Mass Production - Multiple-jets

In spinneret-based electrospinning, a limitation is the number of nozzles that can be packed into a given area. Despite this limitation, studies have shown that several spinning jets may erupt from a single nozzle given the right conditions. This significantly increases the amount of nanofibers that are spun and may offer the opportunity to increase the feed-rate while keeping the spun fibers in the nanometer dimension. Vaseashta reported the generation of multiple spinning jets from a single spinneret tip by using a wavy or highly contoured collector. He hypothesized that this contoured surface alters the distribution of the electric field between the surface of the polymer solution drop and the collector. This may result in pulling forces on the solution drop that act in multiple directions and thus giving rise to multiple electrospinning jets [Vaseashta 2007].

Wavy collector for encouraging multiple jets eruption from single nozzle tip

Multiple jets emitting from a single nozzle was also observed when the high voltage was applied to the collector instead of to the nozzle. This has been demonstrated in multiple nozzle system with plastic nozzles [Varesano et al 2009]. Larger nozzle diameter and higher applied voltage has also been demonstrated to favor the occurrence of multiple jets [Kong et al 2009]. Decreasing the distance between the spinneret tip and the collector while maintaining the voltage will also has the same effect as shown in video 1. A larger nozzle diameter allows a larger pendent drop to form at the tip and this provides more surface area for multiple jets to erupt. For the generation of multiple electrospinning jets from a single nozzle, Liu et al (2019) suggested that beyond a critical voltage, the electrospinning jet recedes into the needle and the electric field concentrates around the tip of the needle instead of the tip of the Taylor cone of the electrospinning jet. Presence of electric field concentrated at the needle tip encourages multiple electrospinning jets to erupt from the tip. However, the input charges from the needle now get shared between the multiple jets and this reduces the electrostatic forces that stretches the fiber and results in higher average fiber diameter. This also increases the fiber diameter distribution of the collected fibers. Tests using poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and polyvinyl alcohol (PVA) showed that their electrospun fiber diameter reduced with increasing voltage from 6kV, reaching a minimum diameter at 20kV before increasing with higher voltage. Their fiber diameter standard deviation also dropped to a minimum at 20kV before rising with higher voltage.

Some polymers may have a greater propensity to form multiple electrospinning jets. Eda et al (2007) showed that with linear polystyrene in various solvents, tetrahydrofuran (THF), chloroform, 1-methyl-2-pyrrolidinone (NMP), and N,N-dimethylformamide (DMF), multiple jets were formed under steady-state conditions. These jets were distributed radially around the edge of the nozzle and they formed solidified fibers around the edge of the nozzle after the spinning has stopped. However, when highly volatile solvent such as carbon disulfide (CS₂) were used, limited jet instability and secondary jet formations were seen. This is unlike cases where highly volatile solvent forms partial solidified solution which gathered at the tip of the nozzle and form secondary jets that breakout from the clump [Abdel-Hady et al 2011].

Breaking up of the electrospinning jet is likely due to high density of charges on the nozzle tip. For this, dielectric or conductivity of the solution will play an important role. Wu et al (2018) did an in-depth study on the dielectric value of the solution and its effect on multiple jets formation. Using a selection of solvents, they showed that as the dielectric constant of the solution decreases, the number of jets decreases. When the applied voltage increase, the number of jets increases due to higher charge density. However, when the solution dielectric constant was reduced to single digit, there was only a single jet despite further increase in applied voltage.