Electrospinning is a novel process that allows the production of nanoscale fibres by using electrostatic force to accelerate a fluid jet. The decisive phase in the process is the region of jet instability, where it is believed that most of the jet thinning occurs. We focus on the onset of axisymmetric instability modes. These instabilities result in the production of beaded nanofibres, which are often seen in practice.

We have recently presented a model for the development of the initial stable electrospun jet, where the laws of electrohydrodynamics (momentum equations and Gauss's Law) are fully coupled with viscoelastic polymeric constitutive equations. We now extend this model, and perform a linear stability analysis on the jet. This analysis allows us to identify two distinct unstable axisymmetric instability modes, and to predict the growth rate and critical wavelength of each. By using an energy analysis, the interplay between base state and perturbation for each instability mode has been identified. For low electrical-conductivity fluids, the dominant instability mode is seen to be capillary-driven, whereas for higher conductivity fluids, the instability modes are more complex, and are driven by the interaction of the electric field with the jet. Our stability analysis also reveals that the viscoelasticity of the fluid has a strong influence on the stability characteristics of these axisymmetric modes, affecting the growth rates and critical wavenumbers.

Axisymmetric instabilities observed during the electrospinning of polymer solutions with various conductivies (PIB Boger fluid, PEO/water solutions, PS-based solutions) have been captured using high-speed photography. The instability characteristics (wavelength and growth rate) are extracted using image analysis, and are compared with the model predictions.