Formation of polymer nanofibers from electrified fluid jets

Abstract

The formation of polymer nanofibers from fluid jets in· an electric field, also referred to as electrospinning, has been studied. Controlling the fiber properties requires a detailed understanding of how a millimeter-diameter fluid jet emanating from a nozzle is transformed into solid fibers that are four orders of magnitude smaller in diameter. To this end, a fiber spinner operating under a uniform electric field and providing a controlled process environment was designed. In the conventional view of electrospinning, the mechanism leading to small fiber diameters has been attributed to the splaying phenomenon, in which a single jet splits into multiple smaller jets due to radial charge repulsion. Using high-speed photography and an aqueous solution of poly(ethylene oxide) as a model fluid, it was shown that the jet does not splay but instead undergoes a rapid whipping motion. The high whipping frequency created the optical artifact of multiple jets. The whipping jet was best observed in the onset region of the instability. Further downstream, the amplitude of the instability continued to grow, and the jet trajectory became more chaotic. This was verified through photography of the entire jet and close-up observations of representative regions further downstream. Based on these findings, an alternative mechanism for the formation of polymer nanofibers is proposed. It is conjectured that the whipping instability causes stretching and bending of the jet. The large reduction in jet diameter is achieved by increasing the path length over which the fluid jet is accelerated and stretched prior to solidification or deposition on a collector. Whipping induced stretching is conjectured to be the primary mechanism causing the jet diameter reduction. To provide a concise way of displaying the stability of electrified fluid jets as a function of the electric field and the flow rate, operating diagrams were developed. These diagrams delineate regions of different jet behavior, and the stability borders for two transitions have been mapped. The first transition is from dripping to a stable jet; and represents the suppression of the Rayleigh instability. For high conductivity fluids, an additional transition from a stable to a whipping jet can be observed at higher electric fields. The experimental findings are supported by a theoretical analysis of the jet thinning and the onset of the instability. To elucidate the fundamental electrohydrodynamics, glycerol was studied as a model fluid. Based on the experimental observation that whipping occurs on a length scale much larger than the jet radius, an asymptotic approximation of the electrohydrodynamic equations has been developed by Hohman and Brenner. This theory governs both long wavelength axisymmetric and non-axisymmetric distortions of the jet, and allows the jet stability to be evaluated as a function of all relevant fluid and process parameters. Three different instabilities are predicted: the classical Rayleigh instability, an axisymmetric conducting mode, and a non-axisymmetric conducting mode. The presence of these instabilities at various locations along the jet has been verified with high-speed video and photography. The particular instability that is observed depends on the jet shape and the surface charge density. To achieve quantitative agreement between experimental and theoretical jet profiles, the jet current and the local electric field in the vicinity of the nozzle had to be taken into account. The electric currents in stable jets were found to be linear in both the electric field and the flow rate Theoretical operating diagrams were developed based on the experimental insight that the instabilities are convective. The dependence of the stability borders on both the electric field and the flow rate is correctly reproduced by the Hohman-Brenner theory. This implies that operating diagrams have the potential to be used as predictive tools to better understand and control the process. The quantitative agreement between theory and experiments suggests that the fundamental process in electrospinning involves indeed a rapidly whipping jet, which is caused by the interaction of surface charges on the jet and the applied electric field. The notion of a whipping jet has also been extended to low viscosity fluids, where the jet disintegrates into fine droplets, i.e., electrospraying. For sufficiently large jet radii, experiments have verified the theoretical prediction that the dispersal of fluid results from the growth of a non-axisymmetric conducting mode along the jet, which subsequently breaks into droplets due to the axisymmetric conducting mode.