Investigations into the mechanisms of electrohydrodynamic spraying of liquids. I: Effect of electric field and the environment on pendant drops and factors affecting the formation of stable jets and atomization

Abstract The effect of an applied electric field on pendant and flowing drops (through a capillary) was studied by observing the droplet profile as a function of applied voltage. With pendant drops of hexadecane, application of the electric field caused a reduction in the apparent surface tension, γ app . In surrounding gases such as N 2 , CO 2 , and Ar, droplet profiles similar to those in air were observed. While gases such as He and Ne have low breakdown potentials, no change in droplet profile upon application of an electric field was detectable. The glow discharge in He was investigated for oil (insulator), water, glycerol, and a brass tip, for both positive and negative potentials. Conducting liquids such as water and glycerol permitted discharge to take place from the surface of the droplet, whereas insulating oils forced the discharge to occur at the metal tip, where the drop is attached. With flowing drops, the liquid conductivity was a major factor in the electrostatic disruption of the liquid surface. With insulating liquids such as paraffinic oil, no disruption occurred due to the lack of sufficient free ions in the bulk liquid. With conducting liquids such as water, very unstable streams were produced. Stable jets having a conical base were only produced with semiconducting liquids (nonpolar liquids with dissolved ionic materials). The cone angle at the base of the jet increased, whereas its length decreased with an increase in applied voltage. At higher voltages, secondary jets were produced from the primary one, whose number increased with an increase in the applied field. The effect of liquid conductivity, applied voltage, flow rate, and capillary diameter on the stability of jets was investigated by measuring the critical voltage, φ c , at which transition from the pulsating mode to the stable jet mode occurred. By measuring the current carried by the jet, the charge-to-mass ratio could also be calculated. Some measurements of droplet size distribution were made using a practical sprayer and a particle measuring system for measuring droplet diameters in flight. These measurements were made as a function of applied voltage, conductivity, and flow rate. The results obtained clearly demonstrate the importance of applied voltage, liquid conductivity and flow rate in the formation of stable jets and the subsequent process of electrohydrodynamic atomization. At a given voltage and flow rate an optimum conductivity range is necessary for producing the most stable jet, the narrowest size distribution, and the smallest droplet size. This could be accounted for in terms of the electric forces acting on the liquid, which are related to the relaxation time of the liquid. The influence of flow rate on the production of stable jets and the subsequent atomization could also be understood in terms of the inertial and electrostatic forces which act in the same direction. At a given conductivity and voltage, stability is enhanced by increasing the flow rate, but at the expense of producing larger droplets.