Nanofibres in a spin
Nano-materials are seen as the future in fields as diverse as medicine, technology and chemistry, but the methods used to create them are not yet fully understood. This is especially true for the latest method of nanofibre production, centrifugal spinning, and a recent paper in JFM has sought to address this issue.
Nanofibres have some of the largest aspect ratios on Earth, with lengths on the order of metres compared to a diameter of only nanometres. This provides them with remarkable properties including an immense surface-to-volume ratio which is invaluable in applications such as drug delivery, filtration membranes and sensors. The production of nanofibers by centrifugal spinning creates a web that mimics the architecture and characteristics of natural tissues, allowing them to also be successful in various biological applications.
There are several methods of nanofiber production, but the method of centrifugal spinning has recently received a lot of attention as it has the potential to eliminate the limitations of previous production methods. The basic principle is to employ fast rotation rates, which then apply high stresses on the fibre leading to a high rate of thinning. A central container is filled with a liquid-polymer solution (approximately 10% powder added to water or another solvent) and then rotated at speeds of up to several thousand rpm. Several nozzles are attached to the container and the liquid is forced out as a thin jet.
As the fibre leaves the nozzle it moves away from the centre of rotation on a curved trajectory, meaning that the distance between two points on the fibre increases with time. This increase in distance creates a large evaporation rate for polymer solutions on the order of milliseconds, which ultimately causes the polymeric liquid to evaporate leaving behind only the solid fibre. One of the fibres produced by this method can be seen in the laboratory experiment video below.
In the recent JFM paper, lead researcher Seyed Mohammad Taghavi and his team at Université Laval in Québec develop a new mathematical model to predict the properties of nanofibers (such as trajectory, speed and radius) generated by the method of centrifugal spinning, as Taghavi explains: “the main idea is to use classical fluid mechanics methods and to apply it to a problem at the nanoscale.”
In contrast to the previous popular method of production by electrospinning, where the main force arises from the electric potential, in centrifugal spinning it is mainly the rotation that drives the fibres out of the nozzle causing them to thin. Most of the other forces present in the problem are ‘dissipative’ as described by Taghavi. “There are many important forces involved, such as viscous forces, shear-thinning and gravity which need to be considered as they all play a role by mainly resisting the formation of the thin fibre.”
Taghavi describes the model as a framework on which to build future models. “It’s an approximate mathematical platform that can be extended to include more complicated elements of the problem such as non-Newtonian effects and mass transfer.” Even considering these complexities, the model proposed by Taghavi and his team enables a fairly rapid solution, which can even be carried out on a regular desktop computer to obtain the fibre trajectory, radius and speed, as well as the effects of key forces on fibre production.
Ultimately, Taghavi and his collaborators intend to build on the original model with the next step the extension to the case of polymer melts. The method of centrifugal spinning is applied in the same way; however, the polymer-liquid solution is now replaced with high temperature polymer melt. In this instance, the nanofibers form when the liquid solidifies due to heat transfer after leaving through a nozzle. “We intend to build upon work from other fields, as we have done here, but the problem is that some of the conventional assumptions may break down at nanoscale and that is the challenge.”
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Noroozi, S., Alamdari, H., Arne, W., Larson, R., & Taghavi, S. (2017). Regularized string model for nanofibre formation in centrifugal spinning methods. Journal of Fluid Mechanics, 822, 202-234. doi:10.1017/jfm.2017.279