centrifugal force field.

Figure 8.

Scanning electron microscope images of polyethylene oxide nanofibers fabricated by centrifugal spinning with different concentrations: (a) 6 wt% and (b) 7 wt%.

At present, research on centrifugal spinning has made some progress. Stojanovska et al. ⁴⁹ found the most suitable processing parameters to fabricate lignin-based nanofibers via centrifugal spinning. Natarajan and Bhargava ⁵⁰ studied the effects of various parameters such as viscosity, chamber temperature, revolutions per minute, and humidity on quality of spinning fibers. Riahi ⁵¹ established a viscosity model for polymeric fluid to study the properties of polymeric jets in the process of centrifugal spinning. The spinning materials available for centrifugal spinning to fabricate nanofibers are mainly high molecular weight polymers such as polyoxyethylene, polyacrylonitrile (PAN), polycaprolactone, polylactic acid, and polyurethane.^52–54 In addition, with the wide application of composite nanofibers, fabricating composite nanofibers by centrifugal spinning has also attracted the attention of researchers. Zhang et al. ⁵⁵ studied the preparation and performance of polyvinylpyrrolidone/polyethylene composite fibers by centrifugal spinning. Agubra et al. ⁵⁶ fabricated binder-free porous Sn/C composite nanofibers via forcespinning and presented that the forcespinning is a viable method for fabricating nano/microfibers on a large scale. Zuniga et al. ⁵⁷ prepared multichannel TiO₂/carbon composite fibers by forcespinning technology and found that TiO₂/carbon composite fibers had better electrochemical performance. They proposed that these composite hollow nanofibers were promising as alternative anode materials for lithium ion batteries. Meanwhile, nanofibers fabricated by centrifugal spinning have potential applications in medicine. Loordhuswamy et al. achieved highly aligned ultrafine polycaprolactone/gelatin fibers with smooth surface by centrifugal spinning. Fiber property results and medical experiments proved that the fibers can be used as a wound dressing material. ⁵⁸

However, in the existing studies about the methods and theories of nanofiber fabrication, the nanofibers with different diameters are usually generated by adjusting the technological parameters and geometric parameters based on experience. ⁵⁹ These studies are limited to fabricating disordered nanofibers, and there are no studies about fabricating nanoyarns by collecting orderly nanofibers. In particular, there are no systematic research about the dynamic problem of polymer jet, nanofiber, and spinning equipment under the action of high-speed centrifugal force and temperature field. Besides, the theoretical and technical foundation of centrifugal spinning has not been formed.

The principle of high-speed centrifugal spinning

The centrifugal spinning device includes a spinneret which can accommodate polymer solution and be rotated by the motor. There is a nozzle connected to the spinneret. Thus, the polymer solution could be ejected from the nozzle by centrifugal force. ⁴⁵ A jet will be formed when the angular velocity is greater than the critical velocity, which is determined by the surface tension and centrifugal force. The schematic diagram of centrifugal spinning technology is shown in Figure 9. If the surface tension of polymer solution is too high, the jet will rupture and form small droplets. ⁵⁹ The material solvent evaporates and the jet moves along a curved trajectory after the polymer solution is ejected from the nozzle. Then, the jet continues to move forward by the inertial force and reaches the collection device finally. Generally, this process could be roughly divided into three stages.

Figure 9.

Schematic diagram of centrifugal spinning.

The first stage of the centrifugal spinning method is to inject liquid or molten polymer material into the container. As mentioned earlier, the nozzle is rotated by a motor. When the polymer material rotates with spinneret, the polymer material is exposed to viscous force, gravity, surface tension, and centrifugal force. ⁵⁰ The viscous force and centrifugal force play leading roles and others can be ignored. As the angular velocity of the spinneret increases, the polymer material will form a Taylor cone at the tip of spinneret when the centrifugal force of polymer material is equal to the viscous force. ⁴² Therefore, there is a critical velocity in the first stage, which is the key to forming the conical droplet. When the jet is ejected from the nozzle, the second stage begins.

In the second stage, the jet is exposed to surface tension, inertia force, and air resistance when the spinning material is ejected from the nozzle. ⁵¹ Then, the jet follows a curved trajectory and continues to be stretched under the action of the resultant force. ⁶⁰ At the same time, the solvent of the polymer material evaporates gradually and the solid fibers are formed. There are many external factors affecting the final diameter and morphology of fibers. The second-stage diagram is shown in Figure 10.

Figure 10.

The second stage of centrifugal spinning.

The third stage of centrifugal spinning is nanofiber formation and collection, as shown in Figure 11. The jet diameter will decrease with the effects of pull force stretching and solvent evaporation in the second stage. Finally, the nanofiber will reach the collection device and be collected.

Figure 11.

The third stage of centrifugal spinning.

