Buckling of an electrically charged liquid jet

Abstract

The mechanism of fiber electrospinning from polymer solutions was theoretically studied. An equationdescribing the destabilization of an electricallycharged jet of a polymer solution was derived. Thebehavior of the jet cannot be explained without takinginto account its elasticity. It was concluded that theelectrospinning is possible only within a certain polymer concentration range. At too high polymer concentration, the jet elasticity is too high and the destabilization is suppressed. At too low polymer concentration, macromolecules are isolated and do not forma single entanglement network. The main parametersdetermining the electrospinning are the linear electriccharge density and Young’s modulus of the jet.Among commercial methods for producing chemical fibers, the electrospinning of nonwoven fibrousmaterials occupies a special place owing to its simplicity, high efficiency, process flexibility, and productdiversity [1–3]. The main advantage of this method isthe possibility of producing very thin fibers of diameter~1 μm and recently even down to 10 nm.Fibers are electrospin from a lowconductivitypolymer solution. A typical polymer concentration is~10%. An electric potential difference of ~3–20 kV isapplied between a receiving electrode and a meteringnozzle with the polymer solution. In nature, fiberelectrospinning is similar to electrospraying. Electrospinning was discovered in 1939 by N.A. Fuks andN.D. Rozenblyum in attempt to produce solid spherical aerosol particles of nitrocellulose from its acetonesolution. However, they suddenly observed a fiber generation mechanism by which a liquid jet flowing froma nozzle changed to strong continuous fibers of a stable cross section of ~1 μm instead of the expectedbreakup into drops. Let us now describe the processmechanism.A fiber is shaped only by electric forces. The electric field in a conductive material is known to be zero.On the surface of an electrically conductive liquid, acharge forms. The charge is acted by an electric fieldopposed by surface tension forces. At low fieldstrengths, the electric forces are weak and the liquidsurface remains stable. As the field strength isincreased, the electric forces increase, and at a certainelectric field strength, the surface tension forcesbecome weaker than the electrostatic forces. As aresult, the drop shape becomes unstable and the socalled Taylor cone forms. In the absence of a polymer,such instability leads to the detachment of the cone tipand the formation of charged drops, and in the case ofa polymer solution, a thin jet forms.Photography showed that, once leaving the nozzle,the jet is destabilized and ta kes the shape of a divergentconical helix [1, 4]. Having been formed, the jet gradually becomes increasingly wider and, at a certainmoment of time, is destabilized again. Up to four successive stages of jet destabilization were observed.The destabilization of the straightline jet isexplained by the Coulomb repulsion of like charges.Once the jet shape deviates from a straightline segment, a resultant of forces emerges (figure), causing anincrease in the fluctuation amplitude and, finally, theformation of the helix.The behavior of a 2% aqueous poly(ethylene oxide)(