Solution Feed-rate

Within a range, the solution feed-rate generally does not have significant impact on most electrospun nanofiber [Beachley et al 2009]. There must however, be a balance in the rate at which the solution is dispensed and the rate at which the solution is drawn off the nozzle tip during electrospinning. At this optimum feed-rate, the distribution of fiber diameter is at the narrowest and any deviation will result in greater fiber diameter spread [Zargham et al 2012]. When the solution feed-rate is too high, periodic dripping of the solution will occur. Electrospinning of polyvinyl alcohol solution by Rodoplu et al showed that at higher solution feed-rate, beaded fibers were observed. Several studies have reported an increase in fiber diameter [Zargham et al 2012, Milleret et al 2011, Wang et al 2009] and ribbon fiber formation [Chen et al 2014] with increasing feed-rate and further increase may result in the formation of droplets and wet fibers [Chowdhury et al 2010]. This is due to increased volume and initial radius of the electrospinning jet leading to reduced bending instability and subsequently increases in fiber diameter. 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. When the feedrate raises above a certain limit, the respulsive force due to the charges on the dispensed solution is insufficient to draw the solution away from the nozzle tip [Chowdhury et al 2010]. This may result in excess solution dripping off the nozzle and deposition of wet fibers. Large beads may also form on the fibers [Zhang et al 2005] although the mechanism may be different from that of low concentration solution. Such beads are likely to come from excess solution gathered at the spinneret tip being carried away by the electrospinning as beads. Conversely, a reduction in feedrate may give smaller fiber diameter. With electrospinning of TiO₂-polyvinyl pyrolidone (PVP) from a solution of titanium tetraisopropoxide (TTIP)/PVP, Someshwararao et al (2018) showed that a reduction of solution feedrate from 1.2 ml/h to 0.6 ml/h was able to reduce the electrospun fiber diameter from 247 nm to 111 nm. The electrospun nanofibers with reduced diameters were smooth and without any beads. This shows that at optimum feedrate, stretching of the solution droplet at the tip of the nozzle can be maximised without breaking the continuous jet. When comparing various methods of mass producing electrospun fibers, the use of nozzles have the advantage of controlling the feedrate. In contrast, free surface electrospinning does not have control over the amount of solution that are drawn off from the surface in the electrospinning jet.

Keeping solution viscosity constant, Veerabhadraiah et al (2017) showed that feedrate has the most significant influence on electrospun polyvinyl acetate fiber diameter based on Taguchi's Design of Experiment. Their experiment compared the parameters, feedrate, voltage and distance between spinneret tip and collector. The feedrate also has an influence on the evaporation rate of the electrospinning jet. At higher feedrate, greater volume of solution on the jet would take a longer time for drying during flight. This would result in beaded fibers. At feedrate of less than 1 ml/h, diameter down to 24.83 nm have been fabricated.

Reduction in feed-rate from a high threshold has been shown to reduce the size of the beads until smooth fibers were collected [Rodoplu et al 2012]. When the feed-rate is insufficient to meet the fiber drawing rate, interruption of the spinning occurs and the continuous formation and disappearance of Taylor cone within the nozzle give rise to a larger deviation of fiber diameter [Zargham et al 2012].

In some cases, increasing the feed-rate does not lead to an increase in average fiber diameter. In a study by Schoenmaker et al [2012], the fiber diameter first increase then decrease when the feed-rate was increased. At the largest fiber diameter, the standard deviation of the fiber was also at its highest [Schoenmaker et al 2012]. While the increase in diameter with feed-rate may be attributed to the earlier explanation, subsequent decrease in fiber diameter must be due to alternative mechanism coming into effect.

Secondary jet splitting off from main jet when there is excess solution. [Abdel-Hady et al. ISRN Nanotechnology, vol. 2011, Article ID 851317, 14 pages, 2011. doi:10.5402/2011/851317. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

The feed-rate which gave the highest fiber diameter and standard deviation may be due to secondary jets splitting off from the main jet at the nozzle [Schoenmaker et al 2012]. This phenomena may be due to excessive solution gathering at the nozzle tip as the spinning rate was insufficient to draw the available solution from the tip. Subsequent partial solidification of the excess solution at the tip encourages the eruption of secondary jets giving rise to an increase in the standard deviation as fibers from the secondary jet have diameter that differs from that of the primary jet. Further increase in feed-rate increases the volume of excess solution and subsequent solidification may lead to the disappearance of a primary jet and replaced by multiple secondary spinning jets.

A high solution flow or feed rate may lead to changes in the spinning jet behavior according to Theron et al [2004] due to its effect on the volume charge density. Consider a set up where the solution is charged using an electrode at the end of the syringe near to the piston/plunger instead of at the spinneret tip. While the rate of charges that are fed to the solution remains relatively unchanged even for higher solution feed-rate given the faster drift velocity of the ions compared to the fluid velocity in the syringe, the rate of withdrawal of the oppositely charged ions from the solution is reduced. This results in a net lower excessive like-charge in the solution at higher solution feed-rate. In solution where the volume charge density is intrinsically low, this will facilitate the formation of garlands where different section of the fibers will merged in flight [Reneker et al 2002].

Another effect of feed-rate is on the phase separation of the solution during electrospinning. A study by Shin et al (2012) showed that increasing feed-rate for electrospinning tin(IV) acetate/polyvinyl acetate solution results in greater phase separation which shows as thin-walled and porous nanofibers whereas low feed-rate give rise to solid and smooth surface fibers after sintering.

In the conservation of mass, when the feed-rate of the solution increases, the excess solution needs to be channeled somewhere. Below is a summary of possible common events that may happen as covered in the article leading to the differences in observation of the resultant fibers,

Differences in the fiber physical characteristics due to higher solution feed rate can be easily explained by its electrospinning behavior. The electrospinning behavior is in turn dependent on other solution characteristics, ambient condition and spinning parameters. Of particular interest will be the solution characteristics and its effect on electrospinning behavior when the solution feed-rate increases and the prediction on its effect on the fiber physical characteristic. This has not been adequately investigated although it is an important consideration on the production rate of electrospinning since a higher feed-rate will contribute to greater fiber mass output.