Novel Insights into Solution Electrospinning for Nanofibers

To understand how nanofibers are formed by solution electrospinning, an in situ observation of the fluid flow is essential. In this study, the flow behavior of charged fluids from the Taylor cone, straight jet part, until whipping (or spiral) jet part along the spinline is explored via particle image velocimetry, light scattering, and high-speed videography, respectively. The results of light scattering reveal that the stretching rate in the straight jet exceeds the intrinsic rates of polymer relaxation in the fluid, derived from the dynamic rheological measurement, supporting the hypothesis of flow-induced phase separation and the resultant evolution of the dissipative structures, the so-called "string" structures, in the straight jet section. Using liquid nitrogen to collect the straight jet followed by freeze-drying, assembled strings with various widths are validated. Moreover, dynamic vortex flow is observed in the Taylor cone using particle image velocimetry, likely generating a swirling flow in the cone apex. The vorticity of the swirl is increased after passing the cone-jet transition zone, at which the electric field is the highest. Thereafter, the enhanced swirl gradually decays (or releases its imposed torsion) during its propagation along the straight jet via a jet twist. The straight jet with internal swirl is considered as the precursor of the spiral jet, given that the preimposed torsion in the straight jet is not completely relaxed at the straight jet end. Using high-speed videography, a transition of the handedness of the spiral jet, rotating either clockwise or counterclockwise, is repeatedly observed in a single spinning line, suggesting the intermittent entry of the swirl with different handedness in the cone apex. Thus, the downstream spiral jet is relevant to the upstream entry flow at the cone apex; this phenomenon resembles the classic extrusion instability. Helical fibers (or coils) are observed on the ground collector, as the residual torsion in the spiral jet is not fully released after solvent evaporation in the spinning line of the spun fiber. Our work shows that important external flow fields are applied to semidilute solutions through the electrospinning process, which self-organizes nanofibers as ordered structures from dissipative structures through thermal concentration fluctuations along the spinning line, starting from the needle tip to the whipping jet.