"Watching the dense jets of solution on the stainless steel mesh spew out and spinning efficiently, I instantly felt that all my efforts were not in vain." For the scientific research "victory" scene in the middle of winter in Beijing, Wu Hui, associate professor of the School of Materials Science of Tsinghua University, said.
(Source: Science Advances)
Recently, his team, together with the research group of Zhao Lihao, associate professor of the School of Aeronautics and Astronautics of the university, jointly developed a new technology of needle-free solution gas spinning that combines the principle of Carmen vortex street, providing a new idea for large-scale production of nanofibers.
Figure | Wu Hui (Source: Tsinghua University)
Because of their unique physico-chemical properties, nanofibers are used in different fields such as environmental filtration, energy storage, flexible electronics, tissue engineering, intelligent fabrics, and personal virus protection. In order to promote the further application of nanofiber materials in end products, more advanced manufacturing science and related technologies must be developed, and only in this way can large-scale production of nanofibers be achieved with high efficiency, low cost, and continuous stability.
Some of the current nanofiber fabrication methods, such as template synthesis, hydrothermal and molecular self-assembly methods, are still in the laboratory-scale development stage due to the complexity of the equipment and low productivity. Meltblown is a widely used method for preparing microfibers in industry, but this method is difficult to prepare fibers with a diameter of less than 1 μm and is only suitable for thermoplastic polymers. Solution spinning methods, including electrospinning, centrifugal spinning, mechanical stretching and solution gas spinning, etc., so that various types of microfibers can be prepared and have attracted wide attention.
In a typical solution spinning process, the polymer solution is ejected from the tip of the needle and stretched by electrostatic, centrifugal or gas shear forces, followed by evaporation of the solvent to obtain fibers. Electrospinning is a widely used solution spinning technique for the manufacture of nanofibers.
However, its main technical shortcomings, including complex electric field design, solvent evaporation and fiber collection difficulties, high-pressure safety issues, needle blockage, droplet formation, and low yield issues, hinder its widespread use in industrial environments.
Centrifugal spinning can achieve higher fiber productivity, but its average fiber diameter is still higher compared to electrospinning. Therefore, despite the rapid development of existing nanofiber manufacturing technologies, it is still challenging to produce on an industrial scale without compromising their quality.
Use an electrostatic field as an alternative
As an alternative to electrostatic fields, high-speed airflow as a driving force for spinning microfibers from solution is attracting more and more attention. In a typical solution gas spinning process, the shear force induced by the air flow at the gas-liquid interface is used to refine the solution extruded from the tip of the needle, forming a jet of liquid along the direction of flow.
Subsequently, the airflow effectively helps the solvent evaporate, resulting in high-quality fibers. It is reported that so far, only needle-based solution gas spinning technology has been reported, while needle-free solution gas spinning technology has not yet been realized.
While much success has been achieved in conveying solutions through needles, issues such as high flow resistance, droplet formation and jetting, and needle clogging that can result due to rapid volatilization or curing of solutions greatly limit the stability, fiber quality, and productivity of the spinning system.
In addition, if the gas shear stress of the liquid jet can be further strengthened, the drafting effect and solvent volatilization during the spinning process of the solution can be enhanced, the solution gas spinning will have more application value. Therefore, there is a great need to develop a new gas spinning system that allows the air stream to interact with the spinning solution in a more efficient manner, resulting in the preparation of high yields of nanofibers.
In 1911, the famous aerodynamicist Theodore von Kármán discovered the Carmen vortex phenomenon, an unusual alternating vortex produced by a fluid flowing through a cylinder, which can be found at various scales of fluid motion. If the gaseous Carmen vortex can be explored to control the liquid jet and further promote fiber formation, this will have great scientific significance and importance.
Construction of a needle-free solution gas spinning system based on Carmen Vortex Street
Based on this, Wu Hui's team designed and built a needle-free solution gas spinning system based on Carmen Vortex Street to achieve high-throughput production of nanofibers. This method uses a newly designed roll-to-roll system to achieve needle-free solution transfer, thereby manufacturing nanofibers. A continuously moving closed-loop nylon wire carries the spinning solution out of the reservoir, and then, driven by a high-speed air stream, forms a Taylor cone and sprays at high speed, forming nanofibers by rapid stretching and oscillation.
The technology precisely designs the airflow structure of the air jet combined with the Carmen Vortex Street. Through the study of fluid theory and computational fluid dynamics simulation, they found that the high-speed air flow through the nylon line will produce strong shear stress and Carmen vortex on the leeward side of the nylon line, strong shear stress will promote the formation of the Taylor cone and the refinement of the solution jet, while the Carmen vortex will disturb the flow field and accelerate the rotation of the air flow from laminar to turbulent, ultimately promoting the volatilization of the solution jet. The combined effect of the two has greatly promoted the high-throughput production of nanofibers.
