On pinchoff behavior of electrified droplets

Electrohydrodynamic (EHD) inkjets produce micro- and nano-sized droplets by using an electrostatic field imposed between the nozzle and the substrate. The conducting droplet about to separate (or "pinchoff") from the continuous fluid undergoes a transitional topological change under the competition between the electrical force due the imposed electrostatic field and the forces arising from liquid's own thermo-physical properties, such as viscosity and surface tension. This study is the first attempt to elucidate the pinchoff topology and characteristics of an electrified liquid droplet experimentally. Aspect ratio, which is defined as the ratio of the vertical (b) to horizontal (a) dimensions of a pinchoff droplet, is reported. The range of the dimensionless charging level (N_e) that yields stable microdripping droplets is also reported for various liquids (i.e., acetone, methanol, ethanol, water, dimethylformamide, and methoxyethanol). We experimentally showed that pinchoff topology is a function of both the charge level (N_e) and the electrical conductivity (K) of the droplet liquid. Despite the different properties of the liquids examined, pinchoff topologies were similar when N_e and K were set as constants.