Extension Rate and Bending Instability of Electrospinning Jets: the Role of the Electric Field

Abstract

Extension rates of an electrospinning jet, in the straight segment as well as in the first jet bend, subjected to different electric fields by changing the tip-to-collector distance (H) at a given applied voltage have been extensively studied through a model solution of poly(N-isopropylacrylamide) in the DMF solvent with a concentration of 17 wt %. Light scattering experiments were carried out to obtain the jet diameter profile along the straight jet (z direction), from which the profiles of the jet velocity vj(z) and the extension rate ε̇(z) (=dvj/dz) were subsequently derived on the assumption of negligible solvent evaporation. In addition, the jet-bending process was also monitored by high-speed video imaging to successfully obtain the axial velocity (z direction) and the lateral velocity of the first bend (wave), from which the extension rate of the first wave was calculated based on a simple equation for the spiral jet. In the straight jet section, the extension rate is found to be the highest at the Taylor cone apex (region I with ε̇I), and it rapidly decays to a relatively constant value in the main jet region (region II with ε̇II), followed by a significant drop to zero near the straight jet end (region III, ε̇III ∼ 0). The presence of the zero extension rate in region III is due to the effective air drag resisting the electric stretching when the fast flowing jet becomes too thin. As the nominal electric field is increased by a smaller H, ε̇II is enhanced. Both ε̇I and ε̇II are higher than the chain retraction rate τe–1 (∼370 s–1) for all the Hs studied, implying that the chain orientation/stretch has already taken place in the straight jet. The residence time of fluid elements in region III is short (<τe), so that the axial elastic strains developed in regions I and II survive downstream in the jet-bending region to stabilize the jet. Our video imaging with high frame rates (>5000 fps) reveals the formation of the “crumpling jet” section at the straight jet end, in which the excitation and relaxation of the dominant Fourier modes of the lateral perturbations of the jet occur in dynamic equilibrium to keep the feature of the straight jet on time average; the lateral perturbations are driven by the compressive force dominantly arising from the air drag (Fdrag). After the conflict between the Coulomb repulsive force (Fc) and the residual elastic tension (Felas), the crumpling jet section finally developed the first bend with a high frequency and a small wavelength, when the section at the jet end slightly shifts downstream. Our results show that both Fdrag and Felas should be taken into consideration at the onset of the jet bending (whipping) process; both promote locally the jet buckling at the straight jet end to generate the crumpling jet, and the latter resists the formation and growth of the first jet bend as well as promotes the shifting of the bending jet upstream. The maximum extension rate of the spiral-growing bend is found to be larger than τe–1. We concluded that the extension rate of the electrospinning jet, from the cone apex to the jet whipping, is position (or time)-dependent, and the process is extremely dynamic due to the fast flow, accompanied by subtle force perturbations along the spinning line.