Beads Formation in Electrospinning

Beaded electrospun fibers are commonly reported when the concentration of the solution or the molecular weight of the polymer used is too low. They are usually considered as "poor" quality fibers and the electrospinning parameters are often optimized to eliminate beads on fiber.

Experiments have shown that the following are the main factors contributing to beads formation,

The cause of beads formation has been attributed to the elasticity of the solution. Fluids with low relaxation time or low extensional viscosity tend to result in beads formation. The mechanism is due to Rayleigh instability driven by surface tension which can be suppressed by viscoelastic behavior of the fluid jet. Where the viscoelastic force completely suppresses or resists the instability, smooth fibers will form [Yu et al 2006]. Instability is caused by surface tension as it forces a liquid to assume a smaller surface area per unit mass which is the form of a sphere [Fong et al 1999]. Adding ethanol to water instead of using pure water has been shown to eliminate beads on polyethylene oxide fiber due to surface tension reduction [Fong et al 1999]. The viscoelasticity of the solution can be increased by increasing the polymer concentration or molecular weight which has been shown to be effective in eliminating beads formation [Fong et al 1999]. However, this also has the effect of increasing the fiber diameter.

Charge density on the electrospinning jet represents the stretching force to elongate the solution. Inadequate charge density results in insufficient elongation viscosity to counter Raleigh instability and this leads to beads formation [Fong et al 1999; Thomspon et al 2007]. Deliberate removal of charges on the electrospinning jet through the use of neutralizing ions has been shown to results in beads formation [Fong et al 1999]. Addition of salt to the solution enhances the charge carrying capacity of the solution and this generally favors the production of beads-free fibers [Choi et al 2004].

Several theoretical modeling has been used to explain the formation of beads on a string structure in viscoelastic filaments [Bhat et al 2010; Oliveira and McKinley 2005]. However, stretching of viscoelastic filaments in electrospinning is slightly different from conventional mechanical stretching or gravitational stretching of viscoelastic fluids as it involves charges that impart a uniform stretching force along the length of the stretched fluid. Zhu et al (2012) studied the evolution of beads formation by electrospinning poly(hydroxybutyrate-co-hydroxyvalerate) on an ethanol bath collector. Depositing the fibers on water collector allows the physical shape of the electrospinning fluid to be arrested at the specific needle tip to collector distance. As the tip to collector increases, the solidified fluid jet physical shape changes in the following sequence, fat fiber, bean-pod beads, individual closely spaced beads and finally beads spaced wider apart.

From the shape of the fibers, the chain of events for fiber formation can be envisioned. During the first stage of electrospinning, the fluid cylinder diameter is large thus no beads are formed. Zhu et al (2012) explained that the formation of bean-pod beads were due to electric field and surface tension without further elaboration. It is likely that Coulomb forces at the end of the beads connecting to the fibers have higher charges and this encourages the bead to be pulled apart when there is sufficient repulsive force. A possible mechanism is illustrated below.

After the bead has separated, charge concentration at the ends of the beads ensures an even distribution of the beads along the fiber length. While the study by Zhu et al (2012) did not suggest that the voltage is varied to maintain the same electric field intensity across the different tip to collector distance, it still provides an interesting insight into the possible evolution of the beads.

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▼ Credit and Acknowledgement

	Stage 1. Charge concentration at the ends of the bead exert a repulsive force on the bead.
	Stage 2. The bead starts to split apart and gathering of charges at the center exert more repulsive force.
	Stage 3. Concentration of the charges at the ends of the split beads caused the beads to move further apart.