Spontaneous self-bundling

If the electrospinning jet is able to self-bundle and be drawn into a yarn, this will certainly make spinning nanofibrous yarn a breeze. Preliminary studies suggest that the conductivity of the solution has a significant impact on the self-bundling capacity of the electrospinning jet. Solution spike with salt to increase its conductivity has been shown to spontaneously self-bundle without any stimulus [Wang et al 2008]. It was hypothesized that with a high conductivity, the charges carried by the electrospinning jet that is closer to the grounded collecting drum will be discharged and consequently, this neutral segment of the electrospinning jet will attract the charged segments of the jet closer to the spinneret [Wang et al 2008]. A similar observation and explanation was offered for electrospinning of poly(p-xylenetetrahydrothiophenium chloride) (precursor of PPV) [Okuzaki and Yan 2010]. Wang et al (2021) reported the auto-twisting of self-bundling electrospun yarn. In their experiment, they used a blend of ethylene glycol (EG)-treated cellulose nanocrystal/polyvinyl alcohol/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (CNC/PVA/PEDOT:PSS) for electrospinning. The electrospun conductive nanofibers stand on the collector and attract newly electrospun fibers to them, causing the charged bundle to twist. As there were multiple strands of fibers standing on the collector, incoming fibers attracted to the free end would cause the standing fiber to rotate around one another as it collides with the standing fiber. Repulsive electrostatic forces between standing fibers prevent the free ends from touching as the twisting and bundling tightens the wrap. With increasing electrospinning duration, the twisting effect becomes more pronounced and the diameter of the yarn reduces. A 10 min electrospinning duration gave a 100 µm diameter yarn but a 20 min duration gave a 60 µm diameter yarn despite having more fibers. Based on the current setup, it seems that only a finite length of twisted yarn can be produced at one time as it is necessary to have a base to anchor the fibers for twisting. This method may be used in applications where a short fiber bundle is needed instead of continuous fiber yarn.

(a) Schematic diagram of nanofiber yarn formation. Stage 1: Individual standing fibers populate the collector plate and lengthen. Stage 2: Fiber bundles twist from electrostatic and mechanical forces to form yarn with neighboring fibers. Stage 3: Newly spun nanofibers attach to the growing yarn structure and continue to twist about the longitudinal axis. (b) Structural transformation from a single fiber to a yarn, demonstrating collection of single fibers into an organized structure in stage 2 and 3. (Scale bar: 100) (c) Time lapse sequence of stage 3 nanofiber yarn formation at 62, 85 and 107 s [Wang et al 2021].

An AC power supply may be used to encourage self-bundling of the electrospinning jet due to the presence of alternating charges on the electrospinning jet. During the flight of the spinning jet, segments of the jet with positive charges will be attracted to segments with negative charges. It was observed that the jet first spreads out from the spinneret tip and subsequently converges in mid-flight to form yarn. Increasing applied frequency was found to increase the fiber thickness and likelihood of beads formation due to reduced stretching of the jet in the presence of differing charges [Maheshwari et al 2009].

Polymer	Solvent	Take-up speed	Reference
Polyacrylonitrile (PAN)	dimethyl formamide and benzyl triethylammonium chloride	0.9 m/s	Wang et al 2008

Pros

• Collected yarn is dry

Cons

• Difficult to control yarn density
• Difficult to control yarn take up speed
• May experience frequent stoppages

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Content [Yarn spinning setup]