Electrospinning and electrically forced jets. I. Stability theory

TLDR

Electrospinning produces solid fibers with diameters below 1 µm from a polymeric fluid stream, and the physical mechanisms underlying jet instability are discussed across various limits. This series of papers analyzes the mechanics of the whipping jet by studying the instability of an electrically forced fluid jet as the field strength increases. An asymptotic approximation of electrohydrodynamic equations is developed to model long‑wavelength axisymmetric distortions and centerline oscillations, identifying Rayleigh, electric‑field–induced axisymmetric, and whipping instabilities. Experiments confirm that a rapidly whipping jet is essential, and as field strength increases electrical instabilities grow while Rayleigh instability is suppressed, with dominance determined by surface charge density and jet radius.

Abstract

Electrospinning is a process in which solid fibers are produced from a polymeric fluid stream (solution or melt) delivered through a millimeter-scale nozzle. The solid fibers are notable for their very small diameters (<1 μm). Recent experiments demonstrate that an essential mechanism of electrospinning is a rapidly whipping fluid jet. This series of papers analyzes the mechanics of this whipping jet by studying the instability of an electrically forced fluid jet with increasing field strength. An asymptotic approximation of the equations of electrohydrodynamics is developed so that quantitative comparisons with experiments can be carried out. The approximation governs both long wavelength axisymmetric distortions of the jet, as well as long wavelength oscillations of the centerline of the jet. Three different instabilities are identified: the classical (axisymmetric) Rayleigh instability, and electric field induced axisymmetric and whipping instabilities. At increasing field strengths, the electrical instabilities are enhanced whereas the Rayleigh instability is suppressed. Which instability dominates depends strongly on the surface charge density and radius of the jet. The physical mechanisms for the instability are discussed in the various possible limits.