On March 16, the paper was published in Science Advances under "High-throughput production of kilogram-scale nanofibers by Kármán vortex solution blow spinning."
Figure | Related Papers (Source: Science Advances)
For the results, one of the reviewers commented: "Wu and his colleagues proposed an original method for improving the solution of gas spinning to produce a large number of high-quality nanofibers. The method has several advantages – in addition to removing existing nozzles and needles, it makes further stretching and refinement of the fibers possible due to the large velocity gradient formed by the leeward surface of the line. The kinetics of the airflow caused by the Carmen vortex also promotes the rapid evaporation of the solution. ”
Another reviewer said: "Using moving lines and Carmen vortexes is a very interesting and innovative idea. Overall, I appreciate the novelty and exquisite use of Carmen vortex jets to make nanofibers at a higher speed than traditional nozzles. This work is fully supported by experimental data and presented in a clear manner. ”
Another reviewer commented: "The large-scale preparation of nanofibers is indeed a challenge worth paying attention to. The author's needle-free solution gas spinning strategy (KV-SBS) proposed in the article is eye-catching by using wire or stainless steel mesh instead of needles. KV-SBS has had a positive impact on the industrialization of nanofibers, which is undoubtedly a valuable work. The author proposes a solution gas spinning strategy with high productivity and universal applicability, and makes a full theoretical analysis of the spinning strategy. ”
Realize needle-free spinning systems
Wu Hui said that when he was doing needle-based solution spinning, he often encountered the problem of the solution blocking the needle, which affected the quality of the fiber sample, so he has been thinking about how to achieve needleless spinning system. At the same time, considering that the air flow in the solution gas spinning is the driving force of the solution being stretched, it can also play a role in assisting the volatilization of the jet, and it has been hoped that the form of gas and liquid action can be cleverly designed to strengthen the shearing and auxiliary volatilization of the air flow.
He noticed that in the Carmen Vortex phenomenon, there would be very pronounced vortex turbulence in the fluid. If the jet is placed in this turbulent flow, this can be very helpful for solvent volatilization and fiber formation during the spinning process.
After having an idea, Wu Hui immediately called the students to verify it together, and DIY developed a simple manual device, which brought a thread out of the beaker containing the solution and purged it with high-speed air flow, and found that the fiber was indeed obtained. Although the quality of the fibers obtained at that time was not particularly good, it still made it very exciting, at least in the right direction.
But the process that followed was not always smooth. In the process of exploring how to make the spinning system stably prepare high-quality nanofibers, many technical problems were encountered, such as how to design the system configuration and construction device, how to better regulate the transportation of the solution, and so on.
During this period, the team and collaborators theoretically explored the mechanism of fiber forming, and used high-speed cameras to conduct a large number of experimental observations on the spinning process, and with the help of the linkage of theory and practice, they explored the improvement of the device and the optimization of experimental parameters step by step. As a result, the stable operation of the spinning system is achieved.
What makes Wu Hui particularly memorable is that when shooting the experimental process with a high-speed camera, in order to shoot better results, he often wears protective glasses for a long time. When using stainless steel mesh as a solution carrier for spinning, higher requirements are put forward for the intensity and strength of the light taken. At that time, they shot for a long time, and finally on a very late winter night, they shot a more satisfactory video.
It has a wide range of applications in environmental filtration and tissue engineering
Nanofibers have great application prospects in many fields such as environmental filtration, energy storage, flexible electronics, and tissue engineering. In order to promote the real commercial application of nanofiber materials, it is necessary to develop more efficient nanofiber manufacturing technologies.
The research group has long been committed to the exploration of basic scientific problems of nanofibers, the research and development of large-scale manufacturing technologies and the practical application of nanofibers. As a methodological paper, Wu Hui hopes that this achievement can promote the application of nanofibers in many fields.
In addition, the research group has explored the application of nanofibers in many fields. In the future, on the basis of existing work, they will continue to develop needle-free solution gas spinning systems to promote the preparation of polymers, carbon, ceramics and other nanofiber materials, and promote the application of nanofibers in many fields such as filtration and adsorption, high temperature insulation and flexible electronics.
In the follow-up plan, on the one hand, the team will further enlarge the needle-free spinning platform built by the research and explore further expanding the manufacturing scale of nanofibers; on the other hand, it will also be committed to further exploring the design and optimization of needle-free spinning systems from theory and practice to further improve the production efficiency of fibers.
-End-
reference:
1、Li, Z., Cui, Z., Zhao, L., Hussain, N., Zhao, Y., Yang, C., ... & Wu, H. (2022). High-throughput production of kilogram-scale nanofibers by Kármán vortex solution blow spinning. Science Advances, 8(11), eabn3690.