Monodisperse Electrosprays of Low Electric Conductivity Liquids in the Cone-Jet Mode

An experimental investigation was performed on electrosprays of low electric conductivity liquids (heptane with different amounts of an antistatic additive) that were operated in the cone-jet mode. The effects of liquid flow rate, applied voltage, and liquid electric conductivity on droplet size and spray monodispersity were systematically investigated. The droplet size was found to be dominantly controlled by the liquid flow rate and secondarily by the applied voltage. It showed no dependence on capillary size. For a given liquid, stable and monodisperse electrosprays could be established only within certain ranges of liquid flow rates and applied voltages, that defined a cone-jet domain. This domain was affected by the electric conductivity of the liquid. As the liquid electric conductivity increased, the domain shifted toward smaller flow rates which implies that smaller droplets were generated in the spray. An increase in the capillary diameter caused a narrowing of this domain. Outside the cone-jet domain, electrosprays were unstable and polydisperse with different instability patterns that depended on the applied voltage and the liquid flow rate. Experiments also showed that the droplet size and the spray monodispersity were independent of the electrode configuration, as long as the electrospray was operated at the onset voltage condition, that is defined as the minimum voltage at which the cone-jet mode was established. By using dimensional analysis the controlling variables were combined into a few dimensionless groups and an empirical fit was derived and was shown to correlate well all the experimental data.