Voltage

Needle-free wire electrospinning initiation by Elmarco

Adjusting the voltage applied have several influences on the electrospinning process since it affect the amount charges applied to the solution. Increasing voltage will accelerate the electrospinning jet and this may result in greater volume of solution drawn from the tip of the needle. If the feed rate of the solution is fixed, this will result in a smaller and less stable Taylor cone [Zhong et al 2002] and may eventually cause the Taylor cone to recede into the needle [Deitzel et al 2001]. There are several contradicting reports on the effect of higher voltage applied on fiber morphology. With a higher applied voltage, it is expected that this will lead to greater stretching of the solution and this in turn should lead to thinner fibers and has been demonstrated in several papers [Lee et al 2004, Buchko et al 1999]. Electrospinning of low viscosity solution at higher voltage may also favor formation of secondary jets and this will also result in smaller fiber diameter. However, exception to this case has also been reported and it is thought that with a lower voltage close to the critical voltage, the flight duration is increased and this allow more time for the solution to be stretched prior to deposition [Zhao et a 2004]. A study by Wu et al showed that the fiber diameters were found to reduce with increasing voltage to a minimum before the trend reverses with further increase in voltage [Wu et al 2012]. 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. Conversely, studies have shown that fiber diameter may initially increase with higher voltage but beyond a certain voltage, the fiber diameter starts to reduce [Mazoochi et al 2012]. While stretching of the solution droplet may increase with higher voltage, the high voltage may also lead to faster acceleration towards the collector due to the increased potential difference leading to less flight time for the jet to stretch prior to deposition. This in turn results in larger fiber diameter.

Figure 1. General effect of increasing voltage on electrospinning process and fiber diameter

Increased beads formation when a higher voltage is applied has been reported by some researchers [Demir et al 2002] although contrary result has also been reported [Jarusuwannapoom et al 2005]. While increased stretching with higher voltage leading to less beads formation is easily understood, the opposite effect is somewhat puzzling. Nevertheless, this may be explained in term of the flight duration of the electrospinning jet. As mentioned in the earlier paragraph, increasing voltage may reduce the flight duration and this in turn may result in less time given for the spinning solution to stretch. Therefore, beads on the electrospinning jet are not given time to be stretched for the formation of smooth fibers.

Electrospnning may also be initiated without the spinneret coming into direct contact with the electrode [Furlan et al 2010]. A solution droplet falling through a electric field could gather sufficient charges to elongate it [Filatov et al]. The density of the charges are influenced by the amount of charges applied to it, the ability of the solution to carry charges and the surface curvature of the solution. The gradient of the potential difference within an electric field is influenced by the distance between two points and/or plates. The surface tension of the solution is a property of the solution but can be reduced by using additives such as surfactants.

Since initiation of the electrospinning process is dependent on the density of charges on the solution, the solution may be dispensed in unconventional methods. Free-surface and needless electrospinning has shown that an orifice is not necessary for this process.

Needle-free drum electrospinning initiation by Elmarco

Most electrospinnings are carried out using high voltage direct current (DC). However, high voltage alternating (AC) current has also been shown to be able to electrospin fibers [Kessick et al 2004]. An additional parameter when using AC high voltage for electrospinning is the frequency of the current. When the frequency is too high, the transfer of charges or mobility of the ion may not be fast enough to sufficiently charge the solution for electrospinning. For each material, the frequency range for AC electrospinning needs to be determined and it is typically between 500 Hz and 1 kHz [Sarkar et al 2007]. When the frequency is too low, the electrospinning jet traveling between the tip and the collector may comprise mainly of a single polarity instead of periodic positive and negative segments. The periodicity of positive and negative segments on an AC electrospinning jet has been shown to facilitate a more stable jet formation compared to DC. When collecting on a rotating drum, the more stable AC electrospun jet produces fiber with greater alignment compared to fibers from DC [Kessick et al 2004]. However, the fiber diameter from AC electrospinning is significantly larger compared to DC electrospinning [Kessick et al 2004]. This is probably due to suppression of bending instability in AC electrospinning.

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Content [Electrospinning Parameters]