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Mass Production

Electrospinning nanofibers at an industrial scale [Photo courtesy of Esfil Techno].

One of the greatest drawback of electrospinning is relatively low rate of generating large mass or volume of nanofibers. Early researchers have tried increase the productivity of electrospinning by employing more electrospinning nozzles. However, it is soon discovered that there is a limit to the density of electrospinning jets that can be packed at the spinning head. Unlike traditional fiber spinning where the spinning nozzle can be packed at high density, the same design applied on electrospinning will only results in spinning from the nozzles at the periphery while the polymer solution will drip out from the nozzles in the middle. This is probably due to unfavorable electric field profile for electrospinning to initiate at the middle nozzles [Varesano et al 2009]. An investigation by Wang et al (2018) using multiple nozzles arranged linearly highlighted the interference between electrospinning jets. In a near field electrospinning setup with a rotating drum collector, deposition distance between fibers from marginal nozzles were greater than intermediate nozzles. The intermediate nozzles experience electric field shielding while the marginal nozzles at the end of the array of nozzles were been forced outwards by the electric field. Further, the electric field shielding is most significant at the middle of the array resulting in the weakest electric field at the middle nozzles. The Taylor Cone formed at the middle of the nozzle arrays is the largest with the same flow rate. In some cases, the electric field strength is so weak that the solution drips from the tip of the nozzle. Even if fibers can be formed from all the nozzles, the variation in electric field will invariably results in poor fiber consistency. Nevertheless,there are a few companies that are able to produces nanofibers are a commercial level using spinneret-based electrospinning.

To overcome the need to optimize the spinning nozzle density, spinning head that do without nozzles have shown to be equally capable of producing nanofibers. Termed free-surface electrospinning, the principle behind this method is to coat a layer of the solution on an electrode or electrified surface. This electrified surface creates a strong potential difference from the collector. Once the solution is sufficiently charged, electrospinning jets will erupt from the surface to the collector. There are several variations to this method such as having a drum, disk and wire string as the electrode where the solution is constantly applied.

Since electrospinning will commence when the repulsive force from the local charge density exceeds the surface tension, disruptions to a calm solution surface may lead to a temporary increase in the local charge density and thus initiate electrospinning. This hypothesis has been tested in "bubble" electrospinning. In this process, air bubbles were been blown from the bottom of a basin of polymer solution. As the bubble bursts at the surface, electrospinning jets are anticipated to erupt from the disrupted surface. A study [Liu and He 2007] have shown that this may be possible, however, it is still inconclusive as to whether this method is able to mass produce nanofibers as only a few strands of fiber were shown in the prototype setup.

A probable limitation of free-surface electrospinning is that the control of fiber diameter may be more difficult due to variations in the size of the jet stream. Evaporation of the solvent on the free surface may also cause local fluctuation in the spinning concentration. The fibers that are spun may be long but will not be continuous as the individual spinning jets are interrupted as the solution replenishes. The voltage required for spinning is generally much higher than spining from nozzle due to greater dispersion of the charges on the free surface.

One of the most interesting mass-production setups is developed in L. Ya Karpov Institute to produce the Petryanov filters. In one version of the setup, instead of having the solution to flow out continuously from a nozzle, a swirled airflow is used to breakup the electrified solution into large number of jets within an annular channel [Filatov et al 2007].

Wire electrode
Drum "spinneret"
Published date: 19 March 2013
Last updated: 4 September 2018



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