The influence of machine and process parameters on fiber diameter and morphology

The final morphology of nanofibers will be affected by many factors in the process of centrifugal spinning, such as the characteristics of polymer material and other factors. ¹³ At the same time, they will affect the nanofiber practical application. These effects can be mainly divided into two categories. One category is the equipment parameters such as nozzle angular velocity, nozzle diameter, and the distance from nozzle to collection device. The other category is the properties of spinning solution such as surface tension, viscosity and spinning solution concentration, and evaporation rate of solvent. ⁵⁸

The rotating speed

It can be seen from the principle of centrifugal spinning that the driving force of polymer solution motion and jet stretching is centrifugal force. According to the formula of centrifugal force, the centrifugal force of spinning solution at the nozzle is related to the quality of spinning solution, the rotation radius, and the nozzle angular velocity. ⁵⁹

The spinning solution must overcome its own surface tension and viscous force at the nozzle, so that it could be ejected from the nozzle. When the motor speed is large enough, the spinning solution at the nozzle will get enough centrifugal force to form jet. ⁴⁹ Some studies have shown that the rotating speed of the nozzle is an important factor affecting the fiber diameter when the other experimental conditions are constant. ⁴⁵ In a certain range, the fiber diameter decreases when the rotating speed increases. However, when the rotational speed increases to a critical speed, the fibers will disconnect and form a lot of beads. It results in failure of nanofiber fabrication. Lu et al. ⁵⁹ fabricated PAN nanofibers by using different rotational speeds. The experiment result shows that the average diameter of PAN nanofibers increases from about 440 to 660 nm when the rotational speed decreases from 4000 to 2000 r/min.

The diameter and direction of nozzle

The nozzle is an important component in the process of centrifugal spinning because its shape, diameter, and direction will affect the jet’s initial shape and the fiber trajectory. ¹² The research emphasis is to determine whether there is an optimal curvature of nozzle orifice so that the final fiber diameter can be minimized. For studying the effect of orifice curvature on nanofiber morphology, Padron et al. ¹³ selected nozzles with different exit angles to represent the orifice curvatures. The experimental results show that when the exit angle is about 30° in the opposite direction of rotation, it is helpful to fabricate coaxial nanofibers. Moreover, as the exit angle gets closer to a right angle in the direction of rotation, the jet initial diameter reduces.

In addition, the nozzle diameter is an important factor to determine the flow rate of polymer solution and the jet initial diameter. ⁵⁹ When the nozzle diameter is small, the flow rate of the spinning solution will be inhibited, which makes it easier to fabricate nanofibers. ⁴⁸ However, reducing the nozzle diameter will increase the viscous force of polymer solution. Thus, the nozzle diameter should be optimized necessarily for fabricating high-quality nanofibers.

Collection distance

The collection distance refers to the distance from the nozzle to the collection device. The jet will continue to move to the collection device after it is ejected from the nozzle. Therefore, the collection distance will directly decide the flight time of the jet. ⁵⁸ The minimum distance between the nozzle and the collection device is necessary because the solvent needs enough time to be evaporated before it reaches the collection device. Therefore, the jet will inevitably extend for a longer period of flight time when the collection distance increases, which is conducive to reduce the fiber diameter. ⁵⁵ The distance between the nozzle and the collection device depends on the solvent evaporation rate and the jet cooling rate. The collection distance is usually 30–80 cm. ⁴⁹ Centrifugal spinning experiments show that as the collection distance is increased from 40 to 70 cm, the average nanofiber diameter decreases from 700 to 600 nm. However, the fibers are contorted and easily broken if the distance is too large. At the same time, the movement path of the jet will be enlarged and some fibers cannot reach the collection device.

The surface tension and viscosity of spinning solution

In centrifugal spinning, two parameters of the spinning polymer are significant. One is the surface tension of polymer solution. The surface tension plays an important role in the nanofiber formation because it is the main force to shrink the jet surface area. In the process of centrifugal spinning, the surface tension, the centrifugal force, and the viscous force interact to determine the final morphology of fibers. ⁵⁹ The jet tends to be stretched and the surface area tends to be expanded with the effect of centrifugal force field. Because the surface tension tends to minimize the surface area of the jet, the surface tension of spinning solution must be limited in a certain range. If the surface tension is too high, the jet will break and form beaded fibers. ¹³

The viscosity of spinning solution is the other important parameter. ¹² It also affects the nanofiber fabrication in the process of centrifugal spinning. The viscous force prevents the jet shape changing rapidly and helps to form smooth nanofibers. Because the polymer solutions used to fabricate nanofibers are very viscous, the solution could not be ejected from the nozzle if the centrifugal force is not strong enough to overcome the viscous force, it is difficult to generate jets and fabricate nanofibers. However, the jet will break or produce droplets if the viscosity is too low. ⁵⁰ At the same time, the flow rate of the spinning fluid will be affected by the viscosity of spinning solution because viscous force could resist the relative motion between spinning solution and nozzle internal wall. Besides, as jet diameter decreases and solvent evaporates, the viscosity of spinning solution will change, which increases the instability of jet.

