Critical condition of electrohydrodynamic jetting from a polymer-solution droplet on a conductive wire

An experimental study was conducted to determine the threshold electrostatic field to initiate electrohydrodynamic jetting from a polymer solution droplet wetting on a conductive wire. The study is crucial to understand the roles of the material and process parameters in wire-based needleless electrospinning for controllable mass production of continuous nanofibers. Two types of polymer solutions, i.e., polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) and aqueous polyethyloxide solution (PEO/H2O), with the mass concentrations of 4%, 8%, 12%, and 16% for PAN and 1%, 2%, 4%, 6%, and 8% for PEO, respectively, were considered in the experiments. Taut thin copper wires with diameters of 0.254, 0.508, 1.016, and 1.524 mm (i.e., 0.01, 0.02, 0.04, and 0.06 in.) were utilized, respectively, as the positive electrodes. The effects of the polymer concentration and wire diameter on the threshold electrical voltage for jetting initiation and nanofiber diameter were examined. Given the droplet volume and spacing between the copper wire and fiber collector, experimental observations show that the threshold electrical voltage increases with increasing either polymer mass concentration or wire diameter. In addition, the effects of polymer mass concentration on the transient shear viscosity and surface tension of the PAN/DMF solutions were examined. It shows the positive correlation between the transient shear viscosity and polymer mass concentration, shear thinning of the polymer solution at a high shear rate, and nearly constant surface tension of the polymer solution in the range of the present PAN mass concentration. Furthermore, a simple electrostatic model is formulated to phenomenologically elucidate the experimental observations. The present study provides useful scaling laws for controllable scale-up nanofiber fabrication by means of a wire-based needleless electrospinning technique.