Electrohydrodynamic instability of a charged polymer jet
Date Issued
2010
Date
2010
Author(s)
Chen, Kuan-Hung
Abstract
Electrospinning is currently the only technique to produce continuous nanofibers with submicron-scale diameters. Although numerous successful applications have found in filtration, medicine, and catalysis industries, the uncontrolled random motion caused by inherent instability during jetting process often leads to the formation of undesirable patterns such as beads-on-string or nonwoven mats, which severely retards the technique toward wider applications. Therefore, a further understanding of the flow instability is necessary to improve the controlling of the movement of electrospinning nanofibers.
This paper aims to carry out the electrohydrodynamic instability of a charged polymer jet to explore the instability mechanisms. The effects of physical parameters on the onset of jet instability are investigated by employing temporal linear stability theory, so as to give a qualitative explanation of observed phenomena. Results show that the liquid viscosity is a mechanism responsible for energy dissipation, decreasing the growth of disturbances and postponing the breakup of the jet. The entanglement effect of polymer in terms of elasticity can enhance the instability by storing the strain energy and releasing the stresses out of phase. Both of the two mechanisms do not trigger more modes to grow. On the contrary, the electrical Coulomb force and the aerodynamic drag arising at the jet surface will be able to induce the growths of more asymmetric modes. The surface tension has a stabilizing effect on all asymmetric modes while it always destabilizes the axisymmetric one. Furthermore, the formation of beads-on-string is an outcome of the predominance of the axisymmetric mode, and the bending instability due to the presence of the electrically repulsive force must be responsible for the chaotic whipping motion. In particular, we also find that, under certain conditions, an increase in the elasticity may lead to the occurrence of branching. The detailed parametric diagrams provide a basic guideline for applications of electrospinning nanofibers technology in future.
Subjects
Electrospinning
Nanofibers
Polymer
Viscoelastic flow
Instability
Electrohydrodynamics
Type
thesis
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