The spinning solution viscosity is usually controlled by selectively regulating molecular structure, molecular weight, and processing temperature. However, the most reasonable and convenient way of controlling the polymer solution viscosity is to adjust the polymer solution concentration. The polymer solution concentration directly affects the polymer solution viscosity, surface tension, and the nanofiber morphology. ⁵⁹ For example, the jet is discontinuous when the spinning solution concentration is low. The nanofibers tend to be uniform and the orderly degree of nanofibers increases when the concentration increases. In the process of fabricating nanofibers using polyethylene oxide with a molecular weight of 1,000,000 g/mol, when the concentrations of polyethylene oxide solution are 3–7 wt%, the nanofiber diameter is relatively uniform. The average diameter of nanofibers is about 400 to 700 nm when the spinning solution concentration is 6 wt%. However, if the spinning solution concentration is 8 wt%, the average diameter reaches about 900 nm.

Research direction of high-speed centrifugal spinning

The movement of jet and nanofiber

The diameter, morphology characteristics, and diameter distribution of nanofibers are inextricably interrelated with the polymer solution parameters and the experimental equipment parameters in the process of electrospinning and centrifugal spinning. ⁵⁰ Studying nanofiber formation mechanism and establishing above relevance could provide a theoretical basis for fabricating nanofibers on the effect of high-speed centrifugal force field. Therefore, it is necessary to study the force and movement of jets and nanofibers in the spinning process.

Many scholars have paid more attention to study the spatial movement and dynamic tension of nanofibers by using applied mathematics and mechanics in recent years. ⁵¹ Faraz et al. ⁶¹ established the unstable fiber movement equation based on the electric field, magnetic field, and various forces of fibers in electrospinning. Xu et al. ⁶² realized the nanofiber fabrication by adjusting the nozzle parameters and solution concentration. The movement of jets and nanofibers on the effect of high-speed centrifugal force includes two typical stages. In the first stage, the polymer solution is smoothly stretched to form a jet under the action of centrifugal force, viscous elastic force, and surface tension. ⁶³ The jet only does uniaxial stretching motion and its shape does not change with time. This is a stable movement stage. In the second stage, the jet starts to bend because it is subjected to the influence of air resistance, gravity, and temperature fields. Then, the jet will undergo an unstable stage of movement before reaching the collection device. ⁶⁴ The process of fabricating nanofibers by centrifugal force is complicated. It involves many different disciplines such as physics and mechanics. In addition, it needs to study the rheology, aerodynamics, mass transport, and heat transport. At present, the research on the nanofiber fabrication mainly focus on using different polymer solutions to fabricate nanofibers. There is little research on the nanofiber preparation process, such as the law of solution motion, critical condition of forming jet, and equipment optimization, especially the influence of temperature field and airflow field on nanofiber forming. Besides, the change rule and prediction model of nanofibers, jet dynamic tension and space movement are rare, too. The relationship model among the technological, geometrical parameters, and the nanofiber diameter has not been established. ⁶⁵ The optimization methods of various parameters, which could provide the theoretical basis for efficient and clean nanofibers fabrication, have not been established. In addition, the effects of the solvent evaporation and temperature field on fiber diameter and morphology are not considered. These influencing factors are often ignored or simplified in practical application, which will make nanofiber diameter uneven. These questions should be further studied.

The study on orderly nanofiber collection and nanospinning mechanism

The continuous single nanofibers which are collected, ordered, and processed like the textile industry yarns can be used widely and have broad application prospects. ⁶⁰ However, the nanofibers are disordered and exist in the form of non-woven fabrics in most existing nanofiber fabrication methods. These defects will limit the application of nanofibers.

After the jet is ejected from the nozzle, it will be affected by the gravity, air resistance, and centrifugal force. So, its movement trajectory is not a straight line but a complex three-dimensional space motion. In order to realize the ordered nanofiber collection, many scholars have made a lot of attempts and gained some experience, which provided useful reference for the subsequent studies.^12,50,60 Badrossamay and Senthilram studied the effect of the relationship between the collection distance and the solvent evaporation time on collecting ordered fibers when the concentration of solution and the motor speed were constant. For different polymer solution jets, the characteristic parameters are quite different. Studying the principle of nanofiber collection to get more orderly nanofibers is the focus of research on nanofiber collection. However, there are a few literature to optimize the design of nanoscale spinning mechanism and process parameters.^12,13 At the same time, nanofibers will be further stretched further in the process of nanofiber collection, but research in this area is still rare. These are questions that need to be studied further.

Conclusion

The technology of centrifugal spinning starts late and develops slowly, so it is still imperfect at present. For instance, the fiber diameter is thick and the fiber quality is hard to be guaranteed because the centrifugal spinning technology and device are immature. Besides, the spinning process needs a high rotational speed, which has potential safety hazard. In the view of nanofiber fabrication process, there are some theoretical problems that need to be studied in depth. For example, the mathematical model of the relationship between technological parameters, geometric parameters, and the nanofiber diameter has not been established. Furthermore, the parameters have not been optimized for efficient green production. These factors have obvious influence on the nanofiber diameter and morphology, but the existing studies do not consider them carefully. In addition, the design and selection of heating device, temperature measurement, and control system are more complicated when the polymer is melt. However, fabricating nanofibers by the high-speed centrifugal spinning could overcome the inherent defects of electrospinning technology. The high-speed centrifugal spinning will be more and more widely used in academic research and industrial application with the further development of technology.