Investigations into the mechanism of electrohydrodynamic spraying of liquids

The mechanism of stable jet formation was investigated by observing the motion of the liquid in a jet close to the capillary tip from which it emerges under the influence of an electric field. Tracer particles of lycopodium were inserted in the liquid and streak photographs were taken of a stable jet formed under the application of 10.5 kV and a flow rate of 30 ml/h. These photographs show an axisymmetric circulation of the liquid in the conical base of the jet, invalidating previous theories that have assumed a uniform velocity profile in the liquid cone. This axisymmetric motion of the liquid in the jet was explained in terms of interfacial electrical shear stresses. Due to the semi-insulating nature of the liquid, there will exist a potential difference between the base of the capillary and the tip of the cone. This potential drop ensures that the interface is subjected to a tangential electric field in the direction of flow and hence an electric shear stress on the surface of the cone. Both tangential and normal fields on the cone were calculated from a knowledge of the jet profile and current. The tangential field on the surface of the cone was calculated by considering it to be a section of a sphere and dividing it into five sections. Each section was assumed to form an equipotential surface normal to the direction of flow and to have a constant current flowing through it. The normal field was calculated numerically using a computer program that estimates the potential distribution within a region subject to given boundary conditions using the finite element method. The results of calculation showed that the shear force acting on the cone was about 5 to 10 times smaller than the normal force. However, the normal force was found to be fairly constant and independent of applied voltage, within the range studied. In contrast, the shear force showed a tendency to decrease with an increase in applied voltage. It was also observed that as the voltage was increased, the length of the jet decreased. This could be explained in terms of the reduction of tangential shear force with an increase in applied voltage, thus reducing jet stability.