Nozzle Diameter

Increasing nozzle diameter has the effect of increasing fiber diameter, distribution and productivity [Heikkila et al 2008]. This may be due to larger amount of mass available for the spinning process. Using regression analysis, Thompson et al [2007] showed that a larger initial spinning radius will lead to a larger final jet radius, in other words, larger fiber diameter. Certainly, a larger nozzle diameter will contribute to a larger Taylor cone and corresponding initial jet radius at initiation. Experimental result has demonstrated that significantly larger nozzle diameter does led to a significant increase in fiber diameter as demonstrated by Kizildag et al (2012) using silk fibroin. A potential benefit of having a larger nozzle diameter is it favours the eruption of multiple jets which may contribute to higher productivity [Kong et al 2009]. For a larger exposed surface area, the likelihood of uneven distribution of the charges over the exposed solution surface at the nozzle opening will be greater and it will lead to regions of higher charge density which causes the eruption of multiple jets. Partial solidification of solution which are not drawn off by the electrospinning jet at the tip of the nozzle may also result in multiple jets or fluctuation in the jet diameter. Diameter instability of the spinning jet may be the main cause for wider fiber diameter distribution from electrospinning using larger nozzle diameter [Chen et al 2014]. A study by Scheideler and Chen (2014) showed that for high viscosity liquid, the flow rate to maintain a steady Taylor cone-jet in an electrified nozzle is dependent on the diameter of the nozzle while for low viscosity liquid, the flow rate is independent of nozzle diameter. A "capillary-viscous" scaling was proposed for high-viscosity liquid which is strongly dependent on nozzle diameter. Since the solution used in their experiment is not specifically targeted for electrospinning, further tests using electrospinnable solution is necessary to verify this hypothesis. In a systematic study to investigate the effect of nozzle diameter on electrospun fiber diameter, He et al (2019) used a shear flow model with polyethylene-oxide (PEO) solution as the model to determine any relationship between nozzle diameter and electrospun fiber diameter. The shear flow rate is affected by the nozzle diameter and the volumetric flow rate and it is well known that increasing solution viscosity will increase fiber diameter. Their experiment showed that as the nozzle diameter increases, the shear rate decreases and the viscosity increases. The increasing viscosity due to reduced shear rate correspondingly led to an increase in fiber diameter. Therefore, a smaller diameter nozzle is recommended for producing smaller diameter electrospun fibers.

The size of the electrospinning jet will vary according to the size of the nozzle. This will affect the specific surface area of the electrospinning jet. A smaller electrospinning jet diameter will have a faster solvent evaporation rate. Liang et al (2019) showed that the pore size on electrospun fibers varies with the nozzle diameter. When the nozzle diameter increases, the pore sizes on the electrospun fibers decreases. This has been attributed to slower solvent evaporation when nozzle diameters are larger which lead to larger electrospinning jet. When the nozzle diameter is smaller, the narrower electrospinning jet and its corresponding higher surface area encourages faster solvent evaporation leading to larger pore size.