Tip to Collector Distance

Fused fibers when insufficient time is given for the solvent to vaporise.

The distance between top and collector parameter has an effect on the jet path and traveling time before resting on the collector. If the voltage is kept constant, the electric field strength will be inversely proportional to the distance. In a typical electrospinning setup, this distance range from 10 cm to 15 cm which generally allows sufficient flight time for the solvent to vaporize such that a dry fiber strand is deposited. If the distance is too short such that the solvent is inadequately vaporized, fused fibers may be formed. In near-field electrospinning, the distance between the tip and the collector is usually less than 10 mm. At such close proximity, the spinning tip is generally much smaller than conventional electrospinning so that the initial jet radius is also reduced. The smaller volume or feed rate allows sufficient vaporization of the solvent such that relatively dry fiber can be collected.

While it is easily understood that when the distance is too short, fused fibers may be formed due to insufficient solvent vaporization, the same fused fiber has been observed when the distance is beyond the optimal range. Ghelich et al (2015) made this observation when they electrospin PVA/NiO-GDC fibers. At electrospinning tip to collector distance of 8 cm, fused fibers were observed. At 10 cm, distinct individual fibers were collected. The diameter of the fibers collected at both distances was similar. At 15 cm, fused fibers were again observed and the diameter increases significantly. The increase in fiber diameter was attributed to reduced electrostatic field strength which leads to less stretching of the fibers. As there are less stretching, the greater diameter increase the amount of solvent trapped within the fibers. The trapped solvents continue to diffuse out after the fiber has deposited and this causes fusion of the fibers.

In cases where the voltage is kept constant while the distance is varied, the electrospinning jet behavior and the collected fiber profile is similar to cases where the voltage is varied while maintaining tip to collector distance. Increasing the distance between tip and collector may reduce the fiber diameter as there is a greater stretching distance [Mazoochi et al 2012]. However, beyond a certain distance, the fiber diameter may increase instead due to the significantly weakened field strength [Ding et al 2010, Bosworth 2012].

In many applications, it is preferred that the fiber deposition is uniform over a specific area. This will reduce the formation of distinct fiber layers which will reduce the tensile property of a membrane due to breakage at weaker interlayer interface. Uniformity of the fiber layer would also give the composite structure a more consistent performance and property. To facilitate uniform distribution of the fibers, a larger deposition area is preferred and this has been demonstrated by increasing the tip to collector distance [Cha et al 2006]. However, there is a limit in which the expanding cone flight profile of the electrospinning jet can cover beyond which the deposition area no longer